Radiant or Geothermal Heat?

[Paddy]

Raspy,

Funny, I started this thread in 2005. We went with Radiant with an ele boiler. The heat is very good, quite. We have many zones so much of our home is un-heated when there are no guests.

I was planning on solar water heating to "feed" the radiant. Now I think I'm best off installing PV solar panels and get power 365 days a year. We have a 3000 sq-ft flat roof so plenty of room for a good size array.

 

[JCByrd24]

Paddy,

I would concur that your best bet these days is PV vice thermal solar. Like you said you can take advantage of the PV all year round. Thermal systems large enough to supplement heat are usually not of much use in the summer unless you want to heat a pool. Thermal is usually sized first for domestic water heating, but even for that purpose PV is getting very good.

Raspy, I concur that there is nothing really particularly complex about a radiant system in the world of typical home heating systems. That being said, in a modest new construction project the quote for a minisplit or two and some supplemental electric baseboards will be likely well under $10K, possibly $5k. A radiant system + boiler will likely be well more, and in most of North America (even here in Maine) the bonus of free AC would be considered huge (I'm running AC right now in my bedroom). If you added some sort of AC to the radiant system it would likely just be a minisplit anyway, at around $3k by itself.

There are numerous articles out that document the study of radiant heat in well (super) insulated homes. The science basically shows that the heat demand can easily be reduced by insulating far enough that the radiant system barely ever has to run, to the point where the floor never actually feels warm. Additionally, the make the point that the difference in comfort is basically unnoticeable. Having friends with super insulated homes, I can attest to the fact that they feel warmer than they are, much like a radiant heated house.

I've never heard anyone get a geothermal quote under $20K. For my money that is a PV array that produces power not just uses less.

 

[Raspy]

JC,

Some good points there. It is interesting that, as you mentioned, the better insulated the house, the less radiant is needed and the floor never really feels warm. In those cases we're not merely trying to keep the house comfortable, but make the floor "invisible" and make it greet them in the morning. The beauty of radiant goes beyond the simple utility of satisfying a thermometer. So, timing it achieves pleasant surprises as it satisfies the heat load. Merely keeping the house a constant temp isn't really the goal as temp changes mean greater comfort and lower bills. Cool sleeping temps, warm bath floors in the morning, warm winter living rooms, cool unused rooms, etc.

Geothermal has never interested me personally with it's over the top complication and electric demand. But PV would help with that, even if it did add yet more hardware to do so.

Solar thermal, as you mentioned, cannot produce 100% of the required load. Mini splits might be a good supplement with their ability to cool too. As I write this we are in Scottsdale AZ, visiting. There is no way I could live here without AC. But in our home in northern Nevada, AC isn't needed. I ducted and wired for it, but I doubt I'll ever install the units and complete it.

PV seems to be really coming of age, but I can't decide which way to do it is best. Probably just research the equipment and do it myself. Our electric bill is so low that I can't justify it yet.

 

[Redneck in training]

We have geothermal with floor heating in our house. If you are barefoot the floor feels cold even at very low temperatures outside. But if you put socks on it suddenly feels warm. We have set entry temperature to 90F. The heating has large time constant (1F per 2 hours) so messing with different temperature setting for occupied/unoccupied makes little difference.
The desuperheater on our unit is able to overheat hot water heater popping the safety. When weather got colder and AC stopped running there was not hot water until the safety was reset. Therefore I added a thermostat that stops desuperheater pump when maximum temperature is reached.
Since all our AC ducting is in attic we used fiberglass insulated ducts. They are very easy to install with only knife and special aluminum tape.
Our air handler for AC is Unico high pressure. We selected Unico because it is easy to install and it it much quieter than low pressure system. Only disadvantage is difficult integration with whole house ventilation/filter.
If I would build house today I would use mini splits and invest the saving in large PV system.

We heat about 3800 scft and AC 2400 sqft. Before we installed PV system we used to have two meters. One for heating only and the second for the whole house. Maximum heating charge (January) was about 90 USD at 4.8 cent/kWh.

 

[Raspy]

Red,

For many years I've designed radiant systems with thin slabs over a standard subfloor. These have, in the bathrooms, tubing spaced on 6" centers. So the response is felt in minutes, instead of hours. It also means the cool down times are less and so, less overshoot. It also means that room thermostats have little value, but timing becomes more important, at least in the bathrooms and kitchen.

In my home I went the opposite direction to bring the floor temp up to just below where I might want it on average and then add some heat as needed. The 8" slab becomes a stabilizing factor and energy storage mass for solar. But even with this large mass, zoning allows a reasonable response time if the energy available is high enough.

In the Bay Area, in CA the heat loss I use is seven BTU/ sq. ft for a break even number. Ten BTUs will run a house OK, but the response is poor. Twenty-five works well even with one thermostat zone in the whole house, but when you have multiple zones, you might have 100 BTU/sq ft available or more for a given small zone. Then timing those zones with priority to lifestyle, such as warm bath at 7AM and warm living room at 6 PM, you can get excellent response. In those cases, "warm" does not mean the wall thermostat is satisfied, it means the floor feels warm and the air temp is relatively cool. It also means the boiler is not overloaded on small zones and can feed a given temp during the whole cycle. This gives excellent response and predicability. A delivery temp of 126 degrees instead of simply a few degrees per pass makes a big difference, even though the few degrees per pass will do the job eventually. And again, the room thermostat is an extremely poor way to run most systems. It only acts as a rough high limit that causes overshoot, especially in smaller zones.

I found a neighborhood with 756 homes that have radiant heat. They were built from the mid '50s to the mid '60s. All were set up the same with a 100,000 BTU boiler and one thermostat. Almost everyone I've talked to has the same complaint. Uneven heat, overshoot, high bills, cold rooms.

I can go into virtually any one of them and do the following: Even out the heat in all rooms, reduce the heating bill by at least $100.per month and eliminate the overshoot, which makes them far more comfortable. This is all done by improving the circulation rate, balancing the manifold, tuning up the boiler and installing a setback thermostat. Then I help them see how to re-think the heating process and realize that the thermostat is a poor measurement of comfort. Universally, they ALL praise the outcome of this process.