Dave, In case I sort of obscured the information, here let me totally louse it up...
NO really, This will help... There are three hot water heaters; one propane at 40 gal and two electric with one being 30 gal and the other is 50 gal. Neither electric is connected to electric power.
The 30 gal tank's thermostat is used to ask the GSHP for hot water and as the system is a heating priority system it will briefly and temporarily interrupt air conditioning (cooling) to satisfy the 30 gal tank. This is about 115 to 120 degree water in the 30 gal tank. This is what is used for hydronic heat.
The 50 gal electric DHW tank has its contents circulated through a flat plate heat exchanger. The "OTHER" side of the heat exchanger is in the loop between the GSHP and the 30 gal tank.
Their are two pumps pumping water through the 30 gal tank. One circuit is fron the GSHP through the heat exchanger, through the 30 gal tank and back to the GSHP. The other circuit is the 30 gal tank, an external pump, and the manifolds for the hydronics, then back to the 30 gal tank.
This is like the traditional joint checking account where one party has the task of putting $ into thte account and the other takes it out with no need for communications between the two parties as long as the capacity to put it in is not exceeded on average by the taking out.
Yet another pump circulates water through the heat exchanger and the 50 gal tank. The thermostat on the 50 gal tank controls this pump. When the 50 gal tank wants heat it starts the pump and "steals" heat from the hydronic storage tank (30 gal tank.) The input to the 50 gal DHW tank is cold water. The output of the 50 gal tank goes to the input of the 40 gal propane fired tank. The 50 gal tank "preheats" the water for the propane fired tank. The 40 gal propane tank should be able to keep up wilth demand since it only has to raise the water from about 115F up top about 140F (at most.)
Deep earth temp here is about 62.5F so water in shallower pipes is sometimes warmer and sometimes colder but probably averages about 62.5. So on average the 50 gal tank provides about 52.5F increase and the propane unit provides about 25F rise. (this assumes 115F in the 30 and 40 gal tanks. If I get 120 then the preheat is 57.5F and the propane unit only has to make 20F rise. I should get pretty fast recovery times.
Some of this is speculative as I have received but not installed the pump for the 50 gal tank... yet.
Cold weather will not make a change in the job of the propane fired unit as it will still get 115-120F feed water and it is located inside a conditioned space.. The equipment room is in the basement along the most burried wall. The basement wall is cast concrete 12 inches thick with R-11 rigid foam on the outside. The floor has R-11 rigid foam under it as well and 16 inches of washed gravel under the foam before hitting dirt (French drains preclude water contact wilth the slab.) There shouldn't be much seasonal difference in operation.
Heat pumps can run about 300 to 400% efficiency as compared to an electric DHW tank which is about 100% efficient. How do you get over 100% efficiency??? Electric water heaters "burn" electricity at efficiency of 100% directly converting electric energy into heat. Heat pumps don't "burn" electricity to make heat. They consume electricity to pump pre-existing heat from one location to another.
[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]