Back up heat can't hold temp

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Old 01-30-14, 06:17 AM
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Back up heat can't hold temp

I have a traneXl14i heat pump with trane XV80 furnace for backup. In colder weather 5-17 degrees the backup heat runs constantly and often loses two degrees or more overnight. When temp rebounds to above twenty five or so it catches up and eventually switches between heat pump and backup. Heating about 2900 sq ft What is wrong?
 
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Old 01-30-14, 08:49 AM
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A natural gas XV-80 furnace ?

It's pretty hard to believe a gas fired furnace and heat pump can't keep up.
 
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Old 01-30-14, 10:23 AM
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Sorry if I misstated the facts. It is an XV80 but powered by propane. I measure the outlet temp at the furthest away duct and it is typically between 82 and85 when the furnace is on depending on the outside temp. Never seen 90 degree outlet temp. Heat pump temps about 3-5 degrees cooler. I would have thought the furnace would do better. Could the fan speed be too high? Or low?
 
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Old 01-30-14, 07:40 PM
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Is the gas furnace running by itself when the HP is not running? or is they both running together?

what is is the t-stat control?
 
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Old 01-31-14, 05:32 AM
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The furnace is running by itself. When aux heat comes on the heat pump is shut down outside fan no running. T-stat is a trane(Honeywell) TCONT 800 series. The system uses something called a fossil fuel kit to switch between heat pump and furnace (I think)
 
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Old 01-31-14, 06:36 AM
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I am wondering if 2nd stag is not kicking in. Turn the power off, and remove the faceplate on the t-stat, and tell us what si wired up.

Then go the the furnace and do the same thing.

There's an outdoor sensor tied into the stat? (see outdoors temps on screen)
 
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Old 01-31-14, 09:05 AM
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I can remove the Tstat if you really think I need to. I have verified that when the aux heat is on the furnace burners are lit up in the attic and the heat pump is completely shut off. It may not be staying on or something like that. The heat pump itself is single stage. Thanks for your advice and I will look at the wiring as you suggested, but I have to go to work this afternoon. Maybe something in the wiring because the tstat and wiring were replaced last spring for another problem. Had the techs back out and they said my fan control board was not compatible with the outside temp sensor controlling the furnace turn on point. Details are fuzzy but they reconnected the fossil fuel kit and disabled the outside temp sensor as a control. It still functions as an indicator. What I wanted was for the heat pump to only run as long as it could keep up and then switch to aux when it fell behind and then switch back to heat pump soon as possible It seems to do this to a point but the falling behind during cold weather concerns me. I'm not sure if my system is undersized for single digit weather or there is another problem. Propane is expensive and I'm looking o conserve it. Thank you so much for your help.
 
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Old 01-31-14, 07:47 PM
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That t-stat should be able to control everything for you. A fuel kit is not needed for this...
 
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Old 02-01-14, 05:08 AM
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When the tstat and wiring were changed last year they added an outside temp sensor and it was wired for that. What started happening this winter was that the aux heat would come on but the fan wouldn't run and the furnace would trip out and the house would loose heat until I turned the system off and restarted. It would then work for 2-3 days and then the same problem. Techs said they researched the model trane unit I have and there was a problem with the fan control board working with the stat? Not sure what but they said I needed the fuel kit reinstalled. That did fix the fan problem and the system switches between heat pump and aux heat but it just bothers me that the furnace running all by itself still falls behind when it gets below twenty degrees overnight and it takes a long time to heat the house back up enough for it it switch back to heat pump. I'm worried that either there is something wrong with the furnace or my system is undersized for that low temp....hope I have described this well enough for you to see what's happening, I'm not very proficient with HVAC operation
 
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Old 02-01-14, 10:25 AM
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it just bothers me that the furnace running all by itself still falls behind when it gets below twenty degrees overnight
I was wondering about that too. Most people use electric reheat coils and they rarely can keep up. Possibly the furnace was sized just as a backup to the heatpump. Can you post the actual model number for the furnace. It's inside on one of the sides near the burner. Should be on an ID tag. XV-80 is just a trade name.
 
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Old 02-01-14, 02:35 PM
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Model number for furnace

The model number I found on a tag beside the burner is

TUD100R9V5K4
Serial number 5405JHL1G
Date of manufacture. 10/2005

I also saw a tag that I think says the furnace is eighty percent efficient. It says manufactured by American standard systems Trane residential units. Hope this helps. I am heating about 2800-2900 sq ft with this system and I have 9 foot ceilings with a vaulted ceiling in the den where I keep the fan running on reverse in low speed for circulation.
 
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Old 02-01-14, 05:03 PM
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Yes....that's what the 80 stands for..... 80%.

You seem to have a two stage furnace....... 65K/100K btu's.
That may be your problem. It may be incorrectly wired or set so that the second stage is not active.

The following link is the service specs for your unit.
documents TUD100R9V5K .pdf
 
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Old 02-01-14, 06:55 PM
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Thanks very much for the info and the link to specs. Can you recommend how I might check to see if it's wired correctly or if the second stage is not set to come on? I'm pretty handy with the electrical wiring end but I'm not sure what the correct wiring is. I think this might very well be the problem since the furnace seems to have a problem keeping up in very cold weather. One last question, does the second stage come on automatically after the first stage fires or is it temperature dependent ? Thanks again for all your help.
 
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Old 02-01-14, 06:59 PM
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I can't answer those question..... I'll see if Jay or Houston, the forum pros, can help us out.
 
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Old 02-01-14, 07:25 PM
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Thank you very much, I'll wait to hear from them.
 
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