Ceiling hydronic heating and cooling
I am building a house in Washington, DC. (Zone 4a). The design hasn’t been finalized yet but a preliminary Manual J has it slightly heating-dominant, about 10 Btu/hr/sf for heating and about 8 Btu/hr/sf for cooling. About 10% of the cooling load is latent, which seems low to me. My intention is to seal and insulate better than code and have a nice tight house.
I have been reading John Siegenthaler’s articles about ceiling hydronic systems, here:
Tantalizingly, his last sentence is:
“By the way, with suitable system controls, this ceiling panel is ideal for radiant cooling applications.”
How can he leave us this way!
In this article he goes into more detail, but still doesn’t get everything:
He makes the point that well-insulated, modern houses need much “gentler” heating and cooling systems than traditionally. He advocates for minimizing thermal mass (which would have been heresy 20 years ago) to maximize responsiveness. I also really like the idea of eliminating (or minimizing) ductwork and the bulkheads, noise and drafts that come with it.
His recommended ceiling construction is 7/16″ OSB, 3/4″ poly-iso, then 1/2″ pex tubing in aluminum heat transfer plates with the tubing facing up, and the whole thing covered in drywall which is attached to the studs with 2 1/2″ screws.
What I’ve figured out is that you’d have to have separate dehumidification to avoid condensation on your ceiling. The recommended control seems to be to have a thermostat and a humidistat in each zone, and to turn off ceiling cooling if the humidity rises so that the dew point is above the water temperature. Let the dehumidifier run and take out the humidity and then turn the ceiling back on again.
Since my house is going to require active ventilation it seems relatively straightforward to integrate ventilation, dehumidification and air circulation in a single shaft that runs from the basement to the top floor.
He recommends a ceiling temperature of 110F (115 supply and 105 return) which gives an output of 30 btu/hr/sf with a room temperature of 70. That would be ample for my house. Using chilled water with a 45 supply and 55 return at a room temperature of 75 would give about 20 btu/hr/sf of sensible cooling in the same assembly, which is still about 2.5 times what it needs to be but lower than for heating. So the assembly would be sized for cooling and about 15-20% oversized for heating, which I think would work. An alternate strategy would be to right-size it for heating and use some of the ventilation shaft/latent cooling to also provide cooling to the rest of the house.
It will probably take me a few posts to lay out the details, I’ve been thinking about this quite a bit. I’m curious in your feedback.
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