Radiant Heat - Open System vs. Closed System Discussion?

Hello Permies Members,

Has there been a discussion on radiant heating, open system vs. closed system?  I have done some searching here and I haven't found much about that.

Here's the thing:  we are installing radiant heat above an existing slab in a pole barn we are converting to an off-grid home.  We have been working with two DIY radiant heat companies for quotes, and they have both suggested an open system for us.  A basic google search brings up tons of personal opinions on how a closed system is safer.  However, both company's claim that open systems are safe and code-compliant.  The open systems uses less energy, which is a major goal for us; in addition, it's less expensive too.

Many members on permies have said that they love their radiant heat (yay!) and we are completely on board with making it happen one way or the other.  We are just struggling with the choice between open and closed.  If you have radiant heat, which did you choose and why?  Are you happy with the system?  Also, if you used a DIY company, which did you use - were you happy with the choice?

I would really love to hear your experiences!
Thank you!

I'm very confused.
Open systems are usually considered safer, as they avoid the possibility of pressurized steam.

We are planning on radiant heat and have received quotes from 2 companies that cater to the DIY market.  We are going with an open system because it has fewer parts and is less expensive.  In the open system, it uses a high efficiency propane or natural gas on-demand water heater to provide your hot water and the radiant heat.  The incoming water for your water heater flows through the system, so the water in the tubing is always being refreshed.  When the system is not providing heat in the warmer months, the supply water for the water heater runs through the system to keep the water in the tubing fresh and actually helps cool your home.  There is no stagnant water in the system.

A closed system uses a separate heat source whose sole purpose is providing heat.  It has tanks to hold the water for the system, and requires some maintenance to keep the lines cleaned.  It also adds a lot of cost.  Since both type systems use PEX tubing, you aren't dealing with high temperature or pressure.  

There are also options to plumb in a solar water heater to preheat the supply water so the gas water heater doesn't have to run as hard.

Both companies recommend putting the thermocouple for the system in the floor instead of on the wall.  That way if you use some type of heat source to warm the air in a hurry it doesn't cause your radiant heat to shut off and not warm the floor.

Ah,  I think I see where I was confused.
Let me explain, it might help someone else.
Radiant heating systems  and solid fuel boilers are  so often discussed together here on permies that I conflated one with the other, but one is the heat distributing system and the other the heat generating system
As Bob illustrated, radiant floors can be heated by conventional means.
The solar preheating connection he mentioned might be suitable for connecting to a wood burning boiler.
Either of those two sources might easily  exceed the working tempature for PEX, so they would probably require the use of a tempering valve.
I don't think there is much actual danger from a closed loop radiant heating system,  but there can be danger from a closed loop boiler.
It sounds like an open loop radiant heating system has few downsides.
In fact,  I wonder if such a system could be driven by a heat pump water heater that itself would be powered by PV solar.
That's a nice thing about  radiant heating systems, the seem adaptable to many diverse heat sources.
You could run a loop through a compost pile or a dragons maw, with the right precautions.

I have a house that is 100% radiant floor heated, DIY, Closed Loop, with Primary and Secondary Loops, with a Metering Valve and PLC; and would not recommend anything else. It really is the most efficient way to heat a home. It is the most expensive set-up, but then again, you only buy a system once, but you pay for fuel EVERY year, so keeping that fuel cost down, makes the most sense.

The effecieny of the whole system is NOT using a hot water heater as a heating unit, and instead going with a purpose built "boiler". I put that in quotes because technically they are boilers due to the presure (12 PSI) but do not actually turn the water to steam. You get that effeciency because with a hot water heater, they lack the controls, and a person can often get short-cyling. A hot water heater is just not designed for the demands of a radiant floor heating system. They are made to provide hot water, at a given temperature, at a moments notice. A hydronic heating system demands varying degrees of hot water, constantly, so the application is drastically different. A hot water heater will work, just not nearly as well as a purpose-built boiler.

