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Purposedly oversized Passive Solar glazing, Heat Pump Water Heater, and exterior shades..

A recent question with regards to heating a basement slab via radiant due to chilled air got me thinking of an idea I had regarding a scheme I had.

We're all more or less in agreement that HPWH are a 'Good Thing' given their ability to extract heat at a fairly high COP. Their main downside in a cold climate being their contribution to the heating load.

We're also more or less in agreement that Passive Solar in cold climates is a 'Great Thing' since the sun is free eh?.

Generally when one is planning Passive Solar houses, I would assume the it would be prudent to calculate heat loads, and plan to avoid overheating due to excessive glazing. However, if you were to merge the two technologies, how difficult would it be to predict how much heat the HPWH could extract/store while minimizing any overheating situations...

I would think this could be stretched even further to have the HPWH (high in the house) hooked up to a fairly large storage tank for buffered heating during dark hours and whatnot.

Overheating could be easily minimized by exterior shades or by simply opening a window. Overheating in the summer... bad, Overheating in the winter... I could think of far more things to complain about.

Would love to hear everyone's opinions on this.

Asked by John O'Brien
Posted Aug 22, 2011 1:55 PM ET

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6 Answers

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1.

I like this idea a lot. The HPWH would be set to operate only if ambient is above 65F.

A solar thermal dhw system has a much higher COP, though, so it should be compared.

Answered by Kevin Dickson, MSME
Posted Aug 22, 2011 3:37 PM ET

2.

Kevin,
Overglazing on the south side of a passive solar house results in overheating on sunny days in March and April. If you stay home all day, and like to adjust curtains, awnings, and windows, that may not be a problem. If you have a job outside the home, however, you may get tired of coming home to an 82°F house.

I think the idea of turning on a heat-pump water heater just when the house gets too hot is fussy and a poor use of expensive equipment.

Answered by Martin Holladay
Posted Aug 22, 2011 3:52 PM ET

3.

Martin is right that a house like this gets a little fussy. There isn't an easy way to prevent operation below 65F inside. The Geospring just has a button that will "stop cold air" for a default period of 3 days.

Here's something that grabbed me while looking at the Rheem Hybrid user manual:
"❑ 1,000 cu. ft unconditioned indoor space."

The manufacturer is basically saying that you may be unhappy unless you have a big unfinished, unoccupied basement. Or maybe a garage that doesn't ever freeze.*

The split system HPWHs promised by some manufacturers will be the better solution, but their installed cost will be relatively higher. Probably at least $2600. Meanwhile, the current crop of HPWHs have been going down a bit, now approaching $1100.

*A well-designed and insulated garage is naturally heated by geothermal: http://www.greenbuildingtalk.com/Forums/tabid/53/aff/13/aft/45292/afv/to...

Answered by Kevin Dickson, MSME
Posted Aug 23, 2011 2:46 AM ET

4.

I can't imagine it would be that difficult to build simple temperature controlled switch to enable/disable the HPWH at certain temperature setpoints within the house but I guess it is a complex system, when a simple electric tank would just work.

Seems a shame to not be able to use such a nice tool just because we live in igloos.

Answered by John O'Brien
Posted Aug 23, 2011 7:26 AM ET

5.

"I can't imagine it would be that difficult to build simple temperature controlled switch to enable/disable the HPWH at certain temperature setpoints within the house"

The thing that makes it a little complex is that ideally you only want to disable the heat pump, not the electric resistance backup. That means you have to open the box and jumper the right wires inside the unit.

But that feature would be easy for the manufacturer to provide, and I'm sure they already have it in their suggestion box. Literally everyone in a mixed heating/cooling climate needs it.

Answered by Kevin Dickson, MSME
Posted Aug 23, 2011 9:37 AM ET

6.

Storing heat from the warmest period of the day I think is do-able. I think all air sources heat pumps could be set up this way and quite simply.

Answered by aj builder, Upstate NY Zone 6a
Posted Aug 23, 2011 10:33 AM ET

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