HVAC slow to reach setpoint temperature in both heat/cool mode under light load
About 3 years ago, we had both upstairs and downstairs residential HVAC units installed in our upstate SC home. The downstairs unit seems to be fine but the upstairs unit sometimes takes a really long time to reach the setpoint and shut off - staying on for 45+ minutes under very little load. It's been this way since it was installed. Both units appear to be properly charged and running well.
When upstairs unit comes on during normal heat mode (for example setpoint @ 70 F and current temp @ 65 F), initially it blows air ~15 to 20 F above ambient. As current temp gradually approaches setpoint (reaching say 69 F), the air coming out of the registers decrease to a temperature of only a few degrees above ambient (~71 to 72 F) and it seems like forever until it finally reaches the setpoint and turns off. This slow response behavior is also exhibited in cooling mode as well. This can happen when outside temp is relatively warm like today (~60 F) and not working very hard at all but can also happen when it's colder (30's to 40's).
I found the technician manual for the thermostat (Honeywell PRO1, T55) and noted the current "Technician Setup" settings. The cooling/heating swing settings (dfco = 0.3/dfhe = 0.4) are the same as for the downstairs unit.
Any idea on how to adjust the thermostat to more aggressively reach the setpoint?
You said, "sometimes takes a long time". So this is not all the time. this happens in heat and cool mode. How do you know the charge is correct. Make sure the filter is clean. You should probably call for service. Is this a heat pump?
If it's always done this it may be sized very close to demand, keep in mind long cycles are a good thing. As long as it"s been determined by a tech that the charge is actually correct and the heat source is staying on through the complete cycle.
Last edited by ckartson; 01-18-2017 at 06:20 AM.
Reason: added new thought
I didn't write the book I just read it!
How about a make and model.
This belongs in the Ask Our Pros forum
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It sounds like you have equipment that is two stage...that is variable capacity. You must be talking about a heat pump. Forget about the thermostat...it's basically a switch. Heat pumps usually operate at a 15 to 20 deg. differential over ambient temperature. A two stage unit will operate at a lower differential because it is designed to save energy whenever possible. Believe it or not, a heat pump running for a longer time is more efficient than other heat sources running for less time. Call your heating guy....let him check it out. What is happening with your energy bill?
Thanks for everyone's help on this one!
@ VTP99: York 2-ton, 15 SEER heat pump, split system (HP: yhjf24s41s1, AH: ahe24b3xh21)
@ ckartson: For the upstairs unit that is problematic, it's not all the time. Unit was just recharged due to a small copper tube breaking off at a solder point during an ice storm, so it should be correct now. Just changed the filter but still operates the same.
@ MissouriBound: Specs say it's a single stage but you bring up a good point about lower temp differential. Tonight it was warm outside (~60F) and again it came on @ 68F trying to get to 70F. Thermometer in register initially showed ~90F but then slowly over about 10 minutes decreased to ~72F and ran for at least 25 more minutes - hovering at 69F. Then I decided to ramp up setpoint to 78F and I heard a click in the thermostat (Honeywell T855) displaying "+1", which manual says is 2nd stage of heat. Register temperature went back up to 92F. Once setpoint reached @ 70F, another click is heard and "+1" disappears. Register temp then drops 10F in about 60 sec. So it sounded like a higher setpoint made the unit kick into a higher differential temp and increased the temperature output. But how can it do this if it's a single stage? Electric bill has always been pretty low (monthly bill shows our costs far below an efficient home) but I normally have only downstairs unit on during day, upstairs only at night. T855 settings: Room temp calibration @ -2F, Cooling swing @ 0.3, Heating swing @ 0.4. One other note, there's no door between upstairs and downstairs and upstairs unit isn't too far from doorway to downstairs. Maybe this is how it's supposed to run but it's annoying when in heating mode it blows somewhat cold air for 45 minutes. Anything I should change for swing/temp calibration settings? I told my HVAC guy about this by the way and he suggested the filter change and maybe a maintenance since it's 3-1/2 years old. I really trust him and don't think he's trying to pull anything over on me.
Most likely that click is your aux heat coming on. That is what gives you the 90 deg. temp. After the differential is met your temp drops drastically. You have a problem with the cooling circuit and I would check the compressor first. Sounds like the valves are leaking or you are way low on charge. Since it's warm out put the unit in cool and see what it does. Check your supply temps and see where they are. I'll bet you get the same temp after 10 min.
With a 47°F outdoor ambient your heat output without heat strips is 22,200 btu/h.
With a COP of 3.68
From what you have told us you have a heat pump (first stage of heat is provided by the outdoor unit) with electric heat as aux heat (thermostat 2nd stage brings this on) during higher demand. When you raised the setpoint and heard the click you engaged your electric heat. Normal operation is outdoor unit only unless outdoor temps are extremely low in which case your system should respond automatically to change over. It seems to me you have a problem with your "outdoor unit". It should stay on and maintain a heating supply air temp of 85° to 120° constantly until the call for heat is satisfied. You need to have your service person check this through a full cycle, it sound to me as if the compressor is shutting off (in outdoor unit) shortly after a heat call from thermostat. I admire your diligence and your observations but at this point there is really nothing else that I can say. Charges need to be rechecked, ampdraws read, controls tested, all to be done by your service person.
I didn't write the book I just read it!
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Thanks very much for your help, getting me to understand what might be happening. I am having my HVAC service guy come over for maintenance and - as you suggest - asked him specifically about running it through a full cycle and watching the register temps the whole time. I'll let you know what he finds Wednesday when he comes to check it out. Thanks again!
Good. Ask him all the questions you asked us. I suspect most of us have pointed out the issues but it will be interesting to hear his prognosis.
He came to do the maintenance and found a loose wire, fixed it and assumed that was the problem. It's still not warming up, so I called and talked to him. He suggested I check the two pipes that enter the outdoor unit - one should be warm and the other cold, if the compressor is working. Here's how the unit ran tonight (which I emailed to him as well)...
@ 7:30 PM (ambient temp @ 68 F, setpoint @ 70 F):
- upstairs unit came on in Emergency Heat mode, even thought it was set for normal HVAC. Register temp @ 90 F, both lines coming out of outside unit cold. Outside unit fan ON.
@ 8:00 PM (ambient temp @ 69 F, setpoint @ 70 F):
- upstairs unit automatically switched to normal HVAC. Register temp @ 72 F, both lines coming out of outside unit cold. Outside unit fan ON.
@ 8:30 PM (ambient temp @ 69 F, setpoint @ 70 F):
- upstairs unit still on normal HVAC. Register temp @ 72 F, both lines coming out of outside unit cold
@ 8:35 PM (ambient temp @ 69 F, setpoint @ 70 F):
- upstairs unit switched to back to Emergency Heat mode. Register temp @ 90 F, both lines coming out of outside unit cold. Outside unit fan OFF. Unit eventually shut off but that's because I lowered the setpoint to 69 F. That's a lot of time to get 2 F when the outside temp is ~60 F!
Upstairs unit ran for 65 minutes, ambient temp only increased 1 F (using emergency heat only) and both lines coming out of unit were always cold. He said he'd come back out after I reported my findings.
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