[jerryd_2008]

Nope, I'm an electrical engineer in the aerospace industry.

A load calc is not rocket science, especially when you have direct control over the design of the house and how it is insulated. HVAC-Calc is designed to be fairly easy to use.

To minimize the required A/C size
1. put the air handler and all duct work in conditioned space.
2. shade all the windows in the summer.
3. make the house air tight
4. adequate insulation

Engineering comes into play when trying to figure out the most cost effective means to do all of the above.

The house is 100% above ground.

I have R13 walls and R49 attic insulation. According to HVAC Calc, I went over board on the attic insulation - but cellulose was really cheap.

Got an excellent energy consultant/architect around here that work's with the electric co-ops that has a $40 video that tells me those things for new construction and as best you can in retrofit situations. And he has been advocating cellulose for 20 years. He says foam is great too but it's several times more expensive.

Paul, I'm not picking on you, but I still don't understand how 2 tons and a bit more insulation does it for that size house in Dallas, right? You must have done some serious house envelope work, including windows, doors, house orientation, etc.

 

[paul42]

Got an excellent energy consultant/architect around here that work's with the electric co-ops that has a $40 video that tells me those things for new construction and as best you can in retrofit situations. And he has been advocating cellulose for 20 years. He says foam is great too but it's several times more expensive.

Paul, I'm not picking on you, but I still don't understand how 2 tons and a bit more insulation does it for that size house in Dallas, right? You must have done some serious house envelope work, including windows, doors, house orientation, etc.

The windows & doors are not as much of an issue as you might think. I went with casement and fixed windows to cut down on infiltration, but they are standard vinyl windows. There are five doors, a mixture of metal cladd, wood, and fiberglass. The total heat gain through the doors & windows is less than 3,500 btu/h.

Orientation is important. My house is a big rectangle running pretty close to due east / west. The garage shades the east end of the house, and we designed the layout so that the only windows on the west end are some small windows right under the eves in the master bathroom. The sun hits them about 6pm in the summer - and I paid for extra low e-coatings on those.

The rest of the windows are shaded by a 3 foot overhang all around the house. My wife likes lots of light, so we have a lot of windows. But, they are right under the eve and even at 5 feet high, they are still well shaded.

I put a lot of thought & effort into air sealing the house. instead of a conventional house wrap, I used a peel & stick roofing membrane. It makes the walls both air & water tight. The walls are insulated with 4x8 sheets of 2 inch thick polyiso foam on the outside of the house wrap. It was more expensive than using cellulose, but a LOT cheaper than spray foam and totally eliminated bridging through the studs. It also eliminated possible problems with condensation etc.. Everything outside of the peel & stick roofing membrane is not affected by moisture. Everything inside the membrane stays at room temperature & humidity.

I put a lot of personal effort into making the ceiling air tight. Now, a better answer would be to have the insulation company spray in an inch of foam to the attic floor before blowing in the cellulose.

 

[jerryd_2008]

...

I put a lot of personal effort into making the ceiling air tight. Now, a better answer would be to have the insulation company spray in an inch of foam to the attic floor before blowing in the cellulose.

Went to an open house with an all-electric air-source HP. They did a lot of foam under cellulose. The energy consultant was there and thought that this was a bit of overkill compared to straight cellulose. Couldn't find my post on that, but I recall that this ~2800 sqft house had a sub-$100/month bill. Also used a separate small (1 ton?) HP for a media room in upstairs. Don't think they got away with 2 tons total though. I was really impressed with how quiet the Rheem inside blower was.

 

[newwindblowing]

Irish, you pose a most interesting question. am a ho monitoring this forum in anticipation of replacing original 20 yr old ng central air system installed in 1989. now am aware of hvac load calc. have 8 different proposals and not a mention of load calc from any. for me, diy calc seems ill conceived. public utilities do not offer. iho, fee for service load calc with dual fuel hybrid incremental cost pay back chart would go a long way to assist prospective clients. how does a ho relate to replacing a 140k btu (probably 0versized) with current equipment. if 3.5 ton ac not fully comfortable for 20 yrs why downsize? if furnaces jump sizes by 20k btu increments, what is load calc all about on a retro? i'd rather be attempting to buy a used car from sam sausage motors. Great Forum in any event--thanx all.

 

[needhelp7]

There seems to be a lot of discrepency within our own industry as to the importance of performing "load calculations" when a representative comes to your home.

I know many of you have come to this site to get advice and direction, and I am curious as to how many of you are aware of the importance of properly sizing your heating and air conditioning system, and more importantly the part that a properly performed load calculation plays in that process whether performed before, during, or after the proposal is made available to you.

The disagreement we seem to have on this subject has prompted me to get YOUR feedback. After all, your thoughts are the most important on the subject and I for one, would appreciate your candid reply.

Let me know if you are aware of the term "load calculation or heat loss/heat gain" and what your understanding of it is.

Secondly, could you let me know the level of importance that YOU apply to this task and where during the process you believe it should be performed.

And finally, let me know if it would make any difference to you whether a contractor performed a load calculation or not in the process of making their recommendations to you.

Thank you.

