this post was submitted on 03 Jan 2024
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The main thing with heat pumps is that they obviously do heating and cooling. For most other systems, those two functions are separate, and can be sized appropriately.
If you live in a climate that is mostly hot, with some cold days, you'd size to accommodate the hottest days. If you live in a cool/cold climate, where you'd only occasionally need cooling, you'd size for the coldest days. Going oversize is bad because it is inefficient and just doesn't work as well. It is often better to size the system to accommodate 99% (or whatever %) of days, and have a little backup heat and/or cooling capability for when the pump can't keep up than it is to oversize and run inefficiently most days.
I lived in an apartment with a heatpump sized to handle the cold, and it kept me warm always with no trouble. It wasn't that cold of an environment, though. During the summer, it would kick on to cool the place, and it would cool very fast. That sounds good, but what happened is the temperature would be fine, but the humidity would be very high, and I would get condensation on my windows and mildew in my closets. I ended up having to increase the hysteresis of the system, so instead of letting the temperature rise a couple degrees above the set point and cooling a couple degrees below the set point, it had to swing much further. That made the system run long enough in one shot to decrease the humidity, but it basically made it so I'd have to either let the place get uncomfortably hot first, or cool it down to uncomfortably cool.
If the system were smaller, maybe there would be a few days a year where I would have to wear a sweater inside, but I wouldn't have to worry about 150 days of too much humidity.
The last thing to think about is air-source vs ground-source. Air source is basically a drop in for an air conditioner, but has a lower limit on temperature it will operate. That limit has gotten a lot lower over the years, but AFAIK, there are theoretical limits it can't beat, depending on the physics of the system and particular refrigerant. Ground source, aka geothermal, should work most places provided you have the space for it, and money to install it. Underground never gets cold enough that the physics of heat pumps aren't effective.
I get both ends of the spectrum. 30+ in the summer and -30(and worse, at times) in the winter.
I currently have a NG burning furnace and central air for the summer, but both are coming up on end of life so Iโm weighing the pros and cons of going with a heat pump system.
Thanks for your reply!
Honestly you could look into a dual-fuel system. This is what I did. The furnace I have was not on it's last leg so to speak, but looking at another 5 years or so left on it.
Dual fuel would allow you to add a heat pump to your existing setup, where it will take on the brunt of the heating and cooling needs. Only when it is so cold that it no longer is efficient will it swap over to the furnace. Good annual maintenance then can extend the life of your furnace for years.
If you opted for a low temperature heat pump they can be efficient down to 0ish degrees F. This may be a very cost effective and heat effective option for you.
Honestly my hope is to be able to remove myself from the gas hookup. NG is fairly cheap where I live so it seems counter intuitive but my delivery charges are always far more than my actual usage costs. Iโd love to be able to be free of that. Plus, Iโd like to do my part and if Iโm going to be spending extra money on delivery/admin charges for not much gas usage, I may as well spend it on electricity.
A noble goal for sure, I'm right there with you. I'm trying to go carbon free myself. The gas furnace is the last thing for me, and it's mostly simply because the heat pump just won't do it up to a certain point. I took the idea that reducing 90% of my emissions was better than not doing anything and staying on it.
I hold to my advice, heat pump will take care of 90% of your HVAC needs, with gas as a backup. Another way of thinking is what happens if there's a -20 degree day and the power goes out? If anything, a gas furnace is a nice safety backup.
I think this winter my gas furnace has kicked on 3 times total so far, and those were only in the early hours of the morning on incredibly cold days. Everything else? Pure electric.
It is not less efficient if it's oversized, that is absolutely not true. It will just run less to keep the room at the desired temp. It's actually more efficient to oversize because they have to run less. Running full on constantly to try to keep the temp, which is what happens if you undersize, it's what's less efficient. You actually want to aim to overshoot your needs by 30-50%.
Have any kind of source? Every source I've seen says that running oversize is worse. In my own experience, it led to humidity problems, too. In fact, every source in seen says its better to undershoot than overshoot.