1. Regular Guest
Join Date
Apr 2006
Posts
67
Post Likes
I've seen two approaches used by techs to calculate the heat load of my 50-60 year old home. Two of five mentioned Manual J or an equivalent software. The others
said that since anyway there's no way to estimate accurately the R value of the insulation, they use the square footage with some formulas. So far I got 3 tons and 3.5 tons estimates.
Interestingly enough, the 3.5 ton value was quoted by two techs using the two different methods.
How to decide which is the right size, and suppose we go with the 3.5 t and it's indeed oversized, how bad would it
be?

2. Professional Member
Join Date
Feb 2003
Location
Huntsville,AL
Posts
4,121
Post Likes

3. Regular Guest
Join Date
Nov 2005
Location
Northern VA
Posts
512
Post Likes
As the owner of a 50-year-old home that's been worked on by several previous owners (and therefore has some weirdness in the insulation, fenestration, you name it) - Man J is still a valuable tool for older homes, but it's only as accurate as the values you put into it. You should have a blower door done, measure the insulation, etc, and get both as much information as possible about your house's envelope AND a good idea of how much you *don't* know.

Close sizing makes sense for new homes where all envelope characteristics are known to a high degree of precision. You still don't want to oversize with an older home, but you do need to be aware that what Man J tells you is based on numbers that might not be quite right. What I found is that the half-ton of "oversize" is generally the way to go for *old* homes. It's really not that much - the big problem with oversizing is when people oversize by a ton or two. And Man J does not fully capture all the holes in an old house's envelope.

My *guess* would be that the 3.5 ton is "right". That is only a guess; it might be wrong. You also need to look at the sensible vs latent load; in my climate I found that was a significant factor in system sizing.

4. ## Don't forget airflow needs and wants

IMO there is not a lot of difference between a 3.0 and 3.5 ton system. I am a homeowner in S.Texas and for better or worse we *do* have a ton of experience with oversized systems. It's hard to get a tech to consider Manual J sizing in these parts, so the low-bid installers apparently get a little more in their pocket by installing equipment on the large size. For example my 1989 house is 3300 sqft and has 6.5 tons AC, a nearby new one has 8.0 tons and appears only about 2500 sqft. It has dual-pane windows etc. and I have lots of old single pane aluminum windows. Sheesh.

Since your house is already built, IMO you should be extremely wary of upsizing any AC equipment unless you make sure your duct system can handle the extra airflow. I think you should not accept excuses such as "it's only a little bigger" or "a VS fan can work fine with those size ducts, it will push harder". The penalty for being wrong, is more on your shoulders than the tech's.

That business about not knowing the insulation of your old house, is a similar bad excuse for not doing Manual J. You pretty much can tell what you have for insulation, and the actual R-value may not make that much difference anyway. If you would spring for \$50 and get HVAC-CALC software, I would not be a bit surprised if you get a heat load number different from *either* of your two estimates. It takes a couple hours serious work measuring your house, not much more effort than that. You can do it.

My own heat load tells me 4.5 to 5.0 tons AC is supposed to be enough. I apparently have 3.0 tons on the west side of the house, and 3.5 tons newer AC on the east side. Based on experience the 3.5 tons is about 0.75 more than I need, and the 3.0 tons is just about adequate. Yet the east side has decidedly slimmer ductwork (291 sqin supply area vs. 400) which really cannot handle 1400 CFM, or the classic 400 cfm/ton. Somebody must have upsized the damn thing without thinking things through.

I think that measuring ESP (External Static Pressure) is one way of judging the adequacy of your duct sizing. Would love to hear the pro's critique that statement of mine. Personally I will be grateful for any pro correcting what I say, if it deserves it.

Best of luck -- Pstu

5. Professional Member
Join Date
Nov 2004
Posts
2,922
Post Likes

## Re: Don't forget airflow needs and wants

Originally posted by pstu

Best of luck -- Pstu
okay stu, what became of your mixed air intake experiment?
did you figure out how to calculate it?

6. ## OT -- fresh air talk

I got convinced that a fresh air (FA) intake in the usual way, would inevitably increase the humidity load in the house. Earlier I did not see so clearly and imagined that it might displace existing infiltration. There are two methods that appeal to me right now: having the FA inlet go directly into a dehumidifier, and having a sensor gadget which would ventilate large amounts when outdoor dewpoint is low, and none at all when it is high. Going slow on either, due to cash flow not being overly high right now.

Best wishes -- Pstu

7. Professional Member
Join Date
Feb 2003
Location
Huntsville,AL
Posts
4,121
Post Likes
ptsu: why don't you add storm windows?

garsh, even I can read a ruler to measure the thickness of insulation

I measured beside an elec box, but the boxes had 0.25" gaps till I closed 'em. go into kit, under a cabinet, make a hole & measure the insul in the exterior wall--

in an old house:
insulate the attic

seal the existing ducts with mastic = 21% savings in my Daughter's house in Fort Wayne IN

install storm windows & doors [which also protects the main doors & win]

seal around all elec boxes, pipe penetrations

as part of painting a room, remove the trim, insulate between frame & window |door, caulk all wall seams & trim.

are the rooms comfortable now?
are the rooms within 3F just after blower shutdown?

run load calc & play what if I add ____ insul in attic, storms, shade trees, ____

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•

## Related Forums

The place where Electrical professionals meet.