Help me understand proper behavior during defrost cycle for my Trane equipment

[supermaxhd]

I have a Trane xl15i heat pump, an XV80 natural gas furnace and a Trane TCONT803 stat with dehumidify feature. I have my heat pump lockout set at 25.

My natural gas service is currently turned off. I monitor the forecast and when I see it is going to get bitter cold I will call and have it turned on. I didn't have it turned on until mid January last year and then didn't need it that much. I save a little by only having my natural gas turned on 2-3 months a year and my local utility does not charge a connection fee if it has been off for 8+ months.

I've noticed a few times recently where the heat pump fan stops and the heat pump makes a buzzing noise. I assume this is a normal defrost cycle. When this was happening I measured the temperature coming out of my registers at 40 degrees (I have black and decker thermal leak detector). This was happening for several minutes. The heat was set at 70 but had dropped to 66 when this happened over just a few minutes. This was with outside temperature of 27 shown on the stat and normal heat mode (not aux heat). Is this the expected behavior? Should my furnace fan turn off during the defrost cycle to prevent my house from being cooled?

thanks for any help with this...

[jimj]

It sound normal, and no the indoor fan should not shut off. Its taking the heat from the home to defrost the outdoor coil.

[supermaxhd]

that makes sense, thanks for the help

[54regcab]

If the gas was turned on the furnace would kick on during defrost so you wouldn't have 40 degree air.

[supermaxhd]

When it was blowing cold air I was still on heat, not aux heat.

[54regcab]

When it was blowing cold air I was still on heat, not aux heat.

When any heat pump goes into defrost it temporarily goes back to "AC mode". The only difference between defrost and AC is the fan doesn't run outside. To offset the cooling effect of the AC running, the defrost board in the outdoor unit will turn on the AUX heat. If gas isn't turned on your AUX heat won't run, therefore you will have cold air out of the registers.

[supermaxhd]

When any heat pump goes into defrost it temporarily goes back to "AC mode". The only difference between defrost and AC is the fan doesn't run outside. To offset the cooling effect of the AC running, the defrost board in the outdoor unit will turn on the AUX heat. If gas isn't turned on your AUX heat won't run, therefore you will have cold air out of the registers.

Gas heat would have kicked in just for the few minutes for the defrost cycle and then return to heat pump after the defrost cycle even though the outside temperature was not low enough for the heat pump lockout setting to kick in the gas heat?

My house is small and tightly insulated. I am on monthly electric budget plan for $79 right now. I will have my natural gas turned back on after November 26 when it can be turned on without a reconnect charge.

[54regcab]

Have you considered just getting rid of gas altogether and running electric for backup heat? I understand the want to get rid of that gas meter fee, it's $28/mo for us with a 12 month cutoff agreement instead of the 8 you have. Our reconnect charge is the $28/mo for all the months you missed !!!

Yes, your defrost board should kick the furnace on for a few minutes while your heat pump defrosts. Exact sequence of operation depends on how the duel fuel kit was wired.

[motoguy128]

I found with my XV80 and XL16i combo at my last home, that by the time the furnace actually warmed the HE and produced any heat, the defrost cycle was over anyway. Still better than nothing. In a perfect world, there would be a dual fuel defrost mode that had a a time delay once defrost is "requested" by the condenser to give it time to fire up the furnace.

[fliks]

I have my heat strips off. The little time it is in defrost doesn't bother me and saves. Heat strips use alot of watts. I realize they are only on briefly but still a little savings.

[catmanacman]

what is needed is a duct heater used during defrost

[dan sw fl]

what is needed is a duct heater used during defrost

Duct heater = ELECTRIC STRIPS.!

LOCATION?
degreedays dot net will assist in determining
1. how many weeks natural gas would be needed.
2. if last year was atypical or not.

How much gas did you use last year?
Conversion to electric should be investigated.
For example,
100 therms =~ $100. natural gas.

100 Therms
100,000 BTU per hr/ therm
10,000,000 BTU/Hr
3412 BTU per Hr/ kW
2931 kW
$0.09 $ / kw or YOUR Rate
$263.77 electric strip backup
$100.00 N.Gas
$163.77 extra over Natural Gas
or
__50 therms
$ _50 N. Gas
$ 131 electric
$ _81 extra for electric strips over N. Gas

[dan sw fl]

http://www.electromn.com/res/html/em.htm

[54regcab]

Duct heater = ELECTRIC STRIPS.!

LOCATION?
degreedays dot net will assist in determining
1. how many weeks natural gas would be needed.
2. if last year was atypical or not.

How much gas did you use last year?
Conversion to electric should be investigated.
For example,
100 therms =~ $100. natural gas.

100 Therms
100,000 BTU per hr/ therm
10,000,000 BTU/Hr
3412 BTU per Hr/ kW
2931 kW
$0.09 $ / kw or YOUR Rate
$263.77 electric strip backup
$100.00 N.Gas
$163.77 extra over Natural Gas
or
__50 therms
$ _50 N. Gas
$ 131 electric
$ _81 extra for electric strips over N. Gas

Getting rid of the $20/mo average (use your numbers) for the gas meter = $240/yr savings.
Also keep in mind HP and strips can run at same time, giving a slightly higher COP.