HVAC question
So, I am admittedly not an expert, thats why Im here 🙂 And I know there are a ton of variables but I hope to sum this up as clearly as possible. This is all relating to heating.
Basically, when the heat pump goes into defrost the blower kicks up to full speed and the temps in the furthest away supplies (8 of them) actually become lower than the temp of the house, and also when it goes into supplemental heat (if its really cold) the blower also goes into full speed, and those 8 supplies are drastically cooler than the other supplies, but unlike the defrost situation, those cooler temps are at least warmer than the house. Ive attached a spreadsheet if that helps. I apologize for my crude spreadsheet and data collection, but Im not a pro, just a homeowner with too much time 🙂 and where the supply temps drop are after the main trunk reduces–before it reduces are the warmer temps.
basic info about the system: installed 2 years ago. its a rigid trunk with flex branches. 17 supplies, 3 returns. 2-stage 4 ton unit for a 3000sq ft one story house entirely on a slab. air handler and ducts are unfortunately in the attic. Im quite certain that there was minimal calculation, if any, done for sizing of any kind. If I only knew then what I know now. Mostly the flex are shorter tight runs with minimal turns.
To my analytical mind, if the problem arises with full blower speed, then that is a good starting point, what that means I dont know… And like I said, Im sure there are a ton of variables–Ill answer any further questions as best as possible.
The worst part about my situation is that when the defrost cycle ends up coming on, it basically creates a feedback loop and the heat runs for the rest of the night trying to make up for pumping cold air into almost half the house…
thanks
rob near asheville NC
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Replies
Rob,
I'm not a heat pump expert, and I look forward to comments from more knowledgeable GBA readers. However, here is my hunch:
1. Something is wrong with your defrost mode controls.
2. You have long, poorly insulated ducts in your cold attic -- and as the air travels through these long ducts, the air temperature drops. By the time the air reaches a distant register, it's cold.
As you may know, it's always best to locate HVAC ducts inside your conditioned envelope.
Or, it may be operating as-designed &/or how the installer configured it to run.
Without even a hint as to the make & model it's impossible to say. The only clues we have is that it's a 2 stage 4 ton with a multi-speed blower and auxiliary resistance heating strips, which only leaves about 87 possible models that it might be.
Some heat pumps can be set up to run defrost cycles by temporarily switching to air conditioning mode for a period, which is more efficient than running heat strips to achieve that end. If this system is zoned with dampers the installer may have configured it to only deliver the chill during that type of defrost mode to just a few of the rooms.
When running auxiliary heating at highest blower speed the exit temps will be lower than when running the blower at a lower speed. (Same amount of heat going into a lower volume of air = higher temp, and conversely). That might be something you can change too.
Read the manual. If you don't have the manual, you can usually download one.
With the ducts in the attic it usually adds at least one full ton of load to the heating & cooling requirements. With ducts in the attic the losses are higher when the blower is running at low speed than at higher speed, since the temperature difference between the ducts and the attic air is larger.
It's a carrier system. the outdoor unit is 25HNB648 and the air handler is FV4CNF. There is no zoning. dampers are installed at every branch, but they are set fully open. From what Ive been told there is no way the turn down the blower speed in defrost, which was the first thing I asked.
And I know the worst place to have ducts is in the attic. But sometimes you inherit a situation that is currently beyond your control to do anything about.
I guess the red flag to me is the inconsistency of temperature when the fan is running in high speeds. If the temp drop in the further supplies was consistent at all fan speeds, then Id probably just accept my fate and be a little bummed, but since its only in higher fan speeds it makes me think something can be done.. .. I can roughly make a photoshop document to explain better, but the physical distance in the house from one of the cold supplies of the warmer supplies is less than 8 feet, and in the attic on the trunk, its less than 6 feet, but the temp drop is there. . . does that make sense? I just feel like theres more to it than "I need get the ductwork out of the attic." but ive been wrong before :)
thanks everybody for responding so far, I really appreciate it
http://www.americancoolingandheating.com/wp-content/uploads/2012/01/25hnb6-9-1si.pdf
The defrost option setting discussion begins on page 8. (Based on a quick scan, it's probably best if you can figure out how to set the defrost to AUTO mode.) The thing should only kick into high mode during a defrost cycle if the outdoor air temps are under 25F, or the thermostat is calling for high stage. If the thermistor sensors for the outdoor air temperature or coil temperature are way off calibration it could be defrosting more often and longer than necessary.
See also:
http://www.refripartesrd.com/uploads/FV4C%20Installation%20Manual.pdf
According to p.3 of the air handler manual the auxiliary heat strips are locked out at temps above 40F except during defrost mode.
On p10 they indicate that there is a minimum air flow setting that's adjustable relative to the wattage of your auxiliary heating strips. If the installer set the air flow higher than the minimum for YOUR heat strip wattage it would result in the too-cool air temps you have been experiencing during both defrost and auxiliary heating modes.
Read up on how different thermostats affect these issues too- it's in there.
Thanks for those links! In just a quick glance, I think youre on to something with the thermostats.... it looks like my thermostat maybe a factor. There is a lot in the manual about communicating vs. non-communicating interfaces. And I dont have an Infinity control, Ive got an Edge thermidistat model # TP-PRH01. Im going to keep investigating, and Ill let ya know what pans out...
but if anyone else has any ideas, please let me know!
thanks again
In case you don't already have it:
http://dms.hvacpartners.com/docs/1009/public/01/tp-prh-10si.pdf
From your spreadsheet, it looks like the problem is not duct insulation or location. In the normal stage 1 or stage 2 operation, the far away supply temperatures are the same or even better than the near ones. But in both cases that the heat strips would be running--defrost or stage 2 with supplemental heat--the hallway supply is much colder than the other supply temperatures. This makes me suspect that somehow the air going to the hallway does go flow over any working heat strips. Maybe the heat strips are physically located just before the ducts branch off to go to different regions of the house, and part of the heat strip isn't working, and the part that isn't working is located in front of the branch that goes to the hall. Or perhaps the heat strips are actually installed after the point where the duct branches, and either there isn't one for the hallway branch, or it isn't working.
In summary, my guess is that the electric heat is only being applied to the air going down some branches of the duct system, either because of the way it was set up or because some portion of the electric heat system has failed.
This could be in addition to a problem with the defrost controls that Dana suspects. It seems like it is entering defrost mode more often than it should, and at higher temperatures than it should.
So just to keep things updated. The airflow was in fact set too high for the amount of heat strips that I have. it was set to 30k when all I have is 15k. Altho the cfm in the manual online doesnt match the cfms on the board in the air handler. In the manual for the air handler it says minimun cfm for 15k is 1305 and on my board under the 15k setting it says 1100cfm. And heres the real kicker----now that Ive reduced the air flow, the air in the cold part of the house is even colder. in defrost now its about 58 degrees, when previously it was low 60's. And I dont know if all the heat strips are working, but I do know that they are installed in the air handler and not in the trunks...
Im attaching a jpg of the layout of the hvac...It might offer some clues, it might not. sorry its so elementary looking. but if the heat strips were located in the trunks like charlie said my problem would make sense, since one of the trunks is cold and one not....but I saw the heat strips in the air handler behind the board i made adjustments on to change the air speed.
any ideas? anybody??
thanks :)
Are the heat strip elements broken?
It still sounds to me like what I wrote before:
"Maybe the heat strips are physically located just before the ducts branch off to go to different regions of the house, and part of the heat strip isn't working, and the part that isn't working is located in front of the branch that goes to the hall"
And it wouldn't have to be "just before", and long as it's close enough that the air doesn't get thoroughly mixed before the two trunk lines branch off.