Please help me read my Manual J calculation report

11 years ago

So after questioning the size unit that was proposed for our new construction house, I asked for the load calculation report. They had not done one so they did it after my request. I don't believe that they actually came to the house but rather used the plans provided by the builder along with some other info. The problem is that they didn't account for some of the changes that we made such as:

First floor:

1. We eliminated a room that was approximatley 150 sq ft.
2. The eliminated room had 2 windows.
3. We eliminated a window from a closet.
4. The plan had a 3/4 glass front entry door, we have a solid wood door.

The report doesn't really tell me what else they put in the calculations but I know my builder said that he sent them the R-values. Of course, he also said that he told them about the eliminated room.

Second floor:

On the second floor, we made some changes as well. We added some square footage but I still think they are about 100+ over. We eliminated two skylights which I don't know that they eliminated from the calculation but will find out tomorrow. We are foaming the roof and I do believe that they factored that in. Also, it says that the upstairs unit will be a multi-zone unit but it will not. We have decided not to zone it based on previous experience. Will the zone (no zone) affect the calculation?

There are several of different numbers for the tons, net required, rec ton and actual. What is what?

Comments (6)

lexmomof3
Original Author
11 years ago
For system 1, the net required is 3.79 but they have proposed a 5 ton unit. Is the net required what it should be (rounded up)? Also, as stated, I believe the 3.79 is slightly over stated because it includes an extra 150 sq ft as well some windows that we eliminated.
mike_home
11 years ago
The changes you pointed out are probably not going to make a big difference, but the input should be as accurate as possible. Ask to make all changes are review it for mistakes. I suggest you also look at the indoor and outdoor design temperatures. These can have a big influence on the calculations.

If the cooling requirement is 3.79 tons, then you would round up to 4 tons.

If you have natural gas, then why aren't you getting a 95%+ furnace and eliminating the heat pumps? Did I miss something from your previous posts?
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lexmomof3
Original Author
11 years ago
No Mike, you didn't miss anything. The hvac company did. We are supposed to have NG for unit 1 (downstairs) per the contract with our builder but apparently the hvac company missed that bit of info.

Is the actual ton (4.83) specific to the unit specified in the report? I assume that is a 5 ton unit????
lexmomof3
Original Author
11 years ago
Also, the units in the report are what was proposed and in the bid but we are planning to upgrade the SEER. Just trying to work out the size of the unit so that we can get the price on the various SEER levels.

Another question: when we decided to foam the rafters, the hvac company said that would allow them to downsize the second floor unit and they gave us a $1000 credit which we're planning to use to upgrade the SEER. Would the foam not also affect the size of the downstairs unit? Our contract only specified the type of unit and SEER rating but didn't give a budget. We were told the foaming would reduce the cost of the hvac equipment but not by how much. I guess I've lost some trust and my concern is that a 5 ton system was budgeted but only a 4 ton system is needed and we won't get credit back for the reduced size needed for the first floor. Is it fair for me to ask what amount was budgeted for the hvac equipment? It is not a cost plus contract but rather a fixed price. Our builder is good about changing things and if it's cheaper, he adjusts the amount in our favor. Of course some things have been more but he's been very reasonable and I think he's just basically charging us the actual amount.
lexmomof3
Original Author
11 years ago
Just talked to hvac guy. He is updating the calculations. I don't think he factored in the low e glass windows either. At the end of our conversation, he said that felt sure the sizing would be more like 3.5-4 tons for for the downstairs unit. I made sure he knew we are not looking for it to be a particular tonnage but the 5 ton unit that was proposed seemed oversized based on some of the changes and upgrades we made to the plan. We want to make sure it was calculated based on those factors. So, for now, I wait for the revised report but I feel so much better about things. I appreciate all of the advice!
mike_home
11 years ago
AC and heat pump units are quoted in nominal sizes. This is in either whole ton or half ton increments. The actual sizes are not exactly the nominal size. In this case the 5 ton heat pump has a size 58,000 BTU. The actual size is also affected by the size of the coil, furnace or air handler model. In general you never get exactly the nominal size. In most cases it is slightly smaller, usually within 5%.

Foaming the rafters has a big effect on the second floor, but no effect on the first floor. The second floor is conditioned air space so the ceiling of the first floor contributes no heating or cooling load to the first floor. This assumes you are going to keep the first and second floors at about the same temperature.

In general builder's know very little when it comes to HVAC design. They rely on the HVAC subcontractor to do the job at the budgeted price.

I think you have every right to talk to the builder about the HVAC budget and fixing the mistakes the HVAC contractor has done so far. If it were me, I would bring up the idea of hiring another HVAC contractor. This outfit has made so many bad mistakes that I am not sure I would trust them to do a good installation.

Please help me read my Manual J calculation report

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Please help me read my Manual J calculation report

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