Building my own radiant (hydronic) system

Greetings. I’m currently building an addition to our 20+ year old straw bale home. Since I’m a thrifty type, most everything is DIY. I’ve buried the PERT tubing in the floor, now having to copy a commercial radiant heat control panel. It looks easy enough and I can build the similar commercial $3k panel for less than $1 k in parts. I can solder.
I’m looking for input on whether my proposed design makes sense; it’ll be powered by a new rinnai lp tankless water heater, feeding (eventually) 4 zones via zone valves. I’ll try to add my schematic below.
Any input appreciated. Thanks. Tom

Tom: "I’ve buried the PERT tubing in the floor"
====
First, Tom, what is PERT? [PolyEthylene Radiant T???]

into what type/kind of flooring?

Terry

Tom Moran wrote:Greetings. I’m currently building an addition to our 20+ year old straw bale home. Since I’m a thrifty type, most everything is DIY. I’ve buried the PERT tubing in the floor, now having to copy a commercial radiant heat control panel. It looks easy enough and I can build the similar commercial $3k panel for less than $1 k in parts. I can solder.
I’m looking for input on whether my proposed design makes sense; it’ll be powered by a new rinnai lp tankless water heater, feeding (eventually) 4 zones via zone valves. I’ll try to add my schematic below.
Any input appreciated. Thanks. Tom

I did a huge whole house radiant floor heating system many many many moons ago. Two floors plus a garage and basement. The latter two had buried in concrete tubing and the house portion, two floors, had the tubing attached to the underside of the plywood floor in a normal wood framed house.

Make your outflow and the return headers for the 4 zones now with the shut off valves/zone valves, etc attached and whatever type of connection you are using, going to use be it barbed, solder, ... so all you have to do later is attach the new feed lines for any new zone you add and of course the return lines so you won't have to shut down the whole system when adding new zones.

One big mistake I made was not allowing for expansion of the concrete floors. Use an [obviously] stay in place compressible form "board" where you are doing a concrete floor pour. Or you will end up with cracked/heaved concrete.

Also stay away from carpeting as it is "fabulous insulation" and it doesn't allow for easy movement of heat to people.

It has been so long that I can't remember the names of the various plumbing fittings though I can see the whole thing in my mind. In those days I think I used Poly B but not even sure on that.

Tom,  there are charts around to determine the size radiant panels most suitable for each room.

This little setup cost about $1,000.
https://www.supplyhouse.com/Caleffi-Mixing-Stations-21113000
https://youtu.be/vXSqEZwK4Ok


$2,000 Tankless Hot Water Heater/Boiler + Pressure Relief Valve + Expansion Tank
$1,000 Pump+Manifold+Air Eliminator+Fill/Drain
$1,000 Piping for 4-zone (each about 250sqft)
$1,000 4-Thermostat + 4-Actuators

Webasto makes diesel boiler hydronic systems for marine applications. Their model ts17 puts out 17k btu's for less than $2500.

Hi Tom,

I probably know just enough to get in trouble

It looks like you are planning to circulate hot water in the loop from the Rinnai and then use the zone pump to inject hot water into the PERT loop(s) in a primary/secondary loop configuration. How will you control the temperature to the PERT? Will the Rinnai turn down low enough? There are suggested limits for different floor coverings but you probably won't want the floor temp over 75-77F. Depending on your heat loss and floor covering the hot water will need to be higher than that temp. If the Rinnai won't turn down far enough, you might want to consider a small buffer tank to keep it from cycling as often.

Resources:

The Uponor Complete Design Assistance Manual (free) has a diagram almost identical to yours (not quite as detailed) on page 141 in the 7th edition. This manual will walk you through the necessary calculations to set the water temp and flow.