With a boiler/primary and secondary loop system coupled with an adequate PLC, the boiler has enough controls so a tempering valve/metering valve is not needed, unless you want to add a high temperature, secondary heat source. You can add a tempering valve, but that never made sense to me; why spend money on fuel to heat up water, just to inject cold water to lower its temperature at the source? I thnk it is far better to go with a metering valve and keep your heat in the primary loop so no energy is lost by cooling it down, but that is more of a powerful argument for a primary/secondary loop system then in using tanks in the system too. And that is also where the effeciency comes in, because now you have the PLC controlling both the boiler, and the metering valve to get the least amount of fuel consumption, and yet heat the house. That is where William;s thoughts come into play; heating water, and controlling it, are really two different things, yet MUST work in unison to get the most per BTU consumed.

How well does it work?

Darn well.

Our zone valves are the dumbest part of the system and never come into play. That is because the PLC works so well with the boiler, the metering valve, the sensor outside, the sensor in the floor, the sensors on the returning water coming out of the loops, that once every minute it predicts the heat loss of the building, what the actual heat loss is, and makes up for it, that our house stays within 1 degree of what the thermostat set temperature. That is pretty good; from closet to livingroom, every square foot of the house is warm with no noisy air blowing on and off, no dust blowing around, and no duct work. The only reason the zone valves are there is in case of over-temperature, but the system is smart enough so it does not do that. Even the thermostats per each zone do not really do much. They tell the PLC if there is a need for heat, and then the PLC takes over. But even there I added some effeciency by ensuring at least two zones call for heat before the system fires up. This further presents short-cycling, and really unifies the house.

The only real drawback to my system is, because the zone valves never come into play (because the system self-adjusts the water temperature circulating throuh the floor based on outside temperature), it means my heating system never shuts off. So I have (3) circulators running from October to May, which causes my electricity bill to go up by $10 per month. Considering what I am saving in fuel, I can gladly live with that.

I built this sytem 13 years ago, and it still is state of the art.

In fact if the government ever gets its act together and does digitalized weather forecasting for residences and not just commercial locations, my heating system could get further effeciency because the PLC would then know what is GOING to happen, instead of reacting to outside weather conditions minute per minute as they happen. I mean if it knew in 12 hours there was going to be a warm front blowing in, it could start backing off the heat output to reduce fuel consumption even more. But the opposite would help too. If a cold front was moving in, it could ramp up heat output so that it did not go into over-ride. (I set my system up so that when it goes to 0 degrees outside temperature, it overides the outside temperatire modulation, and just pumps 100 degree water through my floor. Normally the water in my floor loops operate between 76-90 degrees). If it knew it was going to get very cold, it could run the boiler at 60% instead of 50% before the cold dnap hit so that the boiler did not have to run at 100% when the cold dnap hit. Of course the PLC would then tell the metering valve to put that extra 10% in the floor so that when the cold snap hit, the floor had already been addionally been heated.

It all seems pretty complex, but it is actually pretty simple once you get to understand the components. It took them explaining it to me (3) times before I understood why this was a much, much more effcient system.

(Probably the readability and spelling on this reply sucks, but it is 1:37 AM and this is pretty hard to explain, though a rather simple system).

My radiant floor is just an electric cable. Only a little bit was available when we were laying the floor so I've only got 10 square feet done, and I and my guests love it. I don't trust plumbers here and hydronic floors are not a known thing here so I didn't even consider pipes under the floor of my rammed earth house. Since my house is passive solar heated, I only need a little extra heat, and the low cost of electric cables and their simplicity seemed like a great thing. In winter we only have electricity for a few hours in evening and morning, but the heavy rammed earth floor takes a long time to heat up or cool
down so it's fine.

Travis your expertise is incredible,to the point that I hesitate to describe your system as DIY!
I think you know this stuff better than most sales persons, and probably better than a lot if installers.
A lot of the DIY boiler setups  I see described are very simple, lacking in advanced controls, or many controls at all.
The fireboxes are not well metered or controlled so there is real danger of creating a steam bomb.
A system employing Programmable Logic Controller to control fuel,  air,  and water flow, is worlds apart.


I have been looking for a diagram to go with your explanation, something showing the primary and secondary loops,  and the metering valve.
I haven't found anything showing a metering valve persay,  but the illustrations that show primary and secondary loops make me think that a metering valve controls how much water from the primary loop is injected into the secondary loop.
Can a pump on the primary loop accomplish much the same thing?

Travis, I LOVE your system.  I've made a living doing industrial controls and written or modified many PLC programs over the years.  There are companies like Automation Direct and Phoenix Contacts that sell very inexpensive PLC components and offer free programming software.  My inner geek would love to do something like that.  Optimizing PID loops makes my inner geek purr.

