Need some advice about dual fuel system.

[acobra289]

Hello all.

I had a dual fuel system installed a few months ago. A Trane XR-14 heat pump with a XT-90 furnace. I had propane when the system was installed in the summer. I recently converted to NG and last night was the first night we had used the system for heat. The air coming out of the vents seemed VERY warm for a heat pump so I went to look at the Trane 803 t-stat and I noticed it said AUX HEAT (or maybe just AUX, I can’t remember for sure.) Anyways, it was only about 40 degrees outside last night, so I can’t imagine why the backup heat would be used?

Obviously I am going to call the install company, but being I work all day, it’s going to be hard for me to schedule a time for them to come check it out. So, before I call them out. How can I be sure the backup heat is actually coming on? Is that what the AUX meant when it was displayed on the T-stat? I checked the switchover temp setting, and it is set to 35. Any other settings I should check? Any reason the backup heat would be coming on when the outdoor temp was above the switchover point? Any guidance would be appreciated.

Thanks

[Dent]

I am in pretty much the same situation. I have a Lennox XP19 and a NG backup. I am seeing the furnace fire up at 10C outside temp (50F). The furnace should only come on if the outside temp is below the balance point, you have a lock-out temp set or if the heat pump can't heat the house up to the set temp quick enough.

The AUX definitely means that you are running on NG. If you have a HE furnace, you can probably see steam coming from your exhaust vent. Tying a thin piece of plastic "string" to the grate on your Heat Pump will also give you a good indication of whether it is running or not.

[speedymonk]

My installer told me that 40 degrees is the factory default (at least for Trane) when aux heat comes on. That's anyplace in the country mind you. I called BS on that and set mine to 25 degrees. I have the second lowest bills in the subdivision (64 homes) except for one fellow who has radiant floor heat.

Setting your balance point will depend on your NG and electrical rates and your equipment. Figures for mine show a break even point at 17 degrees. Setting mine at 25 worked just fine last year. I'm going to experiment a little with 20 degrees and see what hapens.

[BaldLoonie]

There's a 5° deadband. If you are set to 35° and it shuts the pump down, it won't bring the pump back on until 40° to avoid short cycling.

Unless you plot a graph of heat loss vs pump output, only thing you can do is experiment. The more oversized the heat pump is, the colder you can go.

[acobra289]

There's a 5° deadband. If you are set to 35° and it shuts the pump down, it won't bring the pump back on until 40° to avoid short cycling.

Unless you plot a graph of heat loss vs pump output, only thing you can do is experiment. The more oversized the heat pump is, the colder you can go.

I think I understand what you mean, but it never got near 35 last night. I'm pretty sure 40 or 41 was the low. Seems to me the furnace should have never kicked on in the first place.

Just as a test I went and raised the target temp so the system would come on and it appears just the HP came on this time. Of course it's only 46 outside right now.

Sooooo does that mean if I want my backup heat to come on at 35 I need to set the switchover temp to 30? Doesn't seem right to me.

[ebierley]

Do you have the outdoor sensor hooked up to your T-stat? If yes you should know what the outdoor temp is. If it got to 40 and the lock-out is set at 40 then the furnace would always come on instead of the HP.

[acobra289]

Do you have the outdoor sensor hooked up to your T-stat? If yes you should know what the outdoor temp is. If it got to 40 and the lock-out is set at 40 then the furnace would always come on instead of the HP.

Yes, the outdoor temp sensor is hooked to the t-stat and it shows. I went through the setup numbers and everything looks ok as far as I can tell.

Setup# 170 set to 7 (2 heat/1 cool)
200 set to 1 fossil fuel backup
210 set to 0 no fossil fuel kit. comfort control controls backup fossil fuel heat with outdoor sensor.
350 set to 35 degrees balance point

According to the manual Setup# 360 is the switch for the HP compressor lockout, but that option does not come up for me. I assume because of how some of the other switches are set up. So if the option doesn't come up, does that mean I don't have a compressor lockout temp?

I guess I will just keep an eye on it and see what it does. If the furnace continues to come on at 40 degrees I will lower the balance point temp to 30. :confused:

Thanks for everyones help.

[Focko]

When switching from propane to natural gas, the gas furnace was fitted with the appropriate kit, right? It's really important that the furnace was converted to NG use. Perhaps that's obvious.

Gas heating is hotter coming out of the supply vents than a heat pump puts out, whether the gas is LP or NG. In some areas it's a tossup whether NG heat or heat pump heat is more efficient money-wise (and at what temp the system calls for aux. heat). Often the outdoor t-stat puts it into aux heat at about 30 for gas, or 20 when the aux is electrical strip heat. That's mostly a question of economics and what you are willing to pay for hot air vs lukewarm air.

