Cold pantry in a Passive House via hot water heat pump air exhaust
We’re building a passive house and one thing we had to give up was the idea of a cold pantry for long-term storage of root vegetables, apples, etc. But after learning about hot water heat pumps and the fact that you can duct the cold exhaust air, I started thinking it could be possible exhaust the cold heat pump air to the pantry. The pantry would have to be tightly sealed and insulated and have a returning duct back to the mechanical room. That would essentially create a cool, dry cold pantry with constant air circulation. Seems like it would be a perfect symbiotic relationship between the two.
There are a few things I don’t know though.
1. Would the pantry get too cold? If so, I guess I could use home automation to monitor the cold pantry temperature and automatically choose one of two paths for exhaust air (mechanical room or cold pantry).
2. Is the exhaust air safe? Could it contaminate or pollute the food in the cold pantry in any way? Assume that the mechanical room is clean and all electric ,and assume it’s not near anything else that could possibly exhaust pollutants like vehicles.
Thanks!
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Replies
Mike,
My guess is that the irregular nature of the cool air provided by a water heater wouldn't match the cooling needs of your proposed pantry. Under some conditions, the room might be too cold. Under other conditions, the room could easily be too warm. The carrots and potatoes would suffer.
What is your geographical location? The easiest way to create an environment to store carrots and potatoes is to build a basement, and to have at least one area of the basement free of heating appliances. In other words, you want a dirt-floored cellar. This approach works best in the northern half of the U.S. or in southern Canada -- not so well in Louisiana or Florida.
Potatoes, apples... need to be as close to 32F degrees as possible for long term storage. 40-48F for most other produce. Our heat pump water heater does bring the basement temp down but certainly not that close. In a small space, it will bring 70F down to 50F.
PK Maintenance & Produce Guru for The Free Market, Appleton, WI
Paul,
I have cabbages, potatoes, beets, and carrots in my cellar right now. The food was harvested six months ago -- and it's all alive and fresh and good to eat.
https://hortnews.extension.iastate.edu/2000/8-11-2000/storepotatoes.html "Cure potatoes at a temperature of 45 to 60 F and high relative humidity (85 to 95 percent) for two weeks. Healing of minor cuts and bruises and thickening of the skin occurs during the curing process. ... Potatoes should be stored at a temperature of 40 to 45 F and relative humidity of 90 percent. Store in a dark location as potatoes turn green when exposed to light. If storage temperatures are above 45 F, the potatoes will start to sprout after two or three months. When stored below 40 F, potatoes develop a sugary, sweet taste. Sugary potatoes may be restored to their natural flavor by placing them at room temperature for a few days prior to use."
Thank you for the replies. I didn’t get the emails that I usually get and just check now to see if anyone replied. I think your right Martin, the temperature won’t be consistent enough over the seasons. We are putting a large fire protection cistern under the garage. That will be the closest thing we have to a basement. I could make a space there for a cellar.
Overall though, I still like the idea of pumping cold exhaust air to a pantry. It can only help with whatever food is in there for short term storage. Long term storage would be a cellar. If we can have root vegetables last for a month, that’s good enough for our main use case.
I've been wondering about a cold storage room for our house as well. While not a certified Passive House design, it will be super insulated.
My concern would be with building a room inside the thermal envelope that was somehow "shorted" to the ground and kept at a reasonably consistent temperature. This room would obviously need to be insulated and sealed off from the rest of the house (basement in this case).
Two ways to do this that I've considered:
1. Bringing all the water in through an air radiator in that room. A flow switch could activate a fan that circulated room air through the radiator, using any warmth in the room to pre-warm the cold ground water. This may not be very effective.
2. Using a small minisplit between the cold room and the basement. This might cost more, but at least you are keeping the heat from the cold room inside the thermal envelope. I don't know if the thermostat controls on a minisplit would work to keep the room cold enough, and would it dry the room out too much?
Just throwing some ideas out there. I'm not sure if I will go through with this or not.
Seems like it'd be a lot easier to just use a refrigerator or chest freezer with an external temperature controller. It's a well tested approach that works (look up kegerators).
A wine cellar chiller would work. They are designed to maintain 50-55 F without dehumidifying too much, and they can probably be pushed down to 45F or so. They are not terribly efficient though. I did recently spec a HPWH for the mechanical room adjacent to a chilled wine cellar. The mass of the room acted as thermal storage, and the water heater and wine room chiller both did their things when necessary. HPWH chilled the room and wine cellar warmed it. Both run more efficiently with moderated temperatures. Still not an ideal solution, with 2 separate pieces of mechanical equipment, but with varying loads and timing, it's probably about as good as it gets.
Depending on the size of your root cellar, there might be some commercial walk-in fridge chiller solutions. Those will control temperatures right down to 32F. Again, dumping the warm exhaust into the WH mechanical room will boost the WH efficiency.
Lance, I think the water method you mention will probably not remove enough heat from the room, even if you were running the water continuously. But of course you won't be doing that, so for many hours at a time you would have no cooling at all. The mini split seems like it would work, but I really can't see it making sense in terms of cost.
As for the original questions, it's just going to be too inconsistent. I don't know if anyone commented on the safety question, but it's literally just the air in the room being passed over aluminum or copper coils, so if the air in the room is safe, so is the air coming from the heat pump.
The most economical solution to a cold cellar in a high performance house is to not have it in the house at all. Dig a big hole near the house, and install a root cellar there. Apparently you can get factory second concrete septic tanks for pretty cheap, which would work pretty well.