Greenspeed report
Last year we purchased a 5-ton Greenspeed to replace a 160k (!!!) BTU oil furnace in our large, long, 1952 CZ4 home. We’d completed many envelope upgrades (nearly complete), and had a ResNet rater compute manual J and HERS. We were also been concerned about air speed, because we have long, original galvanized duct runs and really could have 3 zones (but only have 1).
Happy to say the house is SO much more comfortable than it ever had been before! The house is MORE comfortable in every room. We estimate we’ve reduced costs by 30% by the HP alone (and have several lessons learned if anyone is interested, just ask). The house is warm as toast into the mid-20s, and based on the strength of the unit, I am confident it can heat without backup heat strips at much colder outdoor temps.
My question is, when the outdoor temps are in the 40s, we need to set the thermostat at 70 for my wife to feel comfortable. But when it is colder outside, we can set the thermostat to 68 and feel just fine. Why? I’ve thought about reducing the airflow (I can reduce cfm to about 1200) to increase air temps, but we need 2000+ based on our ducts. And given the length of our house and duct runs, I’m concerned that low cfm won’t get enough air where it needs to be.
Any suggestions?
Thanks.
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Replies
I would think the answer to your question about it feeling more comfortable has something to do with the system running almost contstantly to keep up with demand. This would create more even heat and be more comfortable.
Are the heat strips your only backup? Did you make substantial envelope upgrades when you switched?
My guess is that when it is in the 40s the heat pump is cycling on and off because at its lowest speed it makes more heat than you need. So sometimes it is blowing on your wife and other times it is not and she would adjust to one or the other but the change is uncomfortable.
Some of the problem is about register and return locations and where you are sitting in the room. Your 1950s duct design was not likely done with much math. It was likely let’s put a register in every room under the window and connect them to the furnace.
If you have a room by room manual J getting a manual D calculation should not cost much at all. Then you could measure the air flows and adjust the dampers to each room.
Are you changing the thermostat setting with a program or manually? I think heat pumps run best if you will set it and leave it alone.
When we went from a gas heated house at 68° we found it necessary set the heat pump of the new house to 69°.
Walta
Brian,
Heat strips are only backup - currently locked out until 5 degrees outside. Although temps have only been as low as low 20s so far, we've had not any issues at all. Envelop upgrades have been substantial, but far from deep retrofit. We have stone walls R3 on the first floor, and nothing to do here.
Walta,
I agree that setting and forgetting seems best approach thus far. I did learn that setting back requires manual adjustments, because calls of 2 or more degrees use heat strips (and this option cannot be turned off in settings). I don;t quote follow your suggestion about cfm per room.
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Even though the HP can reduce to 40% max capacity, it is indeed likely that the HP is cycling off, which is why the house feels cool inside. (I don't think blowing cool air is as much of an issue, although I might be wrong.) Either way, I suspect that the easiest fix might be to set the temp to 69 or 70 when the outdoor temps are in the 40s, to keep the system working a bit harder and generate more heat.
If others have experience or suggestions for a systems setting adjustment to make the house more comfortable during "shoulder" weather when outdoor temps are in 40s, please share.
I'd focus on the "why?" before thinking about any fixes. Temperature, temperature variations, cool drafts?
For my part I don't trust my sense of heat and cold very well these days, because I am at the age when my internal thermostat is a bit wonky, and tend to cycle between feeling either too hot or too cold when the thermometer says the same temperature (even apart from actual hot flashes or chills). But I also tend to wear more clothes if it is colder outside, and the house feels warmer after I've been outside, because the temperature difference is greater. In any case, being comfortable at a slightly cooler temperature in the winter is a good thing, as you use less energy. If it were the other way around I would be more concerned.
Regarding setting the stat to cooler when it's colder outside. This might be resolvable by tweaking the thermostat placement, or using an additional remote space sensor place a little closer to an outside wall.
If the thermostat is placed too far from the exterior, in the core of the building, internal gains will keep it satisfied much longer in the shoulder season even though the perimeter rooms might be getting cold. Once it gets colder outside, it doesn't take as long for the stat to sense the cold.
You want the stat close enough to an outside wall to pickup the heat lost to exterior, but not so close that it runs excessively and overheats the house.
Check your thermostat options, there are often settings to block the use of backup heat for recovery. Also, make sure you aren't using backup heat during defrost cycles, I think this is sometimes on by default to counter act the gap in heating during defrost cycles.
Thank you all.
Called the installer yesterday - I will ask him to come take a look.
“Also, make sure you aren't using backup heat during defrost cycles, I think this is sometimes on by default to counter act the gap in heating during defrost cycles.”
I would be interested in hearing other people’s opinions in this point.
To my mind this choice is about comfort verse kilowatts. I may be inclined to disable the strip heat while defrosting if me electric rates were .25 instead of .08 per kWh.
If you make that choice you need to understand you system will be blowing air out about 15° below your set point for several minutes every defrost cycle.
To me it seems like a small cost for comfort as my system is in defrost mode for less than 1% of its run time. My wild guess is that it may cost 6 % more for that level of comfort.
Walta