In indiana with an average temperature of 20 when should the aux heat be engaged - I installed a 24,000 waterfurnace with Williams Comfort Air and was told that it would be normal for the aux heat to be used with temperature in the 20's.

It depends on system design and its relation to the heat load of your home. Aux heat supplementing the heat pump when the outside temp is around 20 is certainly not out of the ordinary

Thanks for the reply - I should have said the system was a geothermal horizontal bore loop system - 3.5 ton unit. 

Vertical or Horizontal bore? Your posts are different in different forums?

Issac,
Technically speaking and in an ideal world your aux heat should not be coming on at 20F. Design specs are 5F (outside air) for heating meaning (again in an ideal world) that your GHP should be running non-stop at or near 5F and the aux heat only on below 5F. However, with that said I noticed my system running nearly 100% of the time at 14F when it was first installed and when it got into the single digits the GHP couldn't keep up (Note: I opted not to install the optional electric backup heat at the time of installation). Needless to say the following year I installed an electric backup aux heat strip in the system.

RJDalga
Kalamazoo, MI

Good point RJ. As an additional example, winds at our location today are sustained at 25-35 mph with gusts up to 65 mph. Normally our system does fine keeping up in first stage with the current outdoor temperature. Today, the system needs to run partially in 2nd stage. I imagine that whether it is a cloudy or sunny day would effect aux usage as well (during the day).

Around 22F is right for Indiana and 18F for southern Michigan (both figures from Geolink). What's important is that over an entire average season, roughly 95% of the heat is from geo. Designing for 5F in Kalmazoo is a little oversized (perhaps by 1 ton).

Posted By Isaac on 09 Feb 2010 06:07 PM
In indiana with an average temperature of 20 when should the aux heat be engaged - I installed a 24,000 waterfurnace with Williams Comfort Air and was told that it would be normal for the aux heat to be used with temperature in the 20's.

This is a variable that depends on your area of the country, your design load and the type of system you have.  In some areas of Canada, 20 degrees is a typical day, aux heat kicking on to that temperature would be very undesirable.  My own system, an open loop geothermal system in New Jersey, has no trouble maintaining comfortable temperatures when it's in the teens and I've had the aux heat disabled since a few months after the system was installed.     

Agreed Techgromit! It varies.... I recently discovered wind has a larger impact than I thought. My open loop system doesn't have aux heat installed, but right now with upstairs and only half the basement heated, 1st stage capacity will handle my house down to around 0 deg F on a still night here in west Michigan. However, a few weeks ago it was about 15 deg F and HIGH winds and my unit ran non-stop on first stage for 6 or 7 hours (obviously right on the border of calling for second stage).

Somewhat off subject, but that indicates that outdoor reset would be improved by taking into account wind conditions.

It would, but I'd K.I.S.S. by allowing for an average wind speed rather than trying to dynamically compensate for measured wind velocity

Posted By jokin on 13 Feb 2010 11:52 PM
Agreed Techgromit! It varies.... I recently discovered wind has a larger impact than I thought. My open loop system doesn't have aux heat installed, but right now with upstairs and only half the basement heated, 1st stage capacity will handle my house down to around 0 deg F on a still night here in west Michigan. However, a few weeks ago it was about 15 deg F and HIGH winds and my unit ran non-stop on first stage for 6 or 7 hours (obviously right on the border of calling for second stage).

Interesting, I would wager that your house isn't as well insulated as you think it is.  My first house, was a converted summer house and was very surprised to find the walls had NO insulation when installing in new outlet for my computer.  I could actually feel a breeze inside the house when it was windy outside.  Icynene insulation in the walls and new windows completely resolved the issue, leading to much lower utility bills and better comfort. You might want to get your house pressure tested to determine where the worst of the air leaks are.

I think wind dependency is very location and house dependent - I analyzed 3 years of wind, temperature, and fuel use data for my house nad found no statistical correlation between heat load and wind whatsoever - load was nearly perfectly linear and dependent only on outdoor and indoor temp. My house is probably tight by 1850 standards but average to below average for a modern structure. I think the biggest factor is that I'm in a village and therefore sheltered on 3 sides by other houses nearby. The 4th side is buffered by an unheated garage.

A co-worker who has a similar house out in the country surrounded by open fields reports a very strong relationship between wind and heat load, though his data consists of how many logs he has to put in his wood boiler in a given day.

Was the wind speed measured at the walls of your house or was it a general figure for the area or something from the roof top? Sounds like the first one is what is needed for accuracy.

A sensor to measure heat loss (vs just temperature) could be simple and small.

I'm not familiar with an outdoor reset, but have heard it mentioned periodically in the forum. How would system control be improved using this?

Techgromit - I really hope I don't have anything larger than very small leaks. I am not a professional or very experienced, but maybe an uncertified energy geek. I was extremly careful to caulk, and foam seal every cavity/crack possible and limit an unnecessary framing, wiring, HVAC, or pluming in exterior walls to maximize amount of insulation in the walls. After 3 or 4 cases of foam cans and caulk tubes, my faithful volunteer helpers were nearly mutinious and were baffled and frustrated with my zeal!
I did the best I could with the budget and information I had at the time, and we settled on: "custom seal package" (lots of caulk and foam by myself and faithful volunteers), professional spray foam in box joist perimeter, professional damp spray cellulous in walls, and profesisonal loose cellulous in attic.

Until now I never doubted that the house was relatively leak free and have been happy with the results... 2300 sq ft heated to 68 deg F, with ~25 mbh @ 0 deg F on a still night (first stage on TTV038 on still night). Another indicator that lead me to belief there was little leakage is that we don't have to humidifiy to maintain comfortable humidity levels right around 50% RH (without the benefit of an ERV or energy recovery unit to recapture house exhaust humidity).

I think the wind has a big impact because our house is exposed on a hill with no trees or other houses/buildings nearby and strong winds... but I'd like to rule out leaks.
If there is a leak I would like to know. I guess I would either have to pressure test the house. Or get out the ol college books and see how to model/approximate the wind facing walls to compare the no wind and high wind convective heat transfer and see if it correlates to the change in my heating load as indicated by how much my unit runs to maintain same thermostat setting. Any other simple/cheap ideas for test / indicators of leakage or lack of leakage ??

How does a house pressure test work (is that a blower door test )? Does the test help find/pinpoint the leak if there is one? And of course I'd be curious how much these things would cost ??

Let's call it "temperature reset", because some devices that accomplish about the same thing don't need to measure outdoor temperature.

The idea is that by knowing the approximate heat load, the heating system can select the mode (eg, first vs second stage or low output rates vs high output rates) used. Ie, if it is warm out, then we know little heat will be needed and the system can work at a more efficient low output. If the heat load is high, then it needs to go to higher output modes to keep up.

Some geo systems have two stages and modulating boilers or variable speed heat pumps have variable stages.   On the other hand, some systems have only one mode and temperature reset can't help them.

I would think that if you purposely pressurized a house, took IR pictures from the inside, then created a negative pressure (causing infiltration) and took more pictures, the leaks would be obvious by looking at the differences in the pictures.

Posted By jokin on 15 Feb 2010 10:10 AM

How does a house pressure test work (is that a blower door test )? Does the test help find/pinpoint the leak if there is one? And of course I'd be curious how much these things would cost ??

Yes, it's a blower door test.  Basically your raising the pressure inside the house and use smoke to see were it gets sucked out.  I thought my house was pretty tight too, until they did a blower test.  You can improvise a test by installing a fan in a window and using a cigarette for smoke.  Infrared thermal imaging would work as well, but I understand the equipment to perform this is very expensive.