Hydronic loops under a wood joist floor are labor intensive, but fairly simple to instal, or so I have heard. Basically long loops of PEX high heat tubing, with aluminum heat exchange fins, insulation to keep heat from going the wrong way, and all joints in accessible spots in case of leaks.
What if such a system ran to an insulated, non pressurized barrel full of a water/ antifreeze mix, with a high temperature pump to flow water through the lines?
What if said barrel was heated from a rocket stove mass, and or a solar panel collector thermosyphon?
Even more simply, what if the barrel was simply filled from a hot water tap, and drained ( for irrigation or other productive uses) once cool?
Remember, I'm not suggesting pressurizing anything; any barrel will have a wide open vent! So no boom squish, right?!
Or would this simplified system (hot water in a barrel pumped through PEX lines) just not work, regardless of how I heated the water?
From what I understand, Hydronic heating needs a very hot source, in order that the pipes coming into the area will be able to continuously absorb heat, since they will be giving off heat throughout their release loop and need to be replenished with heat on each cycle. If a person had a system that was really hot (a hot water tank, for instance), but that hot system overflowed when over temp into another hot water tank, and that second tank dumped into a third (which is over the heating capacity of the system and it dumps back into the initial one, then you would have enough heat in the initial one, maybe to run your pumped pipes through. I'm not sure. There must be a way. Just running off at the brain here.
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Hi Gilbert; I had a similar idea a while back, only i planned on using a hydrolic oil tank and oil to pump thru a pex system. I was even planning on modifying a s.b. chevy oil pump to circulate the heated oil. Oil can handle temps over 400 F . After much investigation and checking the temps on my current rmh mass . I came to conclusion that I would only be able to produce enough heat if the rmh ran nonstop... that includes at night...not gona happen here. Defeats the purpose of having a rmh. Anything can be built (rube goldberg) but is it worth your time and energy... In this case, in my opinion the pex / heated floor idea is not a workable option. Having said that ... I would be thrilled if you can prove me wrong.
It is an interesting proposal, but I am not so sure thermal siphon would work. I have 100% radiant heat in my house and would not heat my home any other way, but it takes rather high flow to accomplish that. Even then, if the temp drops quickly, or alternatively, rises quickly; my radiant floor heating system is unable to recover quickly to compensate, and my computer controller checks outside temp, boiler temp, supply temp, return temp, and floor temp loss every minute. It does this by controlling a variable speed circulator pump. How you could accomplish that vital task via siphon and get livable indoor conditions is beyond me.
As for what Roberto said, he is indeed correct, and yet also wrong on the same front. Please let me explain with apologies to Roberto if he is offended. He should not be offended in the least though, because a radiant heating system is unique in that it reverses itself. It also depends upon the type of system that it is designed for.
1. Reversing itself.
Roberto is correct in that it takes a lot of heat to warm up a space via radiant floor heat...INITIALLY. As an example, it is November in Maine, and yesterday I turned on my heating system. It is 40 degrees outside and so to heat my house ideally I need 95 degree water running through my floors. My heating system is designed to deliver that, but having very cold floors right now, the water flowing into my floors are warm, but the water coming out of my floor loops are cold. In other words every BTU is being sucked up in trying to heat my floors to the ideal temperature. In that way Roberto is right, it takes a lot of hot water to accomplish the task.
After the floors get warmed up; about three days time, it does not take nearly as much heat to maintain them. This is what I mean by reversing itself. The water going into my floor is hot, and the water returning from my floor is warm. Ideally this is 15-20 degrees and called "Delta T". It is the ideal temperature so that whatever is heating your water (boiler, solar, RMH, etc) is not shocked. You control that Delta T by flow controls. When I sayhigh flow, I am talking 1/4 to 1/2 a gallon per minute, per loop. That is a lot!
Now a radiant floor system can also be designed in one of two ways; simple or complex.
