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Manual J opinion, please (x-posted in building)

Posted by cintijen (My Page) on
Wed, Aug 31, 11 at 21:50

This is new construction.

I finally got a copy of the manual J from our HVAC contractor today. (I've never met or spoken to him. It came via email through the builder.) During the bidding phase this contractor specified for our build a 3-ton Carrier geothermal heat pump with desuperheater. We will have electric service only, no NG or LP.
House is 2x4 construction, lots of windows, about 2300 sq ft finished on level 1 and 2 with an approx. 1300 sq. ft. unfinished walkout basement. We are on five very wooded acres and have 18" overhangs on the house.

I'm not sure what numbers off this manual J would be helpful in determining if it was run properly or not. The only thing that jumps out at me is that the Summer and Winter design outdoor temps seem pretty extreme (we're outside of Cincinnati, OH). For summer he has 105 and winter -25. Those are literally record temps here. Can anyone comment if these are the proper temps to use?

He did provide a breakdown for the sensible and latent gain and loss numbers for all the components of the house, and I can list them if they're helpful. The total sensible gain is 34,427, latent total is 2460, for a total gain of 36,887. Total loss is 79,094. Thus the rec for a 3-ton system.

I'm in way over my head on this one. Can anyone help me out here? I suppose most people don't even ask for the manual J, let alone challenge it.

Thanks,
cintijen


Follow-Up Postings:

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RE: Manual J opinion, please (x-posted in building)

I'm glad I don't live in Cincinnati - that is an unbelievable climate.....

Yes the design temps are wrong. There has to be a way to get the correct design temps. HVAC-calc has them but I was quite disappointed in the program so I can't recommend it. In Raleigh, NC our summer design is 96 and winter is 21 - just to give you an idea - although you may have no idea what the climate is here. I found a very complicated chart that had 12 and 90 for Cincinnati.

I am surprised that 2x4 walls meet code unless you are doing rigid foam to the outside. Even if it did meet code - I think it makes sense to be something more than r-13 to the walls. Generally with geothermal, you improve the envelope a lot so that you can downsize the units. You should be able to do $3000 extra in envelope and cut to 2 tons - saving $10k on the geo unit (as a total guesstimate example).

What really doesn't make sense is that heating should size your HPs and for that you need 6.5 tons. So IMO total garbage in and garbage out and then a good guess on size. Those numbers are horrible for new construction.

I've looked at 2 manual J's for new houses and both were done poorly but not nearly as bad as yours. At least the design temps were right. I would never expect tree shading to be done right on a manual J for example. Mine couldn't even count 10 ft porches (2 different contractors).


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RE: Manual J opinion, please (x-posted in building)

Thanks for your reply David. I was thinking of buying the HVAC-calc so I did. It gives outside design temps as 90 and 6 for Cincinnati, so pretty close to what you found.

I don't believe there is any issue with 2x4 construction up here. It seems to be typical. We are paying for some blown cellulose but it was discussed so long ago I can't remember where or how much.

The HVAC guy may or may not be right on sizing the system, but his manual J does not give me much confidence in him. I will be running the manual J myself today.

He's supposed to start running ductwork next week. :-/


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RE: Manual J opinion, please (x-posted in building)

OK, so I ran the manual J myself. The only thing I wasn't sure about was how to properly add in the basement. It's a walkout, and the foundation is 26x42. It's daylight on three sides, and at the rear wall (the 42' wall) we are about 4' out of the ground. On the side walls (the 26' walls) only about the first 3' are in the ground.

So I split the basement into two chunks, the in-ground part and the out-of-ground part and commenced entering the out-of-ground part as I would any other exterior walls. I didn't know what to call the floor for this section so I entered it as "Basement floor, 2' or more below grade". Does this sound right?

Anyway, figuring as best I could, it calculates a total gain of 32,578 (2.5 tons) and a loss of 48,960 (4 tons?).

I got clarification that our HVAC contractor is planning to install a 3-ton Carrier 50YD (27EER).

Any thoughts?


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RE: Manual J opinion, please (x-posted in building)

Probably too late but 2x6 walls or rigid foam to the outside would better balance your loss and gain and make it possible for 3 tons to work (perhaps along with upgraded attic insulation). What you may get with your current situation is an unacceptable auxiliary heat use.

Your treatment of basement sounds ok. Are your basement walls insulated? Basement floor? Basement ceiling?