https://www.uponor.com/en-us/customer-support/order-manuals#radiant-heating

If you really want to jump in the deep end, you can get a trial copy of LoopCAD for free (linked on the Uponor web page) and it will calculate everything. They have video tutorials but some effort to figure it out will be required

https://www.uponor.com/en-us/residential-solutions/loopcad

There is a lot of good info at the hearth.com forums. There are also a lot of knowledgeable people there who might provide useful feedback on your design. Probably the boiler room forum is the most applicable:

https://www.hearth.com/talk/forums/the-boiler-room-wood-boilers-and-furnaces.13/

You might also check out the Caleffi Idronics magazine. There are a bunch of issues covering a lot of topics related to boilers, radiators, radiant heat and more:

https://www.caleffi.com/usa/en-us/technical-magazine

I installed PEX-B in an insulated concrete slab. The manifolds and pumps came from supplyhouse.com. You might also check pexuniverse.com. You can get the Caleffi manifold or for about half the money get a Bluefin stainless manifold. I got the Bluefin with PEX adapters and so far, everything is good. PERT adapters are not listed but may be available. Maybe they are the same as PEX? I got a Grundfos Alpha circulator for the floor loops and a Resideo (Honeywell) circulator with a speed control input for the boiler loop so the wood fired boiler can control its speed. The pumps with ECM motors are much more efficient, a big consideration since we're off grid.

Terry Byrne wrote:Tom: "I’ve buried the PERT tubing in the floor"
====
First, Tom, what is PERT? [PolyEthylene Radiant T???]

into what type/kind of flooring?

Terry

Watts PERT tubing is Polyethylene Raised Temperature. 5 Layer flexible tubing that is not cross linked like PEX.

I am not even sure where to start on this, I guess by saying that while doing what you suggest will work, it will not be very efficient. They make purpose built radiant floor boilers for a reason, but they are expensive up front I know.

As for your pex loops, they need to be EXACTLY the same length, otherwise you will need to put in flow controls for each loop on your return manifold so that you can balance out the system. I would add them anyway as they are nice to have.

While zone valves are needed, they are really only an emergency sort of thing with a radiant floor system. because of the long lag of radiant floor heat, using zone valves like other hydronic systems will be a huge disappointment. By the time they open or close, the rooms will be either overwarm or over cold. A better system is to add a PLC and control a metering valve (variable speed circulating pump). Controlled by temp switches in the floor, outside, and on your return manifold; you will get a much more consistent heat.

If you plan to do domestic hot water now or in the future, your zone valve relay will need to be able to have priority settings.

I did not see any make up water in your plan. That will be required.

I would add plenty of valves to isolate things so that when circulating pumps and the like need changing out, you can easily do so.

Include lots and lots of temperature gauges. Radiant floor is all about temperature differentials and temperature is the hardest thing to control.

But don't let any of this scare you. I built my own radiant floor heating system, four zone with domestic heat priority, and while expensive up front, it is a very efficient, and very comfortable heating system with a lot of possibilities. Since it does not care where it gets its warm water from, later on you can include geothermal heat, compost heat, wood boiler heat, co or tri generational heat, solar heat... really the possibility's are endless. They are a great heating system, just costly up front.

Watts PERT tubing is Polyethylene Raised Temperature. 5 Layer flexible tubing that is not cross linked like PEX.

My understanding is also that PERT cannot be operated at temperatures as high as PEX, so I'm not clear on the benefit of using it over PEX, other than not being able to get PEX?

From the drawing that cold return doesn’t look right. You need to move the water in a loop. Most modern hydronic systems have the pump pumping away from the boiler, supply side.  And then with radiant loops that come off of the boiler supply  T out to supply radiant loops and then there’s a pump on the supply side of the loop pumping that radiant loop so that your radiant loop returns back into the return header.

Wow! I’m actually overwhelmed by the response on my post. I’m certain that I’ll have to rework parts if the design and you all have brought up things I wasn’t even aware of; thank you.
PERT is a less expensive version of PEX; ostensibly it performs as well or better. I was concerned about the crosslinking chemicals in PEX. But potable water won’t be going through it anyway.
I did forget to describe the system in greater detail- 1/2” pert stapled over 2” pink foamboard, buried in 1” concrete/ mortar over which floating engineered hardwood flooring is placed. No carpet! It is in a rather high heat loss setting- lots of windows!
I agree that the system can be expensive,  hence my desire to build it myself.
The Rinnai 199,000 btu cost me 800 brand new ( eBay)
2 loop manifold $74. Two Taco variable speed pumps $120 each. Pert is cheap. Miscellaneous parts available locally. Three zone Taco control 120.  So I can build the controls for a fraction of “professional “ version.
I guess the most difficult part ( of most anything, really) is gaining the knowledge and overcoming inertia.
I greatly appreciate all your ideas and suggestions and will try to look up some of those
Websites.
 Any others are most welcome. I’ll try to respond to each of you as I can.
I love permies!

https://www.pexuniverse.com/differences-between-non-barrier-oxygen-barrier-and-pex-al-pex-tubing

You need to be careful with the components. And many systems have a magnetic scoop or filter on them.

