Observations (n=2) on HVAC sizing

[sktn77a]

We've lived in out current house for almost 20 years. 3000sq feet, two level, in Central North Carolina. The house came with two 2.5 ton (~8 SEER) AC units and two 75000 BTU (80%) furnaces. The systems seemed to work fine although I sometimes felt that they weren't dehumidifying the air enough (?)

Well, we had them both replaced over the last 6 months (ductwork also). The first to go was the downstairs, replaced with a 3 ton (18 SEER) 2 stage condenser and 70000BTU (95%) modulating furnace. Seems to work OK but the airflow is higher than before (and a little noisy). It was sized based on square footage, wall size, window/door size, insulation and orientation. When we replaced the upstairs, I asked if a 2 ton/60,000BTU system would work (to reduce the noise, primarily). They originally proposed the same size as downstairs but as the heat load calculation (same company, same method, not a full Manual J) was between the two, this they did.

The upstairs system has worked just fine in recent 90-100 degree weather with very high humidity. Indeed, the air coming out of there is ICE cold (almost too cold)! The airfow is set to spec (600CFM in low and 800CFM/min on high) and is blowing just fine. It runs for 10-15 min, typically, per hour and shuts off when it reaches setpoint. It's working so well, I'm wondering if I could have gotten away with an even smaller unit! The downstairs unit also works well enough, again ICE cold air (and again, almost too cold - airflows set correctly, 900CFM on low and 1200CFM on high) and seems to be dehumidifying just fine, but it rarely, if ever, kicks in the second stage (neither does the upstairs).

Which brings me to my point. I think most heating and A/C systems are grossly oversized by installing companies (new or replacement). I don't know what the reasoning for this is but I'm guessing it's to prevent callbacks (oversize is better for the installer than undersize). I'm also not convinced that Manual J is the be-all and end-all of HVAC sizing (it may be better than nothing but there are papers in the literature showing that it over-estimates sensible heat load by 30%).

So just how do the Pros on this site size equipment (I hear many calls for Manual J calculations but I can't believe you all do these for every estimate you give). And what about "judgement" and "experience" - how much do you weigh this if a Manual J says "X" and you just know it needs "Y".

Keith
(let me zip up my flame suit all the way...............)

[beenthere]

If manual J says 2 tons. I don't put in a 2.5 or a 1.5 ton.
I put in the 2 ton.

[wraujr]

What do you do if Manual J says 2.5 ton and units only come in 1 ton increments???

or better yet, my Manual J said 36400 and 3 ton xL16i was only going to output 34400 so my contractor wanted to step up to 4 ton putting out 46000......Short story is I insisted on the undersized 3 ton as the ductwork would never have supported 1600 CFM...

In my case, Manual J has caused more trouble than simply saying "have you been happy with your old system?? if so replace it with same size"

P.S. 15 years ago builder's HVAC contractor claimed Manual J cal of 31500 to justify the 3 ton AC he installed... Oh who to beleive????

[beenthere]

Although 2 stage units for the most part only come in whole ton sizes.
Single stage do come in half ton sizes.

Its not the manual J. Its the person performing the measurements, and entering the data.

Garbage in garbage out.

Next. An understanding of sensible and latent heat ratio, and what happens to the unused latent ability of the equipment is required to interpet the results.

[Poodle Head Mikey]

And legitimately.

Around here, the local utility companies want me to size for a twenty degree difference to ambient. Which can be fine. But that's not all there is to it.

Say it's 100 - it's 80 in the house. OK; not a big deal.

But one day it's 105. And the system is ten or fifteen years old. And somehow the house is 87 degrees. That is a big deal.

Another one: I know my customers - they don't. I have a couple (two girls) who each weigh at least 400 lbs. They keep the house at 70 all year round. Even when it's 100-105 outside. ESPECIALLY when it's 100-105 outside. <g>

Their 35 degree difference to ambient heat load is NOT the same as your 20 degree difference heat load.

[sktn77a]

Their 35 degree difference to ambient heat load is NOT the same as your 20 degree difference heat load.

Good point. Does Manual J software allow installers to input all these non-standard parameters?

Keith

[beenthere]

Good point. Does Manual J software allow installers to input all these non-standard parameters?

Keith

Yep..
It may be non standard to you, but thats standard to areas, that have those high temps.

Here, we do 25 degrees difference indoor to outdoor.