Using HVAC-Calc Residential

[infwsdm]

I have owned calc for a few days now, and there are a few things I want to know from those who use it.

1. How long did it take you to trust it?

2. Have you ever miss-sized a unit using calc?

3. Do you change the design conditions from what the program states in a particular area?

I have used it for three houses (including my own which I know is oversized). Deviating from the "rule of thumb", as I have been wanting to do, I find that the calc is 1 ton lower than what I would normally put in for a particular house.

And, my partner is having a harder time believing calc. "there's no way I am putting that unit in, it's way to small" is what I am hearing.

Thoughts please

[htg guy]

I have'nt been using it long either but what I do is bump the design temps up for summer and down for winter design. I add 5 degrees either way. Other wise they do seem pretty low.

[infwsdm]

Originally posted by htg guy
I have'nt been using it long either but what I do is bump the design temps up for summer and down for winter design. I add 5 degrees either way. Other wise they do seem pretty low.

I have been adding almost 10 degrees for summer and it still seems low, but then again everyone I know in this business dosen't do a load calc. In which case, they are oversizing equipment.

Have you done any installs from the calc? How did they turn out? Heating or cooling like a charm or what?

[Airdoctor7]

Here in Alabama it seems like just about all companies oversize. Average 600 sq. ft. per ton on a new well insulated house. When I first started doing Manual J I had to rethink and was nervous on the first 4 or 5 systems. I did one house that was 1900 sq. ft. Manual J called for 2 tons. I couldnt help but bump it to 2.5 My install crew said boss we going to have to tear that one out. The system worked great even in a couple of 100 deg. days. Only complain the customer had was that it ran to much but he loved his utility bills. I told him it was doing exactly right! I have not looked back. I am thinking about getting the HVAC calc advertized here. I already have a laptop. I would like to hear from others who have used it too.

[weathered]

I've been using Don's programs since the mid 80's!! They give you exactly the same answer as a manual Manual J calculation. The Design temps buit in to the program are the same average design temps as you will find in the ASHRAE books. Because they are average, then there are, by definition, colder winters and warmer summers so I too add 5 degrees in summer and lower by 5 in winter. I asked Don about this he he agreed, I think.

I have used it for sizing for actually several thousand jobs with absolutely NO problems. I never round down, for instance if the load is slightly over 2 1/2 ton, I go with a 3. I trust it and it has worked well for me.

Bob

[infwsdm]

So what I am seeing is that you don't go exactly by the calc.

One load I did has a heat gain of 22,622BTUH, the program calls for 2 tons. Would you keep it there or bump it up. I already changed the design conditions for my area. My partner wants me to quote a 2.5 ton (because of the square footage). What do you think?



Airdoc, You said you put in a 2.5 ton in instead of the called for 2 ton. I take it you did not trust the program. Fill me in.

ACBob-- Do you mean if you have a heat gain of say 25,000BTUH you will go with a 2.5 ton instead of a 2 ton (24,000 BTU). What is your "bump up" point?





[Edited by infwsdm on 03-12-2002 at 03:26 PM]

[Airdoctor7]

I still think one has to use some common sense. This job was a lake house. The customer had three children which usually means trips in and out the door. They also entertained small groups of people occasionally. Manual J is a good starting point but IMO one has to look at other factors so as not to grossly oversize or undersize a system The final step of any job is setting the fan speed up correctly. There is a little flexibility and a decision to be made on the final recommendation of equip. I only bumped this job about 1/2 a ton but everyone who quoted the job except me and one other guy recommended a 3.5 ton system based on rule of thumb. I have not had to eat a job yet sizing at or close to Manual J. 1/2 ton over still wasnt close to rule of thumb. Summer will be here soon. Why dont you try one job and prove to yourself. You seem like a professional so I know it will work for you.......David

[Gus HVAC]

I guess I am the only idiot who trust the program, has been
using the program for about one year and do what it says.
I configure the program to roun the tonage and haven had any problems.

[Don Sleeth]

infwsdm, please relax a little bit. HVAC-Calc is a tool designed to help you quickly calculate the heat loss and heat gain of a house based on the proven methodology of Manual J. It will give you the same answer in a fraction of the time.

But it is a tool, you are the contractor who uses that tool in the same way you use your gauges, you read them and then you use your knowledge, skill and common sense to make a decision as you see fit.