In this design, it is dependent upon its zone valves. In this case again, Roberto is correct because it requires a lot of heat. In essence high temperature water flows through the floor, when the set temperature is reached, the thermostat shuts the zone valve. This is simple, but not nearly as efficient on several fronts. It requires higher temp water that requires BTU's whether it be oil, coal, wood, solar, etc. It is also not as comfortable. That is because radiant heat does not heat the air in a room, it heats objects in the room that radiates heat into the air. Because the water is set at high temp and is simply turned on and off, some variation occurs. My father's radiant heating system is this way and as such, just after the thermostat shuts down the zone valve, the air in the room rises so it gets several degrees above set temperature, yet just after the zone valve opens, because it takes awhile to begin warming the room again; the temp drops a few degrees. So there is this constant flux in temps. Yes he sets and leaves his radiant heat at 70 degrees, but his rooms go from 67-73 degrees constantly.
In a nut shell, my system works awesome, never varying indoor temperature by more then a degree to the set temperature, UNLESS a drastic change in outside temp takes place. Instead of relying on hot water, my system only warms the water enough to compensate for the outside temperature. If the temp outside goes down, the house is losing heat faster, so to compensate the water flowing through the floor rises. If the outside temp rises, then the reverse happens and the water flowing through the floor lowers. It does an outside temp check every minute. Therefore I am never heating water more then I have to. It does this by injecting hot water from a main boiler loop that runs at 100 degrees. So what happens is, the temp in my house is not set by the thermostat and zone valves at all, but rather by the computer controlling how warm the water is flowing through my floor. This is so accurate that my thermostats only tell my computer if they need heat or not, and the zone valves are in place, but never stop the flow of water because the temp is so consistent. 99% of the winter my house is 1 degree of where I set the thermostat.
My system is basically using the greatest asset radiant heat has; its large size which is basically one massive sized radiator. Big radiator, less hot water flowing through it to heat the space. Smaller radiator, the hotter the water has to be to heat the same space. Because I am only flowing warm water through my floor, I save a lot of money on fuel. It is far cheaper then my fathers system, however I also paid a lot more for it.
I am not sure your siphon idea would work simply because you are going against the greatest asset of radiant heat; the ability to use warm water to heat a space efficiently. Gathering heat from one area, and moving it to another area via water that is 600 times as dense as air, is simple in concept, but the ability to control that water is where the complexity comes in. With a thermal siphon, you lose all control and are relying on super hot water to move from place to place. There may be places for thermal siphon's, but I am not sure that radiant would be the greatest application.
I am convinced hybrid systems are the best application for radiant systems. As Roberto pointed out, initially heating a space requires ample heat which is where solar, compost, and geothermal heat would be problematic, but where rocket stoves and conventional boilers excel. My hope is to someday use my boiler to heat my building efficiently in the fall, then after the high btu demand tapers off, use compost, solar or thermal heat to keep the house from losing heat.
1. NEVER use antifreeze in any heating system; you lose 15% efficiency in that product alone. If you are afraid of freezing, an automatic back up generator is not only cheaper then buying that much anti-freeze, it will keep your pumps running and keep your system from freezing should you lose power.
2. Thermal siphoning would take away your ability to control water movement; your greatest asset in a hydronic heating system as explained in my post above.
3. It is possible to use a water heater to heat a radiant floor...not the cheapest alternative long term, but can be done, but not as you suggest because of the need for a proper Delta T threshold as described in my post above.
For a really cheap, off grid system where people will be home all day, could one manually turn on and off an electric pump and manual valves based on a wall thermometer reading? Or would this be simply too frustrating? Could this be the way the initial system works before it is upgraded?
As far as thermo-syphoning; I was thinking about using this just for the solar thermal panel part; i.e. the panels could interface that way with the tank if the tank was higher then the collector. This would not work in my case anyway, due to elevational difference. If I did have solar thermal panels, should I use antifreeze in this loop? Why does one loose efficiency with antifreeze?
As far as controlling temperature in the tank manually; could one depend on solar to keep the tank warm, and fire a rocket whenever the tank needed to get hotter or the day was cloudy?
