Temp drop when furnace comes on?

Reply

  #1  
Old 01-14-21, 03:29 PM
K
Member
Thread Starter
Join Date: Jun 2005
Location: USA
Posts: 680
Received 0 Votes on 0 Posts
Temp drop when furnace comes on?

We moved into an existing house at the end of last August. I September I had the HVAC replaced because the original one was 22 yrs old and I thought it was time. I chose a Trane heat pump with propane furnace as backup heat. Here are the model numbers of what was installed.



HP 4TWX6048H1000BB

F S9X2C100U5PSBAA

COIL 4TXCC007DS3HCAA



Once the weather started turning cold I encountered a problem. When the furnace would kick on the temp in the house would drop anywhere from 4-6 degrees in a short period of time, say 15-20 minutes, before it would settle out and start climbing back up. And it never climbs back up as fast as it when down. Can take up to an hour or more to get up to what the thermostat is set at say 70 for example.



I have had the technician out here 4 times now. The first 2 times they thought it may be related to when the HP goes into defrost. But once the problem started being seen even when the HP was locked out due to outdoor temp below setpoint they started looking elsewhere and digging deeper.



Everything they would check indicated that the unit was operating correctly.



Finally the tech looked in the wall behind the return vent. He was amazed as was I how they, the original installer 22 yrs ago, used the wall cavity as the return duct.



The house is a cape cod 1 ½ story house. The furnace and blower are in the attic upstairs. The main return trunk runs from the unit to the top of the wall cavity and then the wall cavity has the return vents in that wall. One 14x14 upstairs and one 14x14 downstairs directly below.



The wall cavity was wide open and once I dropped a remote monitoring temp. sensor in the return I found that the unit was sucking in cold air when it would kick on. I cut into the wall and found that the wall cavity was actually open in one place to the attic. Wow!



So I went to work sealing up anywhere that it could suck cold are from the attic or anywhere else besides the return vents. The size of the wall cavitiy is greatly reduced from what it was too.



With all of this said and done we had hoped that the problem would be fixed. I put the temp sensor back in the return and the temp would not drop when the unit kicked on like it had been.



But….the problem is still there. Not quite as bad but still there. When the furnace kicks on for example when I get up in the morning and bump the temp up a couple of degrees it does the same thing. Drops several degrees before starting to recover.



Now I am looking for ideas as to what it might be or what direction to go next with troubleshooting.



Thanks for any and all help.
 
  #2  
Old 01-14-21, 04:17 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 61,489
Received 1,422 Votes on 1,315 Posts
I don't recognize that furnace model and I can't find anything on it.
Since it's a newer unit.... you'll need to post the model as shown on the ID tag inside near the burner on the side wall.


If your heat pump system is setup correctly...... and you raise the temperature on the thermostat more than 3 degrees..... the heat pump should only run for a very short time, shut down and then switch to the propane furnace. With a heat pump system you should refrain from night setbacks just for this reason.

So based on what you wrote..... is it generally the system where the temps drop or is the propane furnace the issue ? It sounds like you still may have cold air getting into the returns.

You mentioned ductwork issues..... your installers during the furnace setup should have been aware of any air flow problem. An important part of the startup process is measuring air flow thru the system as well as heat rise thru the furnace.
 
  #3  
Old 01-15-21, 06:49 AM
K
Member
Thread Starter
Join Date: Jun 2005
Location: USA
Posts: 680
Received 0 Votes on 0 Posts
Pete,

I will have to get back to you with actual numbers off the label inside the unit but if it helps the HP is a Trane XL16i 4 ton unit and the furnace is a S9X2 model.

I am mostly leaning towards it not being a problem with the HP itself since the symptom happens at times when the HP is locked out due to the outside temp being below 35.

As for still sucking in cold are through the return, I feel that I have ruled that out since closing up the wall cavity leakage. I have put a probe in various places inside the return from the farthest away return vent to inside of the box where the return duct work connects to the unit. I don't see the temp drops like I was when air was being pulled into the return via the attic opening.

Ken
 
  #4  
Old 01-15-21, 10:39 PM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 61,489
Received 1,422 Votes on 1,315 Posts
You have a propane furnace. So the blower moves air heated by the propane burner or the A/C coil. I would expect the furnace to supply a lot more heat than the heat pump. Same return for both.

The blower in furnace mode is supposed to turn on based on a timer setting or actual plenum temperature. The blower should only start when the plenum is heated. If they have the blower controlled directly from the thermostat you may get cold air for a minute or two. That would be a programming change in the thermostat.
 
  #5  
Old 01-16-21, 05:55 AM
Geochurchi's Avatar
Temporarily Suspended
Join Date: Nov 2012
Location: United States
Posts: 5,006
Received 89 Votes on 84 Posts
Hi, how well did the old system work ? What is the temperature difference between the supply air and your return air?
Geo 🇺🇸
 
  #6  
Old 01-19-21, 07:23 AM
K
Member
Thread Starter
Join Date: Jun 2005
Location: USA
Posts: 680
Received 0 Votes on 0 Posts
I don't know how well the old furnace worked. I moved into the house the beginning of Sept. of last year. The system then was old and there was no heatpump. There was a Trane a/c and a propane furnace. I don't know the difference between the supply and return temps. I have measured the temp in the return duct with a time probe that I have that sends the temp to my phone every 5 minutes. Have not tried using it on the supply vents though.