In most places, propane heat is really expensive compared to NG, so propane is used more sparingly. Forgive me if I said stuff that's obvious. There's a lot of difference in cost of different energy sources depending on where you are.

[acobra289]

This morning when I woke up, the outside temp was 38 and the heat was operating in HP mode. I still have no idea why the furnace was on the other day when it was 40 or 41 degrees outside. Who knows, maybe it had something to do with the testing the install company did when starting it up for the first time with NG. (It had never been started with propane either.)

The kit was supposedly installed to switch it from propane to NG. The packet it was in doesn't have much in it now, so I assumed they used it.

I will just keep an eye on it and hopefully I won't have any further issues with the furnace coming on to soon.

Thanks again for all the advice and information.

[woody19]

This morning when I woke up, the outside temp was 38 and the heat was operating in HP mode. I still have no idea why the furnace was on the other day when it was 40 or 41 degrees outside. Who knows, maybe it had something to do with the testing the install company did when starting it up for the first time with NG. (It had never been started with propane either.)

The kit was supposedly installed to switch it from propane to NG. The packet it was in doesn't have much in it now, so I assumed they used it.

I will just keep an eye on it and hopefully I won't have any further issues with the furnace coming on to soon.

Thanks again for all the advice and information.


Are you sure you it wasn't just in defrost? If the temp is low enough and the moisture in the air is high enough it will frost the outdoor coil and then go into defrost, which would bring on the furnace for a max. of 10 minutes. Just a thought.

[cmajerus]

Are you sure you it wasn't just in defrost? If the temp is low enough and the moisture in the air is high enough it will frost the outdoor coil and then go into defrost, which would bring on the furnace for a max. of 10 minutes. Just a thought.
I would have to agree with this scenario. When the defrost kicks in the aux will be displayed. If it happens again watch the condenser, it should be running(you most likely will see steam rising off of it with the condenser fan off, then when the condenser temp is high enough the fan kicks on and will have a big plume of steam pushed out.( Maybe not so much at 40, but will be there for sure at 25)


Also Trane has no "standard" switchover temp, if you want to run your HP at -10they don't care, you won't get much heat from it, but they will operate as low as you want to go, you just find your balance point of where it is cost effective to operate, most of ours run to 25 then aux kicks on.

[acobra289]

Are you sure you it wasn't just in defrost? If the temp is low enough and the moisture in the air is high enough it will frost the outdoor coil and then go into defrost, which would bring on the furnace for a max. of 10 minutes. Just a thought.

I guess it's possible. I have no idea of the conditions required to frost the coil.
Seems odd to me that it would freeze up at 40 degrees. Then again, I don't know much about heat pumps.

[beenthere]

I guess it's possible. I have no idea of the conditions required to frost the coil.
Seems odd to me that it would freeze up at 40 degrees. Then again, I don't know much about heat pumps.
They can frost at 40. Depends on your outdoor humidity.

[Focko]

A heat pump in heating mode makes the outdoor unit like a fridge or freezer. It gets a lot colder than ambient because of how it works. If there's enough heat outside to warm the outdoor coil and keep it from freezing, good enough. But they will easily get frost at 40 degrees depending on the humidity, as beenthere said.

[acobra289]

Thanks for the help guys.

I have not noticed any issues since then. And all the settings and everything appear correct, so the defrost seems a likely option. It's good to know I don't have anything funky going on.

On another note, I'm reasonably impressed by the warmth of the air from the HP. Granted, it's not very cold outside, but it seems to be doing a good job. I think I am going to be very happy with this system. I just wish I had bought it 5 years ago instead of waiting. I could have been saving a ton of money on my utility bills.

Thanks again. This forum has been very helpful through the whole process of getting the new system.

[beenthere]

Setup# 170 set to 7 (2 heat/1 cool)
200 set to 1 fossil fuel backup
210 set to 0 no fossil fuel kit. comfort control controls backup fossil fuel heat with outdoor sensor.
350 set to 35 degrees balance point

According to the manual Setup# 360 is the switch for the HP compressor lockout, but that option does not come up for me. I assume because of how some of the other switches are set up. So if the option doesn't come up, does that mean I don't have a compressor lockout temp?


Thanks for everyones help.

The 8000 series doesn't have a settable droop for dual fuel systems like the IAQ does.
Thats why you don't have 360 displayed as an option.

350 is your lock out temp for the HP.