My new heat pump

[indebt]

I have a question about my new heat pump. When the temp outside is only 18 deg should the outside unit be doing anything or not ? It was cold for two days well below 30 deg and both the outside unit and the electric furnace was on. As I understand when it is this cold outside , the outside unit should not run at all am i correct? /forums/images/graemlins/confused.gif

 

[techman]

Your heat pump will deliver heat more efficiently than straight electric resistance heat when the outside temp is in the low single digits. Most units will deliver 2 watts of heat per watt of electricity when the outside temp is in the low teens.

The problem with heat pumps is that the capacity, or number of BTUs that it can deliver, drops as the outside temperature drops. So a 36,000 BTU (3 ton) heat pump may be able to deliver 36,000 BTUs at 45 deg F, but at 15 deg F it will only deliver 9,000 or 10,000 BTU's. Unfortunately, as it gets colder out, the heat pump's capacity drops, and the heat loss from the structure INCREASES. Where the heat pump output matches the building's heat loss is called the balance point (temperature).

With a heat pump the electric auxiliary heat, the heat pump and the electric heat can operate at the same time. This allows the heat pump to provide some of the heat efficiently and the additional heat needed is by the not so efficient electric strip heaters.

So at cold temperatures the outside unit should be running, as it will be adding efficient heat required for the house. There is a point where the capacity of heat produced is so low that it is not worth running the compressor, but this usually occurs around zero or below. So don't be concerned if the outside unit runs continuously at clod temperatures. It was designed to work under those conditions.

paul

 

[farmerpsv]

Hi techman Paul,
You gave me some great info/advice awhile back when i was looking at geothermal. decided on air to air as i couldn't afford or justify the more than double system cost. went with 2 zone air to air 14 seer w/ two speed air handlers. i am surprised that the auxillary electric strip doesn't kick in very often, even below 20 degrees, though the air feels pretty cool, it still maintains a warmer temp inside. the blower blows so hard that is rattles the heat registers, pretty loud altogether, is this necessary? (ie, the high fan speed). i assume the fan speed can be adjusted down, but would i lose efficiency doing that?
thanks
paul too

 

[techman]

The fan could be tuned down somewhat. As far as the operation goes, with a modern 14 SEER HP it is not surprising that you are not seeing much aux heat. Depends of course on the sizing vs. heat loss. The air feels cool since the air temp at the register may only be 80 or 85 degrees, which "feels" cool. The 2 speed fan will kick into high gear before any aux heat comes on. Usually there is a speed adjust, with a jumper or DIP switch, to set the fan for each range. A higher air flow is usually used with heat pumps to better distribute the not so warm air. Try lowering it a step and see if the house is as comfortable. You should not lose efficiency, but may end up with some cool spots in a room.

As for the geo system, I finished my installation in early November. I put a lot of time into it, and as usual things did not go as planned. I did the ground work in mid September, planning for it to be cooler out, but we had a hot stretch in the 90's That was a challange. Then I did my swap out of my seconf floor unit in November, so it would not be hot when I was in the attic. Sure enough another hot spell in the mid 80's The attic easily reached the high 90's as I worked on my hands and knees. Again I suffered. My new air handler for the attic was the same model as the original, but 19 years newer. Sure enough all the dimensions had changed, so a lot of duct adapters had to be fabbed.

In the end the system is running very well. Register temps are around 95 degrees, so if feels a lot better. Also on the 15 degree nights, instead of the all night non-stop runs of the old air heat pumps, I now get 20 minute runs.

Best of all is the fact that the performance stays up there even when it is 10 degrees out. My amprobe reads about 7 amps per unit, or about 1700 watts. About the same as a window A/C unit.

It was somewhat costly and a lot of time and effort, but well worth it.

paul

 

[indebt]

Thanks for the info that all sounds good . I was just under the impression that the outside unit had its own thermostate that would shut down the heat pump at low temps. /forums/images/graemlins/tongue.gif

 

[jwstewar]

I don't want to step on Paul's toes, and I admit I don't know your system setup. The way our system is setup is we have an outside heat pump and an interior propane furnace. Below 30 or more than a 3 degree variance and it will kick the furnace on - the outside unit won't be doing anything. Right now our HP has big icicles hanging off of it as we had freezing rain Saturday night and it hasn't run since early Saturday morning. BTW, we do not have any of the resistive heat strips for "assistance."

 

[kenmac]

Sounds like your unit is operating like it suppose to.

 

[geneP]

Techman, thanks for reaffirming what my installer told me...He said that the colder the outside temps, the less amps the heat pump draws. Even if the outside unit is running while the auxillary strips are on, the amperage draw is minimal and it's adding heat to the mix. Today the outside temps here were in the mid to upper 30's and the air from the rigisters felt warm. Already tonight it's down to 31 and the register air feels cool. My outside thermostat is set to kick in the electric strips at about 30 degrees.

 

[Bird]

Our "official" temperature yesterday morning was 14 although the thermometer in my breezeway showed 20. When I got up yesterday morning the heat pump was running, the temperature showed to be 4 degrees below what the thermostat was set for and this thing ran continuously until about 1 p.m. This morning the official temperature is 9 and my outside thermometer shows 19, the thermostat shows the temperature inside to be 5 degrees below the setting, and I guess it'll be running a long time today again. /forums/images/graemlins/frown.gif

 

[slapshot]

You must have been reading my mind to post this..
We too have just built a home (1st winter in) with 2 - 14 SEER Heat Pumps. It was down to 3 degree last night, that sun of a gun ran all night. But it kept up and the house was comfortable. I had been wondering myself if the HP portion would kick-off and run on the AUX heat.
Sounds like everything doing as its supposed to.

Thanks for the post,

-dave