You said "One load I did has a heat gain of 22,622BTUH, the program calls for 2 tons.", actually it is the house that requires 2 tons, not the program. The program or Manual J is just a tool to give you a reading on the heat gain of the house. In the case you mention, I would put in a nominal 2 ton unit which usually has a capacity of about 22,000 BTUH.

In the other case you mention, a load of 25,000, well I would look on my spec sheets and suppose I see that a 2 1/2 ton unit has a real capacity of 28,000 and we know the 2 ton has a capacity of 22,000 so I am right in between. I would go with the 2 1/2.

I think we need to keep things in perspective a bit. The wild card in the whole thing is the weather, so how precise do we need to be when calculating something that is diretly proportional to the weather? In the last example above, I oversized by 3,000 BTUH, that is half the capacity of the smallest window shaker. It is not a big deal in my opinion, neither is one whole window shaker if it buys you peace of mind. If you think in terms of little window air conditioners, it helps you keep things in perspective.

Now, all of what I have said above makes the assumption that you have input the information properly. Why don't you give me a call when you are at your computer with that job loaded in and I will check a few things with you. We have excellent support 7 days a week, toll free 888-736-1101 from 8 a.m. until 8 p.m. - you can't beat that!

don

[infwsdm]

Thanks for responding Don, and I am relaxed! :cool:

First I want to thank you for clarifying that a house gets the tonnage not the program :)

Thanks for your examples that helps a lot. And you will be getting a phone call, before 8pm of course :)

As far as entering the data properly, It took me three times to do my house. Ohhhh, I would forget about the ceilings etc, but I got that fixed :D

[Airdoctor7]

One more thing> The duct system and the entire system for that matter has to be right to get proper tonneage. A mfg. rated 2 ton system is rated at that with everything assumed to be installed correctly,duct,charge,fan speed etc.

[mose0]

Don's right,use HVAC-Calc as you would use the rest of your "tools".I use it with the WI code design conditions entered in ( ZONE 3 - 15 deg below zero outdoor ,70 deg indoor in the winter , 78 deg indoor in the summer)They don't give an outdoor summer design temp , so I use 95 deg. I have compared HVAC-Calc results to the results I have gotten on WI's Wischeck(heat loss software)
and the two results are VERY close. The HVAC-Calc is much quicker and easier to use , though, and the homeowner is impressed with the printout! As far as the tonnage
for a.c. goes, I check the sensible to latent ratio specs on the condensing unit and size accordingly. If HVAC-Calc says 25000 btu total gain ( 21000 btu sensible ,and 4000 btu latent), I look at the specs of the 2.5 ton unit and see
that the btu rating is 28000 btu total, with 21000 btu sensible and 7000 btu latent, and see that i'm covered with a 2.5 ton unit! WI code says that we can oversize 15% on the heating , or to the manufacturers next available size.
I'm a big fan of the HVAC-Calc program , and "trust it's results" (because it uses the data that I input, and I trust myself!)You bought it, you might as well benefit from it!!!

[infwsdm]

Thanks for all of your input guys.

And like my other tools, I first have to learn how to use it. And who better to ask than people who have experience with it! And the person who wrote the program of course.

Being an old "rule of thumb" fool. I certainly don't want to go the other way and "undersize". I am getting the hang of things.

Once I get my first job in, I'm sure I will have more confidence. (unless I undersize it :) )

[skibme]

we bought the program a few months ago,we use it for rebates for the electric co.the electric co,gives $320.00 for 13 /14 seer. 500.00 for14 plus.our co.receives100.00 for each manual j . it takes me about half an hour per system.the electric co allows a 1/2 ton swing either way. a maxium 20 degree tempeture split from outside to indoors. i use the given ach.rate,but i adjust the amount of people by 1-3 ,for adjustment purposes... buy yourself a good distance meter, and a compass,to make it go faster. i send my completed forms to the prospective customer,alot of guys oversize,i tell the customer this is the form i will be sending to the electric co.blows your competition out of the sky.(the electric co. does come out to check sizing,and does deny rebates for oversizing)

[infwsdm]

I have seen something about those distance meters- I don't remember where though (feel free to give me any info on those, like how well they work and how much they cost)

The compass idea sounds good too! I get lost as soon as I walk into a building:)

Maybe I should give my electric company a call!

I've been playing with the calc program , I think I have it down to a science now!!

[vt]

An advice from my own experience - RTFM first.

It might make sense to come up with the Rule #0: "If you think your question is not answered, Read The {} Manual".