I realize that electronic brains would make the whole system more efficient, but it would also make it more expensive and also more dependent on big industry. I could imagine a small town or a third world country being able to make plastic tubing, or failing that metal tubing, but I can't imagine them making computers and temperature sensitive valves. Would it be simply impossible to have a completely manual system if one was willing to take some trouble with monitoring it and didn't mind some fluctuating temperatures?
Is running hydronic off a water heater dangerous due to the possibility of legionella? I've heard conflicting information on this. Also, after watching a couple of videos of Hot water heaters undergoing Boiling Liquid Vapor Explosions, I'm quite certain that it would not be a DIY project!
As far as running the rocket too often; I live in sunny Colorado, and was hoping to put up a big solar hot water array, with the rocket being fired for an hour or so a day to boost temperature as required, or potentially running all day on our few cloudy, extremely cold days a year. I'm OK with occasionally having the rocket run all day, since we are always around, and I would also be OK with the house temperature falling at night.
As mentioned a few times here, underfloor radiant heat doesn't need high temperature water. I have a circulating loop with mixing valve so it takes in just enough 140F water from the water heater to keep itself at 100F, circulating constantly when the rooms need heat. The variable water temperature control would be even better, but I don't see a detectable difference in room temperatures. I have a commercial grade gas water heater (Polaris) with condensing powered draft... only warm moist air leaves the house. I designed this twenty some years ago and it still works. The parts embedded in concrete work much better than the parts with aluminum fins under the wood flooring.
The upshot is that any source which can heat a water tank can supply a radiant system, almost regardless of how hot the reservoir is. It will need a pump for circulation to be effective.
Gilbert: Honestly the only way I see something like what you are suggesting work is if you went with compost heat. I looked into this a lot because I had a lot of the resources to do compost heat, but as Thomas said, in the end while I could do it, it was just not worth the time and effort for me, but that is stated "for me", you my friend, may be different.
The reason I suggest compost heat is because it is a low temp heat, simmering around 140 degrees. Simply put you have high mass over here, and through hydronic tubing, bring that heat into the house. Since it would be low temperature heat, and very constant, you could control how much heat is extracted simply by reducing the amount of loops within the compost pile via valves. Flow into the house loops could be controlled by flow control valves or possible quarter turn ball valves. But you would still need to pump that water constantly. Fortunately circulator pumps are not energy guzzlers; my whole boiler system runs off (1) 14 amp circuit. That is the boiler itself, a few circulator pumps, the PLC, etc. Its not a huge electrical load by any means!
The other thing to keep in mind is, how much do you want to babysit your heating system?One surprising fact emerged when I retired; I am just as busy now as when I worked!! Everyone has 24 hours in the day, and no one is busier than the next guy, it is all about what you fill your day with. Do you really want to fill it with monitoring heat, or do you want to build hugels, add onto your home, construct swales and gather food?
Ultimately this is why I never did compost heat even though I have the ideal site for it. TIME! In half the time it would take for me to construct a compost pile and get it situated for winter, I could cut 5 cords of firewood and heat my house in a conventional boiler. (A rocket stove would require even less wood). Even now I don't heat my house with wood, that is because in two days time I can cut two loads of tree length firewood, sell it, and just use the money to pay for my propane instead. Ultimately that is what every homeowner faces, the best way to accomplish heating their home. I use the absolute minimum BTU's to heat my well-insulated home. Others use solar gain with supplemental firewood to accomplish this requirement, where as others use big outside wood boilers that consume cheap fuel, but gobs of it, to make lots of BTU's. Here in Maine, the latter is common, but these people often spend $12,000 for a big outdoor wood boiler when honestly I think if they took that same $12,000 and invested it in new windows, insulation and making their home efficient; they would be much better off.
You really don't need to control the temperature of the storage tank aside from not getting it to boiling temp. A large enough tank should have a good margin between enough heat stored and too hot. I don't know what that size is, but some calculations ought to be practical after looking up specs.
