"I have a little issue with the way they use the DHW heater for the slab. Most towns around here won't let you do that because in the summer that water stagnates in the slab abd then can contaminate your fresh hot water (or DHW) tank and make you ill."
They don't.
Mike, if you look closely at the diagram of the heat pump setup you will see that it DOES show using the DHW for slab heat. You CAN use a DHW heater but CANNOT or SHOULD NOT connect it to the DHW you'll use for drinking, washing etc.
"cold 70" is a term coined for the phenomenon where the air temp is 70* but all the surrounding surfaces are much colder. Walls, floor, ceiling, furniture etc. Even though a thermometer will indicate the air temp is 70 people feel cold. Best example I can think of is a bank once where they had granite counters and brick walls. They would shut the heat off at night and all that rock in there would achieve something like 56* IIRC. When they turned the heat back on in the A.M. the air would warm to 70* but all the rock would still be 56* even by late afternoon. Those girls were FREEZING even though the air temp was 70*.
Why? Cold does not radiate, heat does. In effect their warm little (some not so little) bodies were radiating their heat to the cold surfaces making them feel chilly. You might look at it as the cold surfaces were pretty much "sucking" the heat out of them.
Anyhow, thats how "radiant cooling" works. As a matter of fact, now that Phil mentions it I DO recall something about cooling the ceiling. Honest, they do it in Germany.
I can't imagine that it would be any more effective than using a multi-speed system connected to internal and external thermostats and humidistats, but I can imagine it being a lot more expensive.
This is MY contention. Yes GTHP are considered to be the most efficient heat around however, I can't buy 40% savings over a well designed high efficiency oil fired system. I too would like to see hard numbers but unles you build 2 identical houses side by side one with GTHP the other with a good oil system and compare the reults it is nearly impossible to verify any of this.
40% over an old furnace, definitely. 40% over a high efficiency oil fired hydronic system? Not likely. Maybe 20% I'd buy.
They say 40%. %40 over WHAT!? They conveniently leave that part out.
Nice system Phil! I may be drooling...
I'm going to correct myself. While the diagram shows the DHW hooked to the slab I don't have enough info there to determine exactly what's going on. From what I am reading the heat pump heats either one or the other. As long as there is no way for slab water to find it's way back into DHW then it's ok. A DHW heater is another accepted way to heat a radiant system due to it's being designed for low water temps and there are those that use them. There WERE also those who were using them simultaneously for DHW. This was found to be a bad idea for the reasons described above.