I'm looking for input from others with air source heat pumps (ASHP). I've got a steel SIP house and this is the first autumn we have experienced in it. The steel SIP construction makes it very efficient, but normally we have a cooling dominated climate here in Louisiana. Until now I have not had any need for heat yet. The outside temps have gotten into the 40's F overnight but the inside temp normally doesn't drop below 70 or 69F what with the heat from lights, appliance motors, and human activity.

But last night the overnight low was something like 29 or 30 F so the indoor temp had dropped to 65 when we woke up. I decided to turn on the heat to knock off the chill. But the auxiliary resistance heat kicked in when I turned the system on. I suppose 30 F is pretty cold for an ASHP.

What has been your experience? Does your ASHP only need aux heat until the coils have defrosted? Or does it never work when temps are that low?

I think it is ironic that I installed an ASHP, but in order for it to feel cold enough to turn it on, it has to have gotten too cold for it to function properly. With such an efficient house, I wonder if maybe I shouldn't have bothered with the ASHP in the first place?

Not all ASHPs are the same.

I am installing one right now that will provide efficient heat down to 0F or so.

Was your home or home design analyzed to determine the heating needs before the heating plant was selected?

ICFHybrid,

I just sent you a PM so that I would not hijack this thread.

Alton, I think you hit the nail on the head. After playing with the thermostat I see what you mean. The aux heat only comes on when the difference in temp is greater than a few degrees. Thanks!

ICFHybrid, yes the home was analyzed, but the focus in my climate is on cooling. I don't expect to use the heat much more than a few days a year, and then probably only in the mornings. By comparison my neighbors and family members with conventional construction have been using their heat for about a month already.

Jelly, as Alton hinted at, the use of auxiliary heat is determined by the thermostat. The heat pump has no way to determine otherwise that aux is needed. Normally, the system would be left "on," and the thermostat sends a signal to the heat pump when the temperature is a certain amount below the set point. If the heat pump is staged, then a signal for second stage would be sent either when the temperature has dropped more than some other increment below set point or when the temperature has not been recovering fast enough or for too long a time, indicating that first stage can't keep up with the heat loss. Similarly, electric auxiliary heat is activated either by temperature slipping even further from set point or by second stage (if present) having been on too long without sufficient result. The logic followed may be selectable, as it is on mine, and the temperature differences that trigger staging also may be adjustable if that mode is selected.

As suggested, you may be able to turn off a breaker that feeds the electric auxiliary coils. It would at least be useful to know what thermostat you have and if its operation can be adjusted to give you the control you want. The worst case would be a thermostat replacement to provide the control options you want. I presume what you want is to give the heat pump enough time to do its job in recovering temperature without some other setting of the thermostat deciding it isn't happening fast enough.

Edit: I see you've replied already to Alton's post. You're on the right track.

ICFHybrid, yes the home was analyzed, but the focus in my climate is on cooling

So, the heat pump does fine on your cooling. I assume you have ducting and everything for that. What did the HVAC system cost? Did they consider a ductless mini-split at all when they did it?

Yes, the heat pump does just fine for cooling. This is the perfect climate for an ASHP. It's a central type system, one 3.5 ton unit and one air handler servicing 4000 sq ft of conditioned space (including non-vented conditioned attic) with flex duct and one return (actually two return grilles right next to each other to reduce noise - it works). I don't think the size and layout of my house is conducive to a ductless mini-split system.

The funny part - you wouldn't believe how many people tried to talk me out of an ASHP. The local perception is that they "don't work." I think the concept of cooling a house with something that has the word "heat" in its name is too far fetched for some folks! Also I had to get bids from 5 different HVAC contractors to find one who would bother to do a Manual J calc. The other guys wanted to "rule of thumb" it and told me I needed 8 to 9 tons.

But I finally found the right guys and they did a great job - gotta do the homework...

Most R410A refrigerant heat pumps (ductless or ducted) work just fine even at 20F, but some crap out at +10F, while some of the better mini-splits have operating specs as low as -13F.

Ducted systems are usually lower efficiency than mini-splits due to higher air-handler head, and most do not have the continuously variable speed air handlers and compressors that most mini-splits have. But any decent new single-speed or 2-stage R410A heat pump should be giving you a COP of a 2.0 or more in heating mode @ 20F. (Most mini-splits are better than 2.5 at that temp. Higher-efficiency mini-splits achieve defrost by pulling heat out of the room (at lowest possible speed on the interior unit) rather than relying on resistance heat, which is more efficient, with fewer parts to fail like the defrost elements & controls.) Using a setback strategy is almost always less efficiency with air source heat pumps since it forces either the aux heat to kick on, or less efficient higher compressor speed operation during the recovery periods.

At higher temps, say 40F+, ducted systems will hit COPs of 3 or so but mini-split efficiencies start to soar at that point, when they're running at 1/3 of their max compressor speeds or lower. At 45F even at mid-speed some are already north of 4.5. But it's not usually a solution for a 4000' house with lots of doored-off rooms. (Some 3-4 ton multisplits can handle up to 8 interior heads, but it's kinda silly at that point.)

If you turn off the aux heat coils you'll find out pretty quickly if they're necessary for a defrost cycle, eh? ;-)

Posted By Jelly on 11 Nov 2011 11:48 AM
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The funny part - you wouldn't believe how many people tried to talk me out of an ASHP. The local perception is that they "don't work."

I don't know why people say such things about "heat pumps" when they don't know what they are talking about. What do they imagine an air conditioner or refrigerator is, anyway?

ASHP sounds like a great fit for your home, but persue your study of how to optimize its performance. You should also consider a strategy that monitors outdoor temp to avoid activation of auxilliary above X degrees or a stat that permits X minutes for heat pump to catch up without aux. support.

What thermostat do you have? Most will allow you to set when your auxilary heat comes on. Many have a standard of turning on Auxilary heat anytime there is more than a 3 degree difference between inside temp and setpoint. You should be able to change this back to 5 or 6 degrees and program your thermostat to never let it drop below the temperature when the aux heat comes on. If your HVAC contractor knows his stuff he should easily be able to set this up for you with the explanation you have given us.

It's a Honeywell progammable thermostat, and no I don't have confidence that the HVAC tech would be able to set that up. I will look into it myself though.

I am curious what your heat load calc came out to be at a outside design temperature of 29 F.The problem you may be having is with a reduced COP the temperature difference across the indoor coil iis now at a temperature which will feel cool blowing across the the human skin.Add in long duct runs and by the time the air leaves the register , the temperature is well below 90F. Your heat pump may still be operating and adding heat into the structure but it is doing so at a uncomfortable temperature because of natural drafts created within your home.