Why different size recommendations with load calcs?

[bostonguy]

Ask them to put in writing, what temp it will maintain, and what %RH it is sized to maintain while running.

I will ask about this, thanks for the suggestion.

[bostonguy]

I have a couple of questions if those that have contributed to my thread happen to see this. The contractor that I am seriously considering gave me the following info for things that were suggested earlier in this thread.

1. For design temperatures he said that he used outdoor temp of 95 degrees, indoor temp of 70 degrees and RH of 50%. Sounds reasonable to me, any thoughts or comments.

2. When I asked what values he used for wall insulation in the load calc he said in order to determine wall insulation he would have to poke a hole in the wall to see what is there. My house was built in 1969 and he said that houses built back then used little in the way of wall insulation so he said that he assumed worse case and used zero for wall insulation value. I thought that I had read somewhere that you can take the wall temperatures of an exterior wall and an interior wall and the temp difference would give an idea of insulation in the wall. Am I totally misunderstanding this? The contractor does seem knowledgeable so if he is usung a zero value for wall insulation should I be concerned?

[beenthere]

I would be wondering.

He can pull a wall plate and check for insultaion.

I don't think your design temp in the summer is anywhere near 95.

[jimj]

Is 70* the temp you want to maintain? If it's 72* thats a Hank R. do not do.
I would cut a hole in my sheetrock and check R values before I would assume no insulation in my calcs.
What does Manual J use for your area? don't add degrees to YOUR design temp!
Go here and read Proctor info,http://www.proctoreng.com/articles/better.html

[larobj63]

One may be using a higher infiltration rate then the other.

Infiltration is always an "educated guess" at best. I'm always surprised when people expect that load calculations are such an exact science. It is very difficult to know if a house has 0.5 or 1 air changes per hour or anywhere in between or above or below - and that makes a big difference in load! In fact, infiltration often drives the load more than any other factor - including wall insulation! If you want the load calc to be that dead on - pony up for a blower door test!

It places an element of trust in your contractors experience, in my opinion. We are here in our office chairs - your bidding contractors have seen the house - maybe your windows are new - maybe they suck, we don't know. No-one can tell you what size is right form where we sit.

My house was built around 1962, and it has no wall cavity insulation. But it has a continuous layer of homasote, covered by cedar siding, covered by vinyl insulation (with integral Styrofoam insulation), and lots of shading on the southern exposure. My cooling load is diddly squat - but it would be hard to predict without "knowing the house" - or at least seeing it!

[bostonguy]

I would be wondering.

He can pull a wall plate and check for insultaion.

I don't think your design temp in the summer is anywhere near 95.

You have no idea how I wish I could find a contractor like you or any of the professional contributors to this forum.

Do you know what the Manual J design temp should be for the Boston area?

I'll also suggest that a wall plate be pulled and see what the response is.

[bostonguy]

Is 70* the temp you want to maintain? If it's 72* thats a Hank R. do not do.
I would cut a hole in my sheetrock and check R values before I would assume no insulation in my calcs.
What does Manual J use for your area? don't add degrees to YOUR design temp!
Go here and read Proctor info,http://www.proctoreng.com/articles/better.html

Hank R.?, what's that mean?

Based on my current system I am comfortable at 72 degrees so that would be the temp that I want to maintain. Again my current system could be oversized and with a hopefully correctly sized system I may not need to keep the stat at 72 degrees. More questions for the contractor I guess, thanks for your input.

[adrianf]

Hank R.?, what's that mean?

http://www.amerrock.com/pdfs/slicks/...20Butowski.pdf

[jimj]

Hank Rutkowski, he is the author of Manual J7 and J8.

Your design temp at 1% is (88* DB/72 WB*). At 0.4 its (91* DB/73* B).What the means is the summer design temp will be exceeded only 1% of your cooling hours.

Using 70* indoor and 95* outdoor would be considered oversizing to me!

Here a couple of Manual J Don'ts
Do not design for record breaking heat.
Do not add a safety factor.
Do not design for abnormally low or high indoor temps or humidity.
Do not reduce known ceiling, wall or floor R-values "just to be safe".
Do not apply "safety factors" during any stage of the load calc process.

Here are a couple of does.
Use the outdoor design conditions recommended by Table 1A of manual J.
Use indoor conditions that are compatible with the comfort chart.
Verify all construction details prior to calculating loads.

These are a few of the reasons loads very from company to company. Manual J has a safety factor built into it already, Don't follow there guide lines and you mite as well use the square footage method.

[bostonguy]

Hank Rutkowski, he is the author of Manual J7 and J8.

Your design temp at 1% is (88* DB/72 WB*). At 0.4 its (91* DB/73* B).What the means is the summer design temp will be exceeded only 1% of your cooling hours.

Using 70* indoor and 95* outdoor would be considered oversizing to me!

Here a couple of Manual J Don'ts
Do not design for record breaking heat.
Do not add a safety factor.
Do not design for abnormally low or high indoor temps or humidity.
Do not reduce known ceiling, wall or floor R-values "just to be safe".
Do not apply "safety factors" during any stage of the load calc process.

Here are a couple of does.
Use the outdoor design conditions recommended by Table 1A of manual J.
Use indoor conditions that are compatible with the comfort chart.
Verify all construction details prior to calculating loads.

These are a few of the reasons loads very from company to company. Manual J has a safety factor built into it already, Don't follow there guide lines and you mite as well use the square footage method.

Thanks for these suggestions, really helpful for when I review the load calc with the contractor. If I'm not being a pain I'm a little unsure of what this means:

Your design temp at 1% is (88* DB/72 WB*). At 0.4 its (91* DB/73* B).What the means is the summer design temp will be exceeded only 1% of your cooling hours.

What is the difference between using 88* DB/72 WB* or 91* DB/73* WB).Never mind this question I understand what it means now.