I agree that when you ask a system to do too many things there are compromises that mean it doesn't do any of them particularly well.  I would much rather have a smaller, dedicated on-demand water heater and pre-heat the water using a solar system.  The problem I've read about with solar water heaters is that they sometimes don't get the water hot enough.  With a PLC system controlling a couple valves, especially using proportional valves, you could mix pre-heated and cold water going into the on-demand water heater to optimize for the heat rise available in the water heater.

I have some time before I get to the point of putting in radiant heat.  It will give me some time to explore doing something like Travis' system.

William Bronson wrote: Can a pump on the primary loop accomplish much the same thing?

Well you got some knowledge there yourself, because a metering valve, is just that; a variable speed circulating pump controlled by the PLC.

As the temperature outside goes down (gets colder), the heat the building is losing goes up, and so the temperature circulating through the floor must go up as well. So the PLC tells the variable speed circulating pump (called a metering valve) to increase the ratio of hot water coming out of the primary loop, and inject it into the floor.

It really is very simple.

The effeciency of the system is also like you said, a two computer system, with the boiler's computer running the primary loop, which is the heating aspect, and the PLC running the secondary loop, which is the distribution system. That might mean my boiler is only running at 10%, while my secondary loop might be running at 30%...or any combination in between.

You might note I never said what fuel I am burning, and I said that for a reason: IT DOES NOT MATTER. As long as my primary loop stays beteen 100-150 degrees, the metering valve can inject hot water into the system. This is a Permie dream come true, because it does not matter if that hot water is coming from a Rocket Mass Heater, Solar Hot Water, Compost Heat...or all three, as long as that primary loop is hot, I live in a warm house. In my case I added a wood/coal boiler as an add-on boiler. And as long as that primary loop gets hot water above 100 degrees, the propane boiler does not come on.

Here there is good and bad: The good is that none of these heat sources needed to be protected from freezing. That is because if they are not heating the water, the propane boiler kicks in and sends warm water circulating through them. That means I could put my wood/coal boiler outside, or in an unheated building and do not have to worry about freezing even if the fire was to die. However, the bad is: a back up generator is almost required in case the power goes out. A person could use glycol, but that is silly because it is super expensive, and causes the system to be 10% less efficient...I worked to hard to get what I got to lose 10% just from spending money I do not need too. Glycol is super expensive, far more than what a $400 generator costs from Harbor Freight.

This is my add-on New Yorker Wood/Coal Boiler; 90,000 BTU burning wood, and 120,000 BTU burning hard coal.

Edited to say: You will note the Modine just to the right of Katie's head. An aquastat was set to open a zone valve to that Modine in case of an over-fire situation, so that the water would not flash to steam and go out the relief valve. Obviously I have a few 12 PSI relief valves, but that was just a nice heat dump, sending the heat to an uninsulated shop.

Bob Gallamore wrote:Travis, I LOVE your system.  I've made a living doing industrial controls and written or modified many PLC programs over the years.  There are companies like Automation Direct and Phoenix Contacts that sell very inexpensive PLC components and offer free programming software.  My inner geek would love to do something like that.  Optimizing PID loops makes my inner geek purr.

I agree that when you ask a system to do too many things there are compromises that mean it doesn't do any of them particularly well.  I would much rather have a smaller, dedicated on-demand water heater and pre-heat the water using a solar system.  The problem I've read about with solar water heaters is that they sometimes don't get the water hot enough.  With a PLC system controlling a couple valves, especially using proportional valves, you could mix pre-heated and cold water going into the on-demand water heater to optimize for the heat rise available in the water heater.

I have some time before I get to the point of putting in radiant heat.  It will give me some time to explore doing something like Travis' system.

Bob, I did not read your post, as I read William Bronson's, and replied. But I think you will find some good information on why this system is ideal (but expensive) for Permies, as they can add-on heating units like I did.

Kind of a sad note on the add New Yorker boiler I put in. I never used it. I got it all set-up, but due to use being a Foster Home, then a Day Care and Rental Home, solid fuel appliances are very hard to meet the State of Maine Fire Marshal's stamp of approval. Not that it was unsafe, they just do not like any homes heated with wood, so will seldom license a home with a woodstove/boiler in I, so I took it out.