I've embarrassed myself twice before getting the load done - once before reading the manual, once after :o

[infwsdm]

I have. I got this puppy down now. I read the manual twice :). There were some things on the house construction that I wasn't sure of, but, after speaking with Don, I think I have it down pretty good. Now having said that, I will probably end up in a F**ked up designed house with all kinds of stuff going on :)

[rayr]

Just watch when you are doing 2 story houses that you don't wind up with ceiling loads on both floors.

[infwsdm]

Originally posted by rayr
Just watch when you are doing 2 story houses that you don't wind up with ceiling loads on both floors.

Yeah, I got that right--On my third try:)

[gohigh]

Don, You stated to trust the program but in your manuel you state that you add 20%-25% from your calculated heat gain or loss. I would thinj this is your experiance telling you to do this but if you could would you explain why?

Also the program seems to select the tonnage based off of colling needs, If installing a heat pump would you need to base the tonnage off of the heat loss?

[platchford]

Also the program seems to select the tonnage based off of cooling needs, If installing a heat pump would you need to base the tonnage off of the heat loss?

If installing a heat pump you should still base the tonnage off the heat gain with the possible exception of if you are in a climate that has a very short cooling season. This was discussed in another thread previously. The difference between the system tonnage when sized for cooling vs heating should be made up with the 2nd stage which is generally an electric heating package. The reason you still base the heatpump from heat gain is that a system based on heat loss is generally going to do a poor job of dehumidification in cooling mode. :cool:

[beenthere]

After you get your sensible and latent gains from the program. Go to the spec guide of the brand you install. Check on the performace of the unit at the design temps you have in your area.
If your brand doesn't have extended performance charts. Use Manual S, to determine what tonnage unit you need to use.

Your load calc may say the house needs 22,3000 BTU's for 72* ID, and 95* OD, with 18,160 BTU's being sensible. Your 2 ton A/C may only have a 15,200 BTU sensible capacity at those design temps.
Meaning you would need a nominal rated 2.5 ton A/C to meet the sensible load.

[duckman373]

After you get your sensible and latent gains from the program. Go to the spec guide of the brand you install. Check on the performace of the unit at the design temps you have in your area.
If your brand doesn't have extended performance charts. Use Manual S, to determine what tonnage unit you need to use.

Your load calc may say the house needs 22,3000 BTU's for 72* ID, and 95* OD, with 18,160 BTU's being sensible. Your 2 ton A/C may only have a 15,200 BTU sensible capacity at those design temps.
Meaning you would need a nominal rated 2.5 ton A/C to meet the sensible load.

Words of wisdom. i've been using the program for a few weeks, and am a believer. I also appreciate Don's assistance. Don't buy the program though, just let me know if you want it, and I'll send you my CD..........................

















...................................Just kidding Don ;)

[coolmen]

Don's right,use HVAC-Calc as you would use the rest of your "tools".I use it with the WI code design conditions entered in ( ZONE 3 - 15 deg below zero outdoor ,70 deg indoor in the winter , 78 deg indoor in the summer)They don't give an outdoor summer design temp , so I use 95 deg. I have compared HVAC-Calc results to the results I have gotten on WI's Wischeck(heat loss software)
and the two results are VERY close. The HVAC-Calc is much quicker and easier to use , though, and the homeowner is impressed with the printout! As far as the tonnage
for a.c. goes, I check the sensible to latent ratio specs on the condensing unit and size accordingly. If HVAC-Calc says 25000 btu total gain ( 21000 btu sensible ,and 4000 btu latent), I look at the specs of the 2.5 ton unit and see
that the btu rating is 28000 btu total, with 21000 btu sensible and 7000 btu latent, and see that i'm covered with a 2.5 ton unit! WI code says that we can oversize 15% on the heating , or to the manufacturers next available size.
I'm a big fan of the HVAC-Calc program , and "trust it's results" (because it uses the data that I input, and I trust myself!)You bought it, you might as well benefit from it!!!EXACTLY RIGHT.......the program was money well spent and trust it 100% thanks Don

[teddy bear]

What is the sensible/latent load ratio on 4 people and 75-100 cfm of fresh air on the most humid cool day in WI? How does any of the sizing programs help you size for this condition, while maintaining 75^F, 50%RH? Regards TB

[beenthere]

Teddy, you know the answer.

Load programs are only to determine capacity needed at the design conditions.
Not the entire season.

If you want that changed, you'll have to go to ASHRAE.

[coolmen]

google earth helps pretty fast on house location and figureing where the house faces per compass