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New HERS rated home: Issues with Trane XL HVAC heat pump constantly calling for heat

Wayne Nicholson | Posted in Mechanicals on

Hello guys. Looking for some advice on our new home . Small custom home completed earlier this year. 2 x 6 walls. Single-story. Designed for energy efficiency from the get-go …

1792 square footage living space. 9 foot ceilings. 2 x 6 exterior walls framed with California corners and insulated headers and insulated wall leads.

Complete spray foam encapsulation. Spray foam walls. Spray foam roof deck, and crawlspace is insulated with spray foam. Crawlspace is insulated to an R-15, walls R-22, and roofline is considered R-38.

We participated in the local electric company’s efficiency program and the shell was tested during construction … And has a HERS Index of 45 with a five star plus confirmed .

Blower door test shows 317 CFM air leakage, so extremely tight ….,

We have the GE Geo spring hybrid water heater that stays in the heat pump mode.. We have a Trane XL system which is advertised it as an 18 SEER but in our configuration actually shows as a 17 sear on HERS paperwork. We have the Trane 824 communicating thermostat with a clean effects air filter. We have a Honeywell mechanical air ventilation system that also runs through the Trane thermostat. It is set for eight minutes of ventilation each hour.

We did a Manual J load calculation for the system from the beginning. Of course all ductwork is sealed also even though it is in conditioned space.

It is a 2 ton unit that is single compressor to stage. The first stage accounts for 1.25×1, second stage hosannas are full two time. It is 8 to 10 unit that is single compressor to stage. The first stage accounts for 1.25 tons when second stage in engaged it is the full two ton.

OK, so here’s the problem: Our heat pump is calling for heat sometimes 30 or 40 times a day ! Runtimes are varied from 5 minutes to 16 minutes. I’ve been timing it over the last week and it averages out to be about a nine minute run time, then it makes like a lot for five or six minutes and then calls for heat again.

The funny thing is that thermostat shows a system report which tells us outdoor temperature via remote sensor, outdoor humidity, indoor temperature, to two decimal places, indoor humidity fan percentage, it what it called it’s a load index …

For example I can stand in front of thermostat and watch it. The temperature will not vary… For example if our set point is 72° the system may show 72.06 you will call for heat pump stage one run for three minutes at fan 36 percentage ramp up to fan speed of 50 or 56% for few minutes and then finally it seems after about the nine minute mark it will rip up to 70% fan speed which should match the capacity of first stage … At the end of that cycle it will show indoor temperature at 72.46 for example and shut off so my question is since we were over the target temperature in the first place why did it ever go through that cycle it will turn on with the load index of 28 or 29 one time the next time it might not turn on with a load index of 60 …. So yes I am maintaining indoor temperatures to within a quarter of a degree At all times… The problem is to me this seems unrealistic and totally inefficient which is also shown in her energy usage reported by her local electric company . For the month we are averaging 46KW per day. We have tried many different parameters in the programming of the thermostat but we don’t understand why if you’re over the target temperature it will still call for heat and run for five minutes to bring the temperature up to tenths of a degree and then shut back off the system is running in the early stages where it’s not very efficient….. We have even gotten the representative out to look at things and he is scratching his head also …., also last week I totally turn the system off at 8 AM no indoor heat sources such as an oven going hairdryers clothes dryers or anything else is on original set .72° in six hours time we lost 1.5° temperature when I turn the system back on we rose back up that 1.5° in about 30 minutes runtime With no auxiliary heat used . So I know it’s not the efficiency of the shell we are losing less than a half a degree per hour…. I’m grabbing at straws’s hoping someone has went through something similar to give us a direction thank you for your time

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Replies

  1. Stephen Sheehy | | #1

    Wayne- Where is the house? When the manual J was done, what was the design temp? What was the load calculation at design temp?

  2. Expert Member
    Dana Dorsett | | #2

    When all else fails, read the manual! :-)

    According to the manual you can program the minimum run times or maximum cycles per hour (CPH) in both cooling & heating modes anywhere from 2 cycles to 6 cycles per hour. If the thing is short-cycling itself into low efficiency set it to 2 CPH. (The factory default is 5 CPH.) With fewer cycles you'll have higher temperature overshoot, but so what?

    See section 6.4, p.30:

    http://www.bayareaservice.com/store/XL824_Install.pdf

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