What is the best way to measure the supply and return temps in your opinion?

Thanks
 
  #7  
Old 01-23-21, 06:15 AM
K
Member
Thread Starter
Join Date: Jun 2005
Location: USA
Posts: 680
Received 0 Votes on 0 Posts
Is this a possible cause?

Another idea of where the cold air could be coming from when furnace first comes on. I did an experiment the other morning when I got up before the furnace kicked on and just turned on the fan. When I did that cold air came out of the supply vents.

Could it be that since the supply duct work is in the attic that the air is getting cold when the unit is not running and then when it kicks on all of that cold air is initially pushed into the house causing the temp to initially drop?

If that was possibly the case can I insulate the supply ducts more? What kind of insulation would be best to lay over them?

 
  #8  
Old 01-23-21, 12:58 PM
Geochurchi's Avatar
Temporarily Suspended
Join Date: Nov 2012
Location: United States
Posts: 5,006
Received 89 Votes on 84 Posts
Hi, I would hope that the supply ducts are insulated being in that attic, another thing, does the fan start immediately when there is a call for heat?
Is the furnace that you have? https://www.trane.com/content/dam/Tr...N_11112016.pdf
if so page 98 has the sequence of operation, it states among other things that the indoor blower is delayed on starting by a timer.
Geo 🇺🇸
 
  #9  
Old 01-23-21, 01:43 PM
H
Member
Join Date: Nov 2012
Location: USA
Posts: 2,014
Received 151 Votes on 132 Posts
Originally Posted by kmeyer301
Once the weather started turning cold I encountered a problem. When the furnace would kick on the temp in the house would drop anywhere from 4-6 degrees in a short period of time, say 15-20 minutes, before it would settle out and start climbing back up.
Sounds like you need to check the programming logic on your heat pump.
If I set "heat on for the thermostat of my oil furnace, it runs the furnace to come up to temperature, then starts the fan. However, if I set the thermostat to a specific temperature, the blower will occasionally run to "sample" the ambient air temperature and see if it should run.
In many older homes the cold air returns draw air through the framing, rather than through insulated flexi pipe. It's actually more efficient to feed cool-dense air into a furnace rather than warm-light air, that's why furnaces have heat exchangers.
What you MIGHT do, is see if you can enable a "preheat" setting on the furnace, so that the fans don't come on until the furnace is hot enough that the air through the vents will feel warm.
 
  #10  
Old 02-01-21, 09:17 AM
K
Member
Thread Starter
Join Date: Jun 2005
Location: USA
Posts: 680
Received 0 Votes on 0 Posts
If I wanted to have the duct work checked for air flow and leaks what kind of testing would I request and how should I expect it to be done?
 
  #11  
Old 02-01-21, 10:54 AM
PJmax's Avatar
Group Moderator
Join Date: Oct 2012
Location: Northern NJ - USA
Posts: 61,489
Received 1,422 Votes on 1,315 Posts
There are companies that come out and check the ductwork and can seal it. I've seen online companies that can blow something thru the duct work to seal it. I'm not familiar with that process.

If the ductwork is all metal and is exposed in the attic the best thing to do would be to seal each connection with metal foil tape. Easy to work with and sticks real well. There is fiberglass wrap/insulation that can be put over metal ductwork.
 
  #12  
Old 02-12-21, 07:50 AM
K
Member
Thread Starter
Join Date: Jun 2005
Location: USA
Posts: 680
Received 0 Votes on 0 Posts
Sorry for being away for a bit, had to go out of town. I have contacted the company that did the install and their tech is coming back out next Thursday for the 5th time. First time since I sealed up the return that they have been out. But since problem is still there they are going to replace the thermostat this time. We will see if this helps. Anyway, thanks for all the great feedback and I will let you know if thermostat makes a difference.
 
  #13  
Old 02-12-21, 08:59 AM
K
Member
Thread Starter
Join Date: Jun 2005
Location: USA
Posts: 680
Received 0 Votes on 0 Posts
Here is an example of what it is doing.

I have this info on an excel spreedsheet and tried to copy it to this but isn't working out.
1st number is the time of the morning
2nd number is the indoor temp
3rd number is the thermostat setting
4the number is the outdoor temp.

Sorry it is confusing

4-Feb indoor T setting outside T
6:03 68 68 28
6:07 67 68 28
6:13 66 68 28
6:22 65 68 27 19 minutes to drop 3 degrees 6:31 66 68 27
6:42 66 68 27
6:53 66 68 25
6:56 67 68 25
7:02 68 68 25 40 minutes to raise 3 degrees
7:18 68 68 25 still heating
7:21 68 68 25 still heating
7:25 68 68 25 still heating
7:29 68 68 25 still heating
7:32 68 68 25 still heating
7:36 68 68 25 34 minutes no temp. change
7:38 69 68 25 still heating
7:44 69 68 25 Just now stopped heating (Idle)
8:22 71 68 25 Idle
for
 

Last edited by kmeyer301; 02-12-21 at 09:01 AM. Reason: format
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: