I did a lot of research on compost heat simply because as you have correctly deduced, a radiant floor heat system with variable injection systems currently are the ideal way to heat a space. I have it in my own home, installed about 6 years ago. My only real issue has been finding a boiler tech guy to work on it; here it is so high tech that they don't understand it. I am not trying to sound full of myself, but I just reached the point after paying boiler techs $100 an hour and then have me explain TO THEM how my system works. I just realized I am on my own, but hey I am better off for it!
So why did I need boiler techs?
Well half my house was heated with radiant heat (pex tubing encased in concrete) and the other half was built before the heating system was proven so it used forced hot air. After realizing you can indeed a home in Maine with 80 degree water by using geothermal properties and the monolithic mass of concrete, I had to have it in my whole house and not half of it. In my case I did as Brett suggested and encased my pex in sand on top of an existing concrete slab. I was shocked at how well it worked. There is no difference in the half of the house that is buried in concrete versus what is buried in sand. I did have some issues with "balancing the system" which is where I was hoping boiler techs would help out, but they couldn't.
My system uses a Taco injection pump controlled by a Taco PLC-705-02. Of
course it uses a Taco 3 Zone Relay Switch with priority, and while not really required (a terminal strip will do so as well), a Taco 3 Zone Zone Valve Controller. The Taco PLC is tied into four sensors, one for outside temp, one for slab temp, one for the return temp coming OUT of my manifolds, and my main boiler loop temp. With those, and the PLC taking readings every minute, it controls the Taco Variable Speed Injection Circulating Pump that is withdrawing water from my main boiler loop and out to my floor. When I added the third zone to a formerly 2 zone system, it threw everything off and I had to balance it. I was eventually able to dial it in so I am back to getting phenomenal efficiency. This is the key with these systems,
you want the system running all the time so that the plc controls your room temperatures and not your thermostats in your rooms operating the zone valves. In other words, your thermostats and zone valves are the dumbest part of the system and should only come into play when there is a drastic outside drop or rise in temperature. (more on this in a minute)
So how can compost heat help in this system?
Well in essence you are combining the best of two different systems. Compost heat is a long-lasting low temp heat, it is just that it is in a pile over there, and you need it in your house. The ideal way to do that is by pumping it, and by doing so over a broad area like a concrete slab works exceedingly well. I would think it would work in cob but I know little about that. Still thermal mass is thermal mass. I have 400 tons of rock for added thermal mass under my concrete slab to aid in geothermal heat. Think of it this way; the old cast iron heaters in old houses were heavy, thick cast iron heaters that took steam heat at 212 degrees to work, yet in my house my "radiator" is 3000 square feet, with about 40 btus per square foot, so it is easy to see 80 degree water will really heat a home and so very efficiently. With hydronic heat being 600 times more dense then air, you get efficiency there.
So how does compost heat work for my home? I have no idea, and this is why. While it is possible to heat a home with compost heat, and I do have the sheep manure,
hay and even woody biomass to make it cook, I lack a lot of water since I live on a hill. I could have the local fire department aid in dousing it with the 9000 gallons of water I would need, but it all comes down to resources and choices. This only applies to me, others are far different, but I have plenty of land in which to harvest wood, so in the same amount of time it would take me to construct a compost pile, run pex tubing through it, water it down and then heat my house for the winter that way, I can harvest a few cord of
firewood in far less time. I am not trying to be mean here, I honestly think it is possible to use compost heat to heat a home, and a variable injected radiant floor heating system is absolutely IDEAL for it, but it does not matter where that
energy comes from. In my case it is wood with propane back up, but it could be coal, oil, or yes even compost. It really just depends on what you want to heat that main boiler loop with. As long as it is water over 80 degrees, it will work.
As for even better efficiency that I mentioned earlier. The plc only operates in the here and now, in other words what it senses for a reading outside every minute. By calculating how much heat the slab is losing, how warm the water returning from the floor is, and what the main boiler loop temp is, it gets an idea of how hot the water flowing through the floor should be. Flow controls are only there to get the proper Delta T...which is a 15 degree drop in water temp per loop in the floor. In other words if your target temp is 75 degrees flowing through your floor, you should be getting a return temp of 60 degrees back. If it is 65 degrees, you want to reduce your gallons per per minute to allow your
hot water to absorb better into the floor. If its 50 degrees on the return side, you want to increase the gallons per minute so that you get that ideal 15 Delta T. That is just part of getting your system balanced correctly. Your plc setting also get you close.
Still this is what I want to see. If the government via NOAA would send out the forecast via digital coding over the internet, my plc could interpret what the temperature outside WOULD be. Since it takes 12 hours to make drastic temp differences in my concrete slab, if it is going to be warmer tomorrow then today, my plc would just let my floors slowly cool off and not require so many btus to keep warm knowing it would not be needing it. That would gain some efficiency big time! If I wanted comfort on the other hand, if the temperature dropped suddenly, faster then my concrete floors could react, it would start increasing the water temperature well in time to be warmer when it was needed. I would override that however and glean the most efficiency that I could. That is how the whole system works, by injecting the precise amount of heat through your floor at any given moment, your home stays warm. Not by the thermostats turning on the zone valves, but by the water temperature itself. With the exception of fast spikes in outside temperature differentials, hot to cold suddenly or vice versa, our home stays within a degree of what I set the thermostats.
All of that was not cheap however; it cost me $10,000 6 years ago installed myself, however I also heat a 3000 sq ft home for about $1100 a year in Maine. It has been a good return on investment.
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