It never hurts to bring in a professional and get it done the first time. I’ve seen many have severe headaches and spending money many times over going DIY with this. I’m a plumber and I’m never afraid to say the 3 magic words, “I don’t know”

But I know who to ask or I can find who to ask.

Tom Moran wrote:Greetings. I’m currently building an addition to our 20+ year old straw bale home. Since I’m a thrifty type, most everything is DIY. I’ve buried the PERT tubing in the floor, now having to copy a commercial radiant heat control panel. It looks easy enough and I can build the similar commercial $3k panel for less than $1 k in parts. I can solder.
I’m looking for input on whether my proposed design makes sense; it’ll be powered by a new rinnai lp tankless water heater, feeding (eventually) 4 zones via zone valves. I’ll try to add my schematic below.
Any input appreciated. Thanks. Tom

how many feet of tubing in each of your zones? My hydronic contact was adamant about limiting runs. you might have two runs in a single zone as opposed to a very long run. Is the rinnai able to be activated at low pressure? Most of the domestic hot water heaters I saw require 20psi or more to acivate but my radiant floor runs at 6 psi. I'm told its to reduce turbulence in the pipes and add longevity.

David write:
how many feet of tubing in each of your zones? My hydronic contact was adamant about limiting runs. you might have two runs in a single zone as opposed to a very long run. Is the rinnai able to be activated at low pressure? Most of the domestic hot water heaters I saw require 20psi or more to acivate but my radiant floor runs at 6 psi. I'm told its to reduce turbulence in the pipes and add longevity.


Thanks. The one zone that’s completed consists of two runs of 220 feet joined at a manifold. The two future zones will be about 220 and 150 feet square feet.
I hadn’t thought of the water heater being an issue but will look into it! I may have to rethink the tankless option and go with something else- an efficient tank type water heater perhaps?

I’ve never heard anything about the length of a run. You do need to size the pump on that loop for what it needs to do. I’ve done a mixed system, radiators and radiant loops. The radiator needs a higher temperature while the loops need it lower. Loops got a pump And a mixing valve.

The length of your loops is critical if you do not have flow control valves because the different lengths will screw up your temperature delta on your return manifolds.

Most hvac guys limit the lengths to 200 feet

But with flow controls various lengths are not a problem because you can balance the flows so they are all the same for your temperature deltas

The length of a tubing run is important for at least two reasons:

1. The head loss, or flow restriction created by the pipe. This may be overcome by using a pump which is more powerful but that is not very energy efficient. Parallel loops from a manifold will reduce the head loss making the system more electrically efficient. ECM circulators are also much more energy efficient, although more expensive.

2. Temperature drop (delta-T) of the loop. Usually radiant floors are set up with a delta-T of 10-20 degrees F for the loop. For example, 105F water entering a loop with a 20F delta-T will exit at 105-20=85F. If the loop is too long (or the flow rate too low), the delta-T may be high enough that the end of the loop is not doing any useful heating. The input temperature cannot be increased too much without running into comfort issues or surface durability issues.

The input water temperature, loop area, loop spacing, flow rate and delta-T affect how many BTU/hr of heat the system can provide. These factors are all interrelated so for complex systems using some kind of software to simulate the performance can be very helpful. After a prolonged initial period of confusion in my case

This article goes into much more technical detail on loop length with all the supporting equations:

https://www.pmengineer.com/articles/84671-pushing-the-circuit-length-envelope

PM Engineer magazine lets you read a couple of articles per month for free.

