Improving Thermal performance 2-ton Geo
Last Post 23 Mar 2015 03:04 AM by RobertPat. 7 Replies.
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sbeausolUser is Offline
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19 Feb 2015 11:47 AM
This is cross posted as I realized I put it in the wrong forum...

I live in a 2,000 sqft ranch where I've been slowly improving my house's thermal performance through air sealing and insulation, along with a recent 2-ton geo installation. My attic is around R-60, walls are R-13. I have around 200 sqft of glass which are 1970s double pane casements. This is winter #3 in the house, that last 2 of which were with the new Geo system installed. Of course, both winters seem to be colder than average but overall the system is doing well. I'm realizing I'm not quite there for a true 2-ton system, as I have leakier than expected exterior doors and windows. I also haven't completed my insulation work in the basement which is what prompts this question (more below). I have been logging the electricity usage and some EWT and LWT data, which is why I feel I'm not quite at the 2-ton load yet. I've found that when temps are in the teens, the system needs to run 24/7 to keep the house at 68F. The design temp here is 5F. The EWT is in the low 30s so the system can't quite hit two tons under those conditions (Climatemaster) which probably explains the long run times. I have my aux heat lockout at 12F so backup can kick in when it's cold. This winter (starting Dec 20) I've used 1691 kWh on the compressor and 211 kWh of aux heat against 2,586 heating degree days This leads me to my question...

As it stands now I have 2" of foil faced polyiso glued to my foundation walls, for which I plan to add another inch. The basement is running 57F today, and probably won't get any lower. A couple of mistakes that remain un-addressed are 1) no sealing around the edges with spray foam, and 2) the seams aren't taped. I will do that once the third layer gets up. I've read a bit about the low vapor permeability (or should I say non-existent) on foil faced polyiso, so I'm curios if I should be worried at all about those walls not drying to the interior? About half of my foundation perimeter has a foot to 4' above grade, the other half has less than a foot. My band joist is insulated with 2" of the same polyiso with foam around the edges. I realize I should probably add another 1" to that as well. In my load calculations, taking the foundation walls to R-19 had a big effect on the load, so I'm hopeful completing that will take the house closer to 2-ton. I plan to replace doors and windows this summer, along with adding at least 2" of Roxul comfortboard to the exterior so that should help as well. Any input is appreciated!
joe.amiUser is Offline
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20 Feb 2015 07:21 AM
Dana is the best to answer insulation questions but as to the geo:

Time and again we get folks who are concerned that their geo is running too much or "struggling". If your design temp is 5 then your geo should run all the time when you are in the teens.
In mid MI some of my systems start running most of the hour in the high 20's.

Here's why (and by the way this is true of multi stage and variable fossil systems as well)- A typical multistage thermostat would be set up to activate first stage when you are 1/2 degree below set point, second stage at a full degree below set point and auxiliary at set point -2. A good thermostat will also reverse stage so as auxiliary brings you up 1F the aux shuts off. Then if second stage brings you up another 1F second stage shuts off and just 1st stage runs.

Since your thermostat doesn't show fractional degrees, first stage will run from (assuming 70F set point) 69.1 to 69.9 before shutting off. It will also display 69 the entire time suggesting to those not in the know that it is "struggling". If it can't get it back to 70 it won't shut off but if it keeps it above 69 second stage won't come on. This is very efficient and comfortable heating.
That it is admitedly colder than usual (as evidenced by 40 or so hours on an auxiliary that is not permitted to come on above 12F) and at 15 cents a kwh you have used less than $300 in electricity since DEC 20, why in the world would you think something was wrong?
Joe Hardin
www.amicontracting.com
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www.doityourselfgeothermal.com
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sbeausolUser is Offline
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20 Feb 2015 09:56 AM
Joe, thanks for the reply. I don't feel like the system is running poorly, but I think there is still some room for improvements in the house to improve performance. When I did my load calcs, R-19 on the foundation was important to get me under two tons so I think getting there should help me improve performance even further.

You bring up an interesting point about the thermostat however. Maybe this requires a different thread, but here goes. In my system I have an arzel heat pump pro handling staging decisions. What I've found is that it doesn't seem very smart. As I understand it, it makes decisions based on leaving air temp (LAT) and outdoor air temp (OAT). If LAT relative to OAT is below a threshold it stages up. I still haven't quite found the perfect sweet spot for this setting so I find my system will always go to stage two when it is below 30F outside, and if it is below 12F (my aux lockout temp) it will always go to aux heat. The reason I think it isn't very smart is because I get a lot of solar heat gain during daylight hours, and the house warms up reasonably well, however if it is below 12F outside, aux will come on when it probably doesn't need it. Similarly if it is above 12F, but below 30F I'm guaranteed to run stage 2. A similar scenario comes into play when I'm running my wood stove. Ideally it would make decisions as you described above using the indoor temp to decide on staging.

Does anyone have any experience with the heat pump pro from arzel? My thermostat is set to 68F and never budges from there, but when calling for heat, it stages pretty quickly. It seems to me that it is sensible to stage based on OAT, but not considering inside temp seems like a problem...
jokinUser is Offline
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20 Feb 2015 12:17 PM
If you have a ground source/geothermal heat pump, I'm not sure how the outdoor temperature should be factored into anything the thermostat does...... unless it is trying to do "smart setback and recovery" (optimizing energy usage by balancing energy savings from setback at night against the cost of recovery in the morning).
To me it really, sounds like the thermostat you have might be setup for an air source heat pump that would likely not be cost effective below a certain OAT (for some types of air source heat pump that might be around 12°F). If that is correct i would get this reconfigured for geothermal or get a new thermostat.

joe.amiUser is Offline
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21 Feb 2015 07:52 AM
I'm not familiar with the stat but does it have an "economy" vs "comfort" setting? try that if it does and it will stage more slowly. Few tstats are perfect though we are talking to a hardware builder right now to see if we can put something together. No doubt I can build my dream stat, but I don't know what it will cost yet.
Joe Hardin
www.amicontracting.com
We Dig Comfort!
www.doityourselfgeothermal.com
Dig Your Own Comfort!
ChrisJUser is Offline
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21 Feb 2015 02:01 PM
I read some of the tstat manual.

Stages based on LAT and outdoor air temp. Default LAT is 100*F, For every degree below 30* the tstat raises what the LAT should be by .5*, for example: 20*outside the tstat wants to have 105* LAT and stages up to sense that high temp.

My GSHP never has a LAT even close to 100*, unless my aux is running. Hopefully the a lower number is programmed in.

ChrisJ
sbeausolUser is Offline
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21 Feb 2015 02:28 PM
ChrisJ nailed it. I have the setting somewhere around 85, which basically forces me to stage up no matter the indoor air temperatures. It seems counter intuitive to me...
RobertPatUser is Offline
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23 Mar 2015 03:04 AM
According to the comments and the discussion above gone, I don't think that machine needs any improvement it is just good.But still nothing is perfect or fills the total requirements.The Geo Heat pump is all around good,people should consult it as per their requirements.
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