Using a RMH you might want to consider reading about vapor-vacuum steam systems. (I wouldn't use in floor, but instead use large cast iron rads) You would need to use threaded black iron, as it needs the threads to allow for the slight expansion and contraction of the metal as it heats and cools, most regular soldered connections will quickly fail. With steam, there are very few moving parts, no need for circulators and pumps if it is piped correctly. My system (uses a regular gas boiler, not RMH) runs on ounces of pressure. The rads continue to heat long after the fire is out due to their mass. Unlike regular steam heat, a vapor system was designed to pull a vacuum as the fire died down (think old coal fired boilers) and that might translate well to a RMH. With a vacuum, water converts to steam at a lower temp at no vacuum 212F, at 25.98 125F. Course to fly in the face of all the original vapor system designs, there is a guy that designed a modern system (which didn't use black iron) and was very efficient https://heatinghelp.com/systems-help-center/a-new-look-at-vacuum-heating/.
100 cubic yards seems high, but you may be right. I remember doing the calculations several years back and I came up with 2 acres of land to produce enough silage for composting. If that is the case it would be around 48 tons. I can sell silage for $32 a ton so I would be just as much ahead to sell off the silage and buy propane as making compost heating piles. Plus I live on a huge hill and am nowhere near water, so the 9000 gallons it would take to kick start the compost would be problematic. I could get my local fire department to pump in enough water for it, but then I would be adding additional costs to a questionable return on investment venture. That is why I say it just never worked for me, but that is not to say it would not work for someone else.
With hydronic heat, you have a lot of options because the water temperature required is so low. My propane boiler, because it unto itself is controlled by computer could easily control the water flowing through my floors without the need for a mixing circulator controlled by its own computer. However when I put the system in I planned to add an wood/coal boiler to it to save on propane. The interesting thing is, because it is so efficient the amount of money I would save on propane is so little, that the return on investment for a new wood/coal boiler is too long of a time span. In other words it makes no sense to buy one because by the time I recouped my money, it would be time to buy another wood/coal boiler...best to just keep buying propane since it uses so little of it. Still without the ability to control the water temperature precisely, a mixing valve is needed to knock down the temperature of the water. With it, it does not matter how the water is heated, or how hot.
Now if you use a hand-fired stove like a wood boiler or rocket stove you need to contend with an over-fire situation, such as you just loaded the stove and the power goes out and circulators are no longer pushing the water off the waterjacket. It is very probably that this could flash to steam. Because of that you would need a relief valve on your system to prevent a steam explosion. But this is not the only thing you should have, as you should have a dump zone. As Denise suggested, a big radiator in a room that does not get below freezing would be the best answer. When the stove went into an over-fire situation, the aquastat would open the zone valve, and the radiator would suck up all that excess heat almost instantly. A modine and circulator would work as well. If you went only with a relief valve (not recommended), at the very least you should have an automatic fill from your fresh water line so that your boiler is always filled with the right amount of water. More than likely you would have that anyway on your expansion tank, autofill, spitter valve and air scoop.
I personally like hydronic heat for a lot of factors besides overall cost. First it is super quiet and without fans kicking on and off, and all that nonsense, you never have to adjust the sound on the TV. I also like the fact that every room in the house is the same temperature; from the great room to the hallway closet, they are all warm. NO COLD SPOTS. But I also love the fact that the humidity is less. The bath mat outside the tub dries within an hour after taking a shower, and to dry dedicates like pantyhose and stuff, my wife puts them on the floor to dry. Wet boots in the winter on our mudroom means mud turns to easily swept dry dirt in an hour. The same goes for leaks and spills. They quickly dry up. At the same time since radiant heat only travels 8 feet, my home does not lose as much heat into the attic as a conventional home. My house never shifts because there is no frost to heave doors, crack drywall, or make windows stick. Finally there is the lower temperature factor. You can set your thermostats at 67 degrees and feel like 70 degrees because radiant heat does not warm the air in a house, it warms the objects in a room, thereby heating the air. Because you are an object, you absorb that radiated heat and feel warmer. 3 degrees does not sound like much, but when you add up how many btus it takes per degree day, it amounts to a lot of money saved.
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