I have hired a TECA certified hydronic designer (local requirement). Things seem to be going fine but he has called into question my desire to use an ASHP coupled with under floor panels such as Joist Trak. He claims that ASHP struggle to deliver water hot enough to work with them and is suggesting instead a 1-1/2" concrete topping system.
 
I thought that was the whole point of heavy aluminum extrusions - they run on lower water temps (and therefore your ASHP efficiency improves).
[Rob, I believe it was you that got me thinking this way - can you comment on Joist Trak water temperatures? I think my heat loads are minor.]

Our local building code says 5310HDD(f) and design temperature 18(f).

The house design is aiming at "code plus 50%" insulation levels, and very air tight (with HRV). 1000 sq ft basement, 1000 sq ft main floor, 850 sq ft second floor. (As soon as I have the heat loss calculation from him I will add that to the house description.)

Thanks for your input

John

 

You would have to know what your water temperature requirements are to answer that question. In your climate, if you're using the altherma and you have a heat load higher than the backup element's capacity, you would want a maximum supply temperature of 113 or less to keep the heat pump "in the game". 104 would be even better and would keep the heat pump in the game down to about 5 degree F.

with heavy plates under wood finish floor and 3/4" subfloor that's about 14 to 17 BTUs/sq ft in a 70 degree F room. If your Per square foot loads are higher than that you would want to put in supplemental emitter (wall, radiator perhaps).

Thin pour of concrete is about 5 BTUs/sq ft higher at 6" o.c.

If your heat load is lower than the backup element's capacity, or if you have a secondary heat source, then the water temp could go higher... though lower is always better.

Heavy extruded aluminum panel will increase output by 175% vs. bare tube but that doesn't put it in GSHP performance range in most applications. Your TECA guy is right.

"5" Btuh Rob?

I do heavy plates, and even suspended tube with GSHPs fairly regularly in good envelopes. 110 max design there, aggressive but very achievable.

as I said, a thin pour of concrete with tubing 6" o.c. will give about 5 BTUs/sq ft/hr more than heavy plates in a joist for a given water temp. do you disagree? that's what my output charts say and they've worked well for me...

We are still fine tuning the heat loss a bit but its looking like well below 30,000 BTU.

Excluding stairs etc that works out to approximately 11 BTU/sq.ft.

Flooring is typical 3/4" plywood subfloor with 12mm engineered wood flooring, some areas tile, some lino.
Would it be reasonable to assume R 1.2 for the eng. floor?
What about the tile and lino areas?

I am a bit confused. If its only 11 BTU/sq.ft., the flooring pretty typical and using heavy plates wouldn't my required water temperature be reasonably low?
Is there another variable I am overlooking?

Any useful input is much appreciated.

John

max temp 100 degrees or lower with heavy plates and 11 BTU/sq ft with wood floor. but... is that peak load, or building average? room by room calcs is where the devil lies. you might have one room with 30 BTUs/sq ft and others with only 5.

if that's the peak room load though you're in great shape.

"a thin pour of concrete with tubing 6" o.c. will give about 5 BTUs/sq ft/hr more than heavy plates in a joist for a given water temp. do you disagree?"

Sorry, I missed the "more than". I should have known better.

Nothing beats the output of a wet radiant floor heating system, save radiant ceiling and oversized panel radiators.