Thanks. That’s good information but what I really need is advice on design of the controls.
The present tubing layout adheres to the specs you indicate.
I have two Taco 007e pumps which are ecm units ready to go to work. Tom

Tom Moran wrote:what I really need is advice on design of the controls.

You can go with something super simple and see what happens. At our previous home I had a large shop building. I used a 40 gallon electric water heater as the heat source (bad idea - very expensive to run). There was a single circulator that pumped water from the water heater through the floor loops which were all on a single manifold. The pump was controlled by a 120V thermostat on the wall. The water heater was set pretty low so it wouldn't shock the floor. After the pump ran a while all the water was at the floor temperature which slowly increased until the pump turned off. Normally it was set to about 40F so there was no risk of overheating on a warm day. In your house, overheating on warm days due to the slow reaction time might be a concern.

Just thinking through your design, the secondary pump (floor loops) will need to be controlled by either air temp or the floor temp. Once it turns on, it will also need to turn on the primary pump to fire up the Rinnai if the water temp is below its set point. So you need a thermostat to turn on both pumps at the same time. The Rinnai will cycle as needed based on its internal controls. If the Rinnai will turn down to your desired water temp you should be good to go. If not, you could add something like a 20 gallon hot water storage tank (look for indirect water heater tanks that don't have heating elements, or probably save money and just get a regular water heater for storage), set the Rinnai to a higher temperature and use a mixing valve to set the floor loop temp.

Simple thermostat controls like the Azel DST-777S (picked randomly as an example) could possibly be used to control the pumps. Or even a mechanical 120V thermostat to start out.

More advanced systems use something called outdoor reset. It adjusts the water temperature to the floor loops based on the outdoor temperature. The idea is to match the heat delivered to the heat required by the outdoor conditions. My system will eventually use an LK110 motorized mixing valve with outdoor reset to adjust the floor loop temperature drawn from a large amount of thermal storage. The secondary circulator runs most if not all of the time in these systems. The water temp is controlled instead of the pump.

Using a tank type gas water heater might be easier than the on-demand water heater if you don't already have it. Keep in mind that having some part of the system hot enough to kill legionella is a good idea.

You can find more info on controls and how to manage a large thermal mass in the Caleffi Idronics issue 14. They have a bunch of proven system designs in issue 19. https://www.caleffi.com/usa/en-us/technical-magazine

Places like Supply House and Tarm USA sell controls for radiant systems so you might check out some of their options.

Sorry I don't have a simple "do this" answer for you... I'm an electrical engineer with very limited HVAC experience so I'm also figuring this stuff out as I go.

This has turned into quite the wild goose chase for hydronics system design and controls info. I'll add a couple more helpful links that I had forgotten about.

John Siegenthaler is a consulting engineer who has written a huge amount of material on hydronics including a text book. PM Magazine put together 5 volumes of his articles which you can find here:

https://www.pmmag.com/articles/102245-modern-hydronics-done-right
https://www.pmmag.com/articles/102246-modern-hydronics-done-right-volume-2
https://www.pmmag.com/articles/102247-modern-hydronics-done-right-volume-3
https://www.pmmag.com/articles/102414-modern-hydronics-done-right-volume-4
https://www.pmmag.com/articles/102550-modern-hydronics-done-right-volume-5

Tekmar also has some technical essays on topics like "Hydronic Requirements", "Heat Curves and Reset Ratios", "Control Strategies" and Control of Radiant Floor Zones" among others:

https://www.watts.com/our-story/brands/tekmar/references/essays

Sorry about turning the info firehose on you, but this is great stuff for anyone wanting to know more (a LOT more ) about hydronic system design and operation.

