GT Heating--Do I need to insist on a schedule J Calc.?

vespadaddyMarch 10, 2009

I talked to a GT heating sales guy today, and when I spoke with him, he said he could size a unit for my home with just some climate data (which he has), and the natural gas therm usage for the past year.

Does this sound right? Is this typical? Is this the typical, ballpark way to size a unit for a bid, and if so, should I expect and demand that a schedule J be completed before I sign off on the bid, as a part of the system design?


Dan in Mpls

Thank you for reporting this comment. Undo

Manual J. Yes.

    Bookmark   March 10, 2009 at 9:50PM
Thank you for reporting this comment. Undo

Yes meaning I should insist on it?

Thanks for the reply,


    Bookmark   March 10, 2009 at 9:58PM
Thank you for reporting this comment. Undo

He can size it for the bid using any method he wants (though you could be unpleasantly surprised later) since you're not paying him for the bid, but you should insist on a Manual J calculation as part of the contract. Just put in the right to back out if final cost is more than X% of the estimate so he doesn't low ball the bid to get the job. Get at least 3 estimates if you can find 3 contractors in your area who know (at least somewhat - some may be better than others) geo. Get lots of references - try nearby town building depts, they may be able to tell you who has geo. Most owners are thrilled to share their experiences and prob. wouldn't mind you contacting them (though I don't know if building dept would give that info to you). Tax records often don't show GT b/c they are so new still - our tax record shows electric heat and no AC - we haven't tried to correct it. That's another thing - here in CT, GT systems are exempt from property taxes.

We got 3 "estimates" or I should say bids on size of our unit based on house plans alone (new construction) but we did provide details on insulation, windows and fiberglas front door we had. We went with company my builder had worked with (conventional system but he vouched for neatness in running ducts to maximize attic space, quality of ductwork) who gave detailed estimate. Other 2 people highly recommended by acquatances didn't get back to us after initial verbal contact b/c I guess they had more than enough work. Another company gave detailed estimate but I believe oversized system - again, didn't do calculations. But I knew from talking to the "experts" who were too busy to actually bid (or do calculations, but had so much experience that I trusted their recommendations, esp. since they agreed on 4 ton unit) what the right size for us would be.

Good luck.

    Bookmark   March 10, 2009 at 10:54PM
Thank you for reporting this comment. Undo

Bingo. Thanks very much for the information. I'm a dedicated DIY guy, so I have little to no experience dealing with contractors.

Geothermal is out of my DIY league though, so I know I need a competent designer/installer. Your info will help me to pick one. Thanks.

    Bookmark   March 11, 2009 at 12:21AM
Thank you for reporting this comment. Undo

It wasn't too hard to make a choice once we found the 2 experts weren't responding to our RFPs, though talking to them they said w/o seeing the house they'd say prob. 4 ton would be enough. Our builder had a 3 ton for same size house and was supplementing with woodstove, complaining about system so we didn't even ask people he used to bid. But rejected the detailed estimate that said 3 ton right away. Then the person who wanted to put a 3 ton for downstairs (app. 1500 sf) and 2 ton upstairs (1200 sf) we thought would be oversized. So we went with the neat, well-laid out ductwork guy who bid on 4 ton unit, talked to some of his GT references (though I prefer to ask around and find out from building dept, GC's who best subs are now that I've been burnt from tile layer who provided me with glowing references but clearly didn't know how to prepare subfloor). I could have gone back to other contractors and asked them to bid 4 ton unit but I felt that if they couldn't get it right the 1st time, who was I to tell them - any problems with the system and they'd say it was what I picked.

The 4-ton guy did say in his bid "The heating and cooling loads will need to be designed on Manuel (sic) J software and will be designed to meet the energy star guidelines. This will require the home to pass an energy star blower door test for maximum energy savings."