Irishmist

Just finished getting information via different thread for choosing an hvac contractor. Based on recommendations from these website discussions,we screened potential contractors with the simple question: "How do you size your units." Before we learned to do that, though, the contractors who came out didn't offer to run one, and based unit sizing on current unit - just added a ton to our what they thought was the current unit to accomodate our closing in our garage. Because of tightening up the house a bit, the fact our cureent 3.5T unit is oversized for existing area, and the better efficiency of today's units, they would have oversized us by 1-1/2T - priced 5T units, and we only need 3.5T (and some on this list think we could go even smaller) per manual J. That question weeded out 7 of 11 contractors, and the one we ended up with will guarantee we are satisfied.

The contractor we went with was the only one to do a Manual J before he gave us a quote - he might have done Manual N as well - and that is how he quotes ALL his jobs. Our elec co. ran one for us as well, but strongly suggested that whoever did our job do his own, to confirm the numbers. How could a contractor even make a proposal without load calc, if he is going to use load calc at all?

Thinking load calcs are VERY important in choosing a system that will provide comfort, and not just "well, it's blowing cold/hot air so it's working".

 

[ILDAIP2]

Dave, I checked your other thread but I didn't see the size that you finally installed. May have missed it but I'm curious as to how you got to what you picked when you had proposals in the 2-5 ton range!

What did you have previously? If it did a decent job, did you at least tell the contractors to not to exceed the current system?

Had a old NG furnance I was replacing so no rule of thumb to assist with the sizing. Ended up with a 3T. So far works fine with heating the structure but the noise (line set and return air) is way higher than expected. Installer just said that the only silent heat is radiant so get used to it. hum..... not quite what I wanted to hear.

IMHO in the area Vancouver we don't have qualified installers. At least none of the 5 I talked to could (I mean would) perform a load calc. They figured they could size it just using a rule of thumb..... 1T per 1000 sqft.

It's also typical to set the balance point at 34-35 degrees despite the efficency of the outdoor unit still being better than NG.

Dave

 

[pstu]

I am way late in responding to this thread, but I do believe a Manual J is only one way to get good sizing information. No argument that Manual J is the best method for new construction, and for when the person has not lived in the house. My point is that a homeowner has experience information not available to a visiting pro.

Grudgingly, I can see the merit in sizing by what was experienced to work before. Yes there are times when the old system did not do its job well, but not always.

I wish professionals would pay more attention to ductwork leakage and/or airflow issues, if we had to pick just one topic to focus on.

Thank you for asking -- Pstu

 

[gstein]

Man J

Speaking as someone who has spent an ungodly amount of money on HVAC issues and been burned nearly 100% of the time, I think an honest manual J is fine and dandy, but system size requirement is as much a function of duct design, installation quality, and similar factors as it is a function of heat load. For example, our current residence is 4200 SF with 10 tons of AC in Houston, which was supposedly determined with a Man J. Definitely oversized, but the ductwork is restrictive and unbalanced, 80% of air flow passes through only one side of each A coil, duct insulation is poor, etc. We need the full 10 tons to cool the house, dehumidification is poor, and our electric bills are painful. I suspect 5 or 6 tons would be more than adequate with a better install. Another example, our previous home in Florida was 3200SF with a 4 ton A/C. Properly sized according to Man J, maybe, but the ductwork is restrictive and runs at very high static pressures and can't be balanced making one side of the house 3 or 4 degrees warmer than the other. The A/C can't keep up on hot days. Our fancy 19 seer system gave us $350+ cooling bills (not including other electrical usage) in the summer.

I am not in the HVAC business, but it is obvious to me that sizing a system depends on much more than simple heat load. The problem I see is that Manual J calculations and, for that matter, manual D designs, are based on overly-simplified assumptions. Whether one is dealing with Global Warming, Nuclear Physics or a heat load calculation, a mathematical model is NOT reality. It is a useful tool that provides information, like a gage set, but it is simply a guess based upon certain incomplete assumptions. The results of a heat load calc might be useful or might be less-than-worthless BS, depending on the assumptions and how the results are interpreted. For example, we are currently renovating a house in Florida which the initial Man J calculation specified 5 tons of cooling. The original A/C was 4 tons, which was able to keep the house cool and mostly dehumidified on a hot day with single pane windows AFTER all of the insulation and sheetrock had been removed. The remodeled house will be very tight and super insulated. I don't believe it, 3 or 3.5 would likely be more than adequate. The Man J doesn't take into account the shading effect of the garage roof, roof pitch and convection, how many windows simultaneously face the sun, and tons of other variables. Of course, an inflexible county code enforcement department is involved so common sense does not apply.

Our contractor has a good reputation so I am sure we will be able to negotiate a workable design, but my point is that I think common sense and experience are far more important than the results of a calculation. I think the difference between a pro and a hack is a thorough understanding of his tools and their limitations.

Peace

 

[beenthere]

Speaking as someone who has spent an ungodly amount of money on HVAC issues and been burned nearly 100% of the time, I think an honest manual J is fine and dandy, but system size requirement is as much a function of duct design, installation quality, and similar factors as it is a function of heat load.

Manual J can't account for a bad install. That is not its purpose.