Tom Moran wrote:Greetings. I’m currently building an addition to our 20+ year old straw bale home. Since I’m a thrifty type, most everything is DIY. I’ve buried the PERT tubing in the floor, now having to copy a commercial radiant heat control panel. It looks easy enough and I can build the similar commercial $3k panel for less than $1 k in parts. I can solder.
I’m looking for input on whether my proposed design makes sense; it’ll be powered by a new rinnai lp tankless water heater, feeding (eventually) 4 zones via zone valves. I’ll try to add my schematic below.
Any input appreciated. Thanks. Tom

At the old house we had an amazon manifold that was stil working fine after 10 years. the zone control adjustments meant we only ever needed one pump and one thermostat. We were heating a thick slab though and entire floors under the plywood. I had built my own manifold at first the the flow gauges alone made it worthwhile to install a ready made one.
https://www.amazon.ca/s?k=radiant+floor+manifold&crid=1VZ0CJKXNNVTE&sprefix=radiant+floor+manif%2Caps%2C867&ref=nb_sb_noss_2

Update for anyone still interested..
Just found the manual for my rinnai ru199i tankless water heater- it indicates that minimum temperature is 98 degrees and minimum flow is 0.26 gpm. Does that make it usable for the hydronic system?  Thanks

Tom Moran wrote:Update for anyone still interested..
Just found the manual for my rinnai ru199i tankless water heater- it indicates that minimum temperature is 98 degrees and minimum flow is 0.26 gpm. Does that make it usable for the hydronic system?  Thanks

Hi Tom,

It depends on your heating demand. I might be willing to try it and see what happens but keep in mind that I'm just some dude on the Internet

For example, our new place is very thermally efficient with low air leakage (I hope). The worst case heating demand (coldest daily average temp for our location of 8F) is estimated at about 6 BTU/Hour per square foot. Looking at a single zone (multiple loops / rooms) of 2300 square feet with a target temperature of 68F, that will require about 1.75GPM at 85F to generate a surface temp of about 70F on an insulated concrete slab. The finish floor is polished concrete so there is no heat loss due to floor coverings. Many days it will be warmer outside so the input water temperature would be less than 85F due to lower heat demand.

In my case, the Rinnai would need to cycle on and off quite a lot even at its lowest setting. Using an air temperature thermostat there will be some overshoot. A PWM type controller and a floor temp sensor would reduce the overshoot but the Rinnai would cycle on and off more frequently. Check out the Tekmar essay on controls to see how this might work.

It sounds like your heat demand is greater than mine (you said lots of windows) so the Rinnai minimum 98F water temp might be OK. You might need to turn it up more, but with engineered hardwood flooring you have to be careful. Check the flooring warranty and their requirements on the max temp allowed. The hardwood will reduce the heat output but your thin 1" slab will have less loss than my 4" slab and it will be much more responsive due to less thermal mass.

Calculating everything out to get a better idea of what will happen is always a good idea. Especially if you plan to add more zones and an outdoor wood boiler in the future. Or, you could just wing it and see what happens

If you want to calculate it, the Uponor Complete Design Assistance Manual (CDAM) will walk you through the process with examples you can follow. Initially, I built a spreadsheet using the same equations provided in the CDAM. The results were surprisingly close to what I got with LoopCAD.

Tom Moran wrote:Thanks. That’s good information but what I really need is advice on design of the controls.

Small thing you may be aware of....  your main feeder should be a 1" or larger too feed your floor lines.  Do not expect a 1/2 inch feeder to feed 3 -  1/2 inch floor lines.

Here is what i have in my home....

Many year ago, I installed a 3 ton ground source heat pump.  Instead of going with the forced air option, I installed my own radiant floor heat - under plywood subfloors.
Using aluminum plates that staple under the plywood, then adding R20 insulation, it heated the house not too badly - that is,until the outside temp hit - 2 or  -3  C.  then the house slowly cooled off....
Now, I noticed that the heat pump was cycling on and off way too much, so I made some improvements.

First, I added another 60 gallon storage tank to the exsisting 40 gallon tank - bringing the storage capacity up too 100 gallons.  When it is - 20 outside, the heat pump cycles about 20 minutes every hour - with a 15 degree temperature differential.

Second.  With 5 zones of different lengths, I used a store bought manifold - that included spring loaded flow rate indicators - so I could balance the system easily.  As a backup to the radiant floor, I installed a low temperature radiator directly in line with the 1.5 inch return line to the storage tanks.  
The logic there, is the water goes through the floor pipes @ 110 degrees or, does not loose a whole lot of heat - since wood is an insulator  - and the house was cooling off below -2 F - so the return water - that is still at 105 - 110 at its coolest goes into the low temp rad, and a thermostate turns it on when the radiant floor cannot keep up.  keeping the house warm.