I don't know if they ever did the blower test (guess I should call them) since it was new construction and I wasn't around the whole time but I can tell you that we don't even turn on the breaker for the Aux heat until late Nov (2007 & 2008). We have only 18 months experience living with this system but so far we think it's good. Just looking for a way to reduce our energy usage during the long NE winter. Maybe next year we should try keeping the heat at 65 instead of 68, but we get so used to being warm in Oct/Nov we hate to turn it down for Dec-Feb!

    Bookmark   March 11, 2009 at 8:29AM
Thank you for reporting this comment. Undo

ajsmama - you should ask for the details of the blower door test. it will be useful for you to know how "tight" the house is - it is a measure of the competency of the contractors. Also, a good result might even increase resale value.

    Bookmark   March 11, 2009 at 7:17PM
Thank you for reporting this comment. Undo

vespadaddy - He didn't run blower test, suggested I ask electric co. about blower or thermal imaging. Since his contract says he'd need a blower test to do calcs, I don't know if he just based them on house design data (window ratings, etc.) but he is going to get me the numbers. I will post them if small enough, I can email them to you if larger and you're interested.

klaire - I would be interested too, he suggested waiting til all our casing is up b/c it does make a difference. DH checked last night and all plumbing (vent) penetrations in attic are firecaulked just like the bathroom fans are. Of course all the ductwork is insulated and taped. So I think windows and doors are the only things we can make tighter. Resale value not an issue - we built on family land and my dad would disown me if we sold!

    Bookmark   March 13, 2009 at 7:45AM
Thank you for reporting this comment. Undo

I have had 2 contractors here so far, a company that installs Waterfurnace, and a company that installs Econar.

The waterfurnace guy sized everything based on our natural gas use for the past year, using a program called Geolink Design Studio, IIRC. Given that this is a retrofit, might that be as accurate as a Manual J calc, given the R-value of the walls would have to be estimated? He said he wouldn't do a manual J, as the design program was accurate.

The Econar rep just pulled a number out of the air as we sat together, as far as I could tell.

I have more contractors coming out next week. I'll post up what I hear then.

    Bookmark   March 23, 2009 at 2:37AM
Thank you for reporting this comment. Undo

I would also rely more on the NG data than the Manual J. Actual usage data is always preferred to estimates. Of course, you need to correct for heating degree days and programmeable thermostats/life style issues. Personally, if I was considering a new heat source, I would have been closely monitoring my heat usage during the previous winter. I think this is even more important in the case of a heat pump, where heat load vs temperature is useful design info. This also provides a baseline and a way of making useful comparisons. If you said my heat load is 30 kbtu/hr for a 2 story 2000 sq/ft at an average temp of 30, we would know you could use some more insulation/air sealing.

    Bookmark   March 25, 2009 at 12:57PM
Thank you for reporting this comment. Undo

A proper Manual J calculation takes into consideration such variables as the exterior color of the structure, The color of the roofing shingles, elevation above or below average terrain, the presence of trees or other nearby structures and how they will effect wind Wind and solar loads, the orientation of the structure to determine how much frontage it presents in the direction of the prevailing wind and solar load etc.

If one were to build ten identical homes from the same set of prints and material specifications he/she would still get ten different results when conducting a manual j calculation on each structure.

While there are contractors out there who will dismiss the manual J calculation as an unnecessary expense, in the end over the life of the structure you will pay far more in wasted energy to operate a miss-matched system.

    Bookmark   April 10, 2009 at 9:54AM
Sign Up to comment
More Discussions
Critique of House plan
I am still trying to figure out the perfect plan for...
Ideas on how to capture gray water runoff from A/C?
The pipe that comes out of our crawl space which empties...
Radon system
Can somebody tell me why there's condensation outside...
Solar hot water vs. tankless systems
I live in southern Louisiana. We're renovating a very...
PV installation ROI review
My wife and I are researching solar panel systems for...
People viewed this after searching for:
© 2015 Houzz Inc. Houzz® The new way to design your home™