Posted By engineer on 01/10/2009 11:38 PM
The next 3-6 days look to bring a severe cold snap to most of eastern half of US, and probably most of southeast Canada. In addition, we are presently attempting, here and at geoexchange to assist several folks with some or all of undersized, short looped, or poorly ducted geo systems.

Given that forecast I have a system + loop design question for the pros in heating dominant areas:

Please comment upon, clarify or refute the following two-part design statement:

1) It is acceptable, indeed typical for a system to be sized such that on days at or below winter design temps, and especially during January cold snaps, moderate use of auxiliary, backup, or stage 3 heat is expected.

2) It is NOT acceptable for the loop to be designed or installed such that the system compressor EVER cuts out on low EWT (entering water temperature). In other words, the loop has to support virtually continuous stage two operation over a period potentially lasting several days without EWT dipping below the cutout temperature (15 deg F for Waterfurnace Envision, probably similar for other major brands).

3) As EWT decreases during a cold snap, system capacity decreases at a time when building load is at maximum - it is the role of aux heat to make up the difference for those few days. However, as long as EWT stays above cutout, the compressor keeps running and all three stages contribute to meeting the heating load.

4) If I'm wrong about the above and it is expected for geo system compressors to drop out on low EWT during cold snaps the only alternative I see is for aux heat to be sized and wired to meet the entire house load at temperatures at or somewhat below design temperatures. Is that common practice?

I've posted this at geoexchange as well just to catch a wider audience - don't bother to reply on both forums unless you feel like doing so...