I then switched from a Grundfos pump - that used 100 watts per hour, to an El. Sid pump that uses 10 watts per hour.  1 pump for everything.

Been up and running for 15 years now, and no matter what the temp outside - works flawlessly.

Some improvements ?  Not enough gauges - both pressure and temperature.  Should have added a few more here and there....

Hope this helps.

Thanks.  Believe me, all input is helpful!  One big radiant heat company indicates that tank type heaters are good but that tankless can be used, so I’m still searching for the ideal heater, and hence for the proper controls. Back to the drawing board!

M Rives has really given you great information because he is saying the stuff I also know but dared not say: going with a hot water heater will work, and lower your upfront costs, but will make it REALLY inefficient. It took the guys explaining to me three times why going with a hot water heater was a very bad idea, but once I went with a tiny wall hung boiler, I realized it was well worth the money. That system just sips fuel...

One thing you must really understand conceptually is, the more controls you put into your system, the more control you will have of it. That saves fuel and is what you want over the long-term, so being cheap up front is just going to cost you dearly later. That is because of anything to control, temperature of something is the hardest.

Today... it is probably too late to convince you, but I would never put in radiant floor heat. It is heat to die for as every room, even closets are the same temp as every other square foot of the house, the system has no cycling sounds, no hot or cold spots, sips fuel, and all that, so it is THE perfect type of heating system for sure. It is just...

I no longer have a need to heat 100% of my house when I am not in various rooms. Radiant floor heat is not conducive to that lifestyle.

If I was to heat 100% of my house all winter there is no better way than radiant floor heat, but I just don't like that way anymore. I have three daughters that spend a weekend a month here on different schedules. To heat all their spaces all the time is just heat I don't need. Yes, it is nice to walk into a room that is warm and not chilly, but most of the square footage here is just not used enough. By not heating it, and only when occupied, I save a ton of money; $750 per year instead of $2800 for a 2200 square foot house in Maine.

My new house is 3000 square feet and I will heat it the same way; only when occupied. That requires a different mindset I know, but something to consider before you waste too much money on an efficient heating system, because this is a simple ruth; heating only occupied spaces is far more efficient than the best heating system you could ever devise.

Tom Moran wrote: tank type heaters are good but that tankless can be used, so I’m still searching for the ideal heater

I know what you mean.  Dug up some of the loops for the heat pump last fall, and spent all winter thinking " if this thing freezes up on me.... what do i do "....

I have 100 gallon storage, so i could put in an on demand water heater in place of the heat pump, on its own thermostate - so it would .... say .... come on at 110, and turn off at 130  - and just cycle on and off keeping that 100 gallons hot.... you could do that with a regular 60 gallon tank.

Or, i could look into an on demand that would go low enough ( electrics can ) and plumb it directly ( but will it handle the flow, and how much power would it use....

My two options are electric and propane.... electric takes alot of amperage, low flow while the propane has high output, gas lines, tanks ect.. but will it go low enough ?

Oh, ya.  Know what ya mean.

Dave Lotte wrote:  I know what you mean.  Dug up some of the loops for the heat pump last fall, and spent all winter thinking " if this thing freezes up on me.... what do i do "....

Just a thing I wonder about from time to time- What if you buried the loops below frost line, where the ground is always ~50 degrees, and just circulate that water, with no additional heat added. Would that not give you a mass/dwelling at 50 degrees? And now you only need to warm the house another 15-20 degrees which would take relatively little energy.
I know there has to be a flaw in my simple thinking or people would be doing it already!

That is what I did.

Mine was embedded in concrete on grade so it basically took the heat from the center of the building and mixed it with the cooler areas on the outer fringes. I got exactly what you would expect, a building that was 44 degrees inside, vacant all winter and yet -7 below zero outside. I did not even have a warm body in there adding to the heat load and it still stayed above freezing.

Julie Reed wrote:

Just a thing I wonder about from time to time- What if you buried the loops below frost line, and just circulate that water.

I tried that with my system, but on the house side of the unit using only water.

