Energy audit and Manual J

[Bailey'sMom]

I have an energy audit scheduled for Monday. Once that report comes in and the repairs are complete, I'll schedule a HVAC quote with a Manual J.

Is there anything specific I should be looking out for or any questions I should be asking?

[tipsrfine]

Hopefully you didn't go with an inexpensive audit from a company that isn't even certified by BPI or RESNET. If the company doing the energy audit also does work on the home like insulation & hvac etc..., be prepared to read between the lines on what they recommend. Start going through the websites of local hvac companies and find a few that actually advertise on their site that they perform manual j calcs. Let those few companies know you've had a blower door test performed during your energy audit, and the one that ask to see the infiltration numbers so that he can enter those numbers into the manual j software is the one to go with.

[second opinion]

Hopefully you didn't go with an inexpensive audit from a company that isn't even certified by BPI or RESNET. If the company doing the energy audit also does work on the home like insulation & hvac etc..., be prepared to read between the lines on what they recommend. Start going through the websites of local hvac companies and find a few that actually advertise on their site that they perform manual j calcs. Let those few companies know you've had a blower door test performed during your energy audit, and the one that ask to see the infiltration numbers so that he can enter those numbers into the manual j software is the one to go with.

Times 2 unless repairs have been done

[Bailey'sMom]

The company is BPI certified (as verified on BPI.org) and will provide a report on inspection and after the repairs are completed so we can see the difference. They can perform the repairs or we can find someone to do them ourselves. They do mention insulation on their website, but I don't see anything about HVAC.

http://betterbuildingworks.com/

With the Manual J, I've seen it mentioned a few times that contractors will pad the figures in the software to compensate for other things, and the homeowner ends up with an oversized unit. Is it safe to say that I want them to use the exact figures and to use average temperatures instead of extreme temperatures?

[second opinion]

With both pieces of information you do not need to worry.

[Shophound]

Manual J for Roanoke, VA has your summer outdoor design temperature at 91 degrees with a coincident wet bulb of 72. In layman's terms this means that someone in your area designing an HVAC system by Manual J standards will operate under the assumption that your cooling system will keep your house comfortably at 75 degrees/50% relative humidity when it is 91 degrees at 40% relative humidity outside your house (known as a "design day" in engineering parlance). For winter, this same designer will size your furnace or other heating source around a 16 degree day.

A Manual J done well accounts for how leaky your house is...which is what the blower door info should do for you. It also attempts to make an informed estimation of how much heat will enter into your house in summer or leave the house in winter during the above named "design day" conditions, and how big your HVAC needs to be to keep you comfortable during those extremes in temperature and humidity.

The only "padding", IMO that should occur arising from a thorough Manual J calculation is if the results render a cooling BTU requirement somewhere in the gray zone...IOW if the Manual J calls for 4.45 tons of cooling, a five ton unit should be chosen because there are no 4.5 ton units made. For heating, if you are talking natural gas I'd seriously look at modulating furnaces, with the "high fire" stage sized according to your Manual J results for maximum expected heating demand. If you're looking at a heat pump your designer will need to know how much backup heat you need to meet your 16 degree day because heat pumps are sized for the cooling load, not heating.

The hazard you may come across, and it is unfortunately a common occurance, is finding a residential HVAC contractor who is comfortable with Manual J's. Most do not like to deal with it, thinking either it's a waste of time and their experience is sufficient, or they may not know how to do a Manual J at all. The ones who do tend to be more expensive, but often they deliver a better installation and stand behind their work. That's what you want in the long run. You want right-sized equipment, a quality installation, and a contractor who stands behind their work. If you find this person or company and they hit well on all three points, shout their praises from the rooftops so others in your area can benefit from their expertise.

[Bailey'sMom]

I thought the general rule was when in doubt, go smaller. Doesn't that mean I'd go with a 4 ton vs. a 5 ton if the Manual J called for a 4.45 ton?

[Shophound]

I thought the general rule was when in doubt, go smaller. Doesn't that mean I'd go with a 4 ton vs. a 5 ton if the Manual J called for a 4.45 ton?

Manual J is sizing your system for the "normalized" extremes for your area. IIRC, it is built on weather bin data for your area that has been "normalized" for as many years as are on record. Meaning that the cooling load is taking into account the highest temperature AND humidity expected for a "design day" condition. If you have a day that is higher than your area's design day 91 degrees, but the humidity is lower, you should have enough capacity to still stay comfortable.

What I meant in my last post is that manufacturers do not make a 4.5 ton system. If you're over four tons, rounding down to four tons might leave you short on a few days of the year, but otherwise would probably keep you comfortable the rest of the time.

That said, it's more complex regarding proper equipment sizing than I've discussed here, especially when outdoor design day temperatures are above 95 degrees, and/or the area is a hot-humid region. To make matters more complex, while you may think a four ton system delivers four tons of cooling any time it runs, this is not the case. That four ton rating was derived in a laboratory and does not account for duct heat gain, air leakage, etc. Also, the conditions that ARI rates tonnage at seldom occur in residential settings, at least indoors. How many people want to maintain 80 degrees at 50% relative humidity when it is 95 degrees outside? Not many.

So, what's the bottom line? Someone who knows how to run a Manual J should understand all of this, plus more that I have not mentioned, and be able to size your new system correctly. Even at that, Manual J is a reasonable estimate of the expected maximum heat gains and losses for your house year round. It's far better than guessing or going by seat-of-the-pants experience, but it is not exact. I would still want it done, and done by someone who knows what they're doing, than not.