The idea is, you have an antifreeze solution in your  ( house ) system so it will not freeze ( like the ground loop ) and in the summer, you flip it over to A/C , and the heat pump will pump the heat back into the ground - cooling the house.

With my system, i told the installer i was using straight water ( 100 gallons ) and did not want to buy 50 gallons of antifreeze ( $$$$ ).  So what he did ... he left a little jumper wire attached inside the circut board of the heat pump - so that when the unit hits just above freezing - it shuts it off - as a safety measure to keep it from killing itself.

So, just for the heck of it,  i switched it over to a/c and ran it till the safety kicked in.  It did get really cold, just not cold enough on a 30 degree day, and you wouldnt want cold water in your floor anyways - condensation, mold ....

Answer : for the money spent on digging and installing the ground loop, you would want the heat pump to pull as much heat out of the ground as possible.
Also : if you were to pump 50 degree water through the floor and heat the room at the same time, all the heat would go into the ground loop - acting like A/C - it goes both ways - heat always moves towards cold.

Steve Zoma wrote:

I no longer have a need to heat 100% of my house when I am not in various rooms. Radiant floor heat is not conducive to that lifestyle.
heating only occupied spaces is far more efficient than the best heating system you could ever devise.

Just curious on what type of system you have ??

With my Hobbit Home Build, I specifically designed and drew up the plans, so that seperate rooms can be temperature controlled - or completely shut off - if needed.  Can you not shut off the zones you are not using ??

I suspect, that once built, the main bedroom and main bathroom will carry most of the heat load for the house....

Wanted to post this with my last posting, but could not find the pictures....

Here is the electric ODWH i use for my shower.  I had originally purchased it as a supplemental heater to my Apricus Solar water heater.  No more solar, so it is now the primary water heater.

Looking at the pictures, if my return water temp is already 100 F.  And i need 115 - it should keep up with the flow rate.

Thing i love about this unit, is it has a temp dial on the front.  Set it at 42 and enjoy your shower - using NO COLD WATER mix.  All the energy is just for the hot water, no having to cool it down before you use it.

Tempra 12 ODWH needs a dedicated 60 Amp breaker.

Edit :  acorrding to the calculator, 12 KW is equal to 40,000 BTU.  My main heat source is the heat pump - cycles on and off evey 20 minutes in -20  weather, and it is rated at 36,000 BTU.  This water heater should do the job easily....

But will it ? 😁

Indoor Air Temp = 65F
Floor Temp = 75F
Water Temp in PEX Piping = 100F
Flow Rate, this will control how often the heater cycles, the slower the flow rate is the longer it take for the place to heat up and the longer the cycle, the faster the flow rate and it is faster the system.

Dave Lotte wrote:

Steve Zoma wrote:

I no longer have a need to heat 100% of my house when I am not in various rooms. Radiant floor heat is not conducive to that lifestyle.
heating only occupied spaces is far more efficient than the best heating system you could ever devise.

Just curious on what type of system you have ??

With my Hobbit Home Build, I specifically designed and drew up the plans, so that seperate rooms can be temperature controlled - or completely shut off - if needed.  Can you not shut off the zones you are not using ??

I suspect, that once built, the main bedroom and main bathroom will carry most of the heat load for the house....

My home has pex embedded in the concrete floor, so while I can shut off zones, the monolithic nature of concrete means it effectively acts as one radiator: the good and bad of radiant floor heat. Yes, you have what amounts to a Huge radiator instead of tiny baseboards with fins, but one zone will also absorb heat continuously if it is set lower than the other.

It is why my system must have 2 calls for heat before it kicks on the heating system, not only does it prevent a cold spot in one place from kicking on, it also heats up a larger segment of the floor.

S Bengi wrote:Indoor Air Temp = 65F
Floor Temp = 75F
Water Temp in PEX Piping = 100F
Flow Rate, this will control how often the heater cycles, the slower the flow rate is the longer it take for the place to heat up and the longer the cycle, the faster the flow rate and it is faster the system.

That is not how radiant floor heating systems work though.

Using a plc and a tempering valve, it checks outside air temp, returning manifold temp, in floor temp, and then calculates what the water running through the pex loops needs to be to reach the desired temp in the particular zone.

It dies this via a predetermined chart, basically the lower the outside temp the higher the water in the pex tubing needs to be. As the temp outside rises, the pex tubing water lowers to meet that need.

In the end the boiler only cycles to keep the main boiler loop hot. From that loop the mixing valve draws out the hot water needed to make up the difference. In doing that it is a very efficient system for whole house heat.

The mixing valve and plc does everything. The thermostats are dumb, they just tell the system to come on, and what you want various zones at temp wise. The zone valves are there just in case, but rarely come into lay, if at all.

The plc is so accurate that it keeps the temp to within a degree up or down of what you set the thermostat for. All this keeps the boiler from being shocked by too much temperature difference.

Steve Zoma wrote:

S Bengi wrote:Indoor Air Temp = 65F
Floor Temp = 75F
Water Temp in PEX Piping = 100F
Flow Rate, this will control how often the heater cycles, the slower the flow rate is the longer it take for the place to heat up and the longer the cycle, the faster the flow rate and it is faster the system.

That is not how radiant floor heating systems work though.

I am finding there are 2 different classifications for heating systems, although there are a wide variety of systems out there....

The first type is based on a high temperature boiler, which in this case, would use a tempering valve and related pieces...

The second type, is the one i have, using water that is just hot enough too do the job, directly feeding the system.  Thermostate comes on when the house cools off - feed water is always between 100 and 115 - give or take.

Of course there are a hundred different options between the 2 different types.

I will point out, that in my research, you need the flow rate to stay fairly low.  My flow rates are between 1 and  1.5 litres a minutes - whether that is low enough or not, that is what its is.   Any higher flow rates, and you do not give the heat time too transfer to the concrete, and also could run into interior pipe abrasion - where the water actually wears away the piping - its just going too fast.

If the pipe you have is not heating the area, the loop is either too long, or you need a larger pipe.  Think of an ice melt system that uses 3/4 inch pipe.  You need that size of flow to carry enough heat to melt the ice.

Spring update: I’m still researching options on the floor heat. I’m convinced of using a tankless water heater ( a smaller one)  along with a mixing valve.
The responses regarding you tube videos were mostly useful and informative. It looks like lots of folks have pulled off a RFH system with very simple components and not a whole lot of money.
I’m sure there are lots of options and types of systems; so many choices to make.
Perhaps some who have suggested programmable logic controls could make some recommendations…
We’re also looking into a system using both or either lo water heater and outdoor wood boiler, since wood is very available here in mid Missouri. Thanks so much.  Tom

Greetings. If anyone is still interested, here’s a view of version 2 of my quest to build my own hydronic heat control system. I’ve tried to take into consideration many of the suggestions offered so freely here on permies.
Does it look functionally suitable?  Thanks in advance.  Tom

Hi Tom,

The concept looks good to me. Using the Rinnai for both domestic hot water and space heating is clever.

Here are some things to consider:

1. Ideally, it's best to "pump away" from the expansion tank. This would require moving the expansion tank connection  to the pump input (after the loop return manifold and drain). Search "pumping away" for all the details.

2. A small expansion tank on the input side with the Rinnai is probably a good idea You may already have it to prevent water hammer.

3. You may want dirt separators or some kind of screen/filter on both sides of the heat exchanger. Flat plate heat exchangers have narrow passages can be clogged by particulates in the system.

4. You may want isolation / clean-out valves on both sides of the heat exchanger so you can more easily flush / descale it when you flush the Rinnai (required maintenance). In this open system where you have combined domestic hot water with space heating, more scale-causing minerals will be introduced all the time. Maybe include a summer bypass loop around the heat exchanger input since it's only one more valve?

5. The storage might not be required depending on the heat demand and the ability of the Rinnai to turn down low enough. If it short cycles, the storage tank will help prevent that.

6. When the floor is calling for heat the Rinnai might not be able to keep up with the total demand. Like if too many showers are running with the washing machine, etc. This seems unlikely, but an unexpected warm / cool shower could result.

7. Make sure the air bleed is at the highest point. You may need more than one (manual air bleeds don't cost too much). The manifolds I bought include air bleeds.