Just to repeat - cellulose in a 2x4 wall is really unacceptable in your climate. Upgrading would absolutely have a good ROI.


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RE: Manual J opinion, please (x-posted in building)

David,

Yes, we are dried-in, so too late for 2x6 and foam board. Given that, what options should we be considering? Is it worth pricing foam?

The basement walls are 2x6 and 10" thick concrete. I have emailed builder about insulation in floors, wall and ceiling.

Thanks,
Cintijen


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RE: Manual J opinion, please (x-posted in building)

I wouldn't bother with foam. I said "cellulose in a 2x4" but really foam doesn't offer enough advantage to justify the cost.

I am still surprised that r-13 is code. I am sure than 50 miles up the road it isn't since you border a climate zone change.


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RE: Manual J opinion, please (x-posted in building)

My Carrier software indicates Cincinnati OH design temps are 6°F winter and 92°F summer. Is your contractor planning to install a 3-ton Carrier 50YD split unit or a mono block?

Which thermostat?

SR


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RE: Manual J opinion, please (x-posted in building)

SR,

I don't have a copy of the bid, but I googled the 50YD split and it mentioned a separate NG/LP powered air handler. We will only have electric service so I'm going to venture a guess and say monoblock.

I don't know what thermostat. I haven't asked about that yet. Do you have a recommendation?

Thanks for your response!
Cintijen


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RE: Manual J opinion, please (x-posted in building)

The split geothermal heat pump would be matched up with the Carrier Infinity FE4ANF00300 fan coil unit and Carrier Infinity thermostat.

SR


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RE: Manual J opinion, please (x-posted in building)

SR,

I will have to work on getting the specifics of the bid. I did however find out that the contractor is going to run the man j again and also have his sales rep run it. He knows I ran my own and got different results. He said his sales rep has more sophisticated software.

Another question, our ceiling will have r-38 insulation and I don't see that as a choice in the HVAC calc. Right now I have it entered as r-30. Will this make an appreciable difference?

Thanks again. I'm up against a very steep learning curve.
Cintijen


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RE: Manual J opinion, please (x-posted in building)

You could always take the line item for ceiling loss/gain and do some math. It is about 25% more insulation on one of the larger loss areas. Like I complained about before, HVAC-calc is really pretty bad. Can you not do r-40 which is closer at least?


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RE: Manual J opinion, please (x-posted in building)

If you start with the wrong numbers for the design you will get the wrong answer at the end.

My ASHRE handbook is at work, but the design numbers are not hard to find.
You need the winter number and both a temperature and humidity for the cooling load.

Those numbers do not look correct though.


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RE: Manual J opinion, please (x-posted in building)

Here is my latest manual J. I made some adjustments (figured out the roof, I was in the wrong category). I used SR's suggested summer design temp of 92 (HVAC-calc said 90). I also adjusted our indoor temperatures to what we are accustomed to using, although I'm not confident in our current thermostat. If anyone has any comments I'm all ears.

Thanks!
cintijen


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RE: Manual J opinion, please (x-posted in building)

Your infiltration number seems high for a new house. What did you use? The ceiling number also seems low. Not that it matters in your situation, but some guests over on a hot day would change your gain. It looks like you have 2 people in the house, you have to remember, you are looking at maximums - not the usual number. By necessity, the contractor has to put a higher number here.

My biggest complaint with HVAC-calc was no place to enter the SHGC of a window. Such an important large line item and no way to get it right.

A well designed new house will have more moderate thermostat settings than an older house. A drafty house will make you set it higher in the winter and because of humidity, a leaky house, will make you set it lower in the summer.
I've gotten my wife to be comfortable at 77 in the summer in our new house - at least 2 degrees more than before.


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RE: Manual J opinion, please (x-posted in building)

David,

For infiltration, I let HVAC-calc set those for me. It used .2 in the summer and .3 in the winter. I selected "Best" for construction tightness.

You're right, I had the people wrong. I thought I had put 4 downstairs and 2 upstairs but only the two upstairs were in the calculation. I changed it to 6 downstairs and 4 upstairs. We are a family of four but do have extended family who visit regularly (usually only two at a time). We rarely entertain more than a couple of people at a time, although I'm sure this will change as our kids get older.

I had wondered what HVAC-calc uses as default stats on double-pane Low-e windows. Just to add a bit of information, we have 56 windows in the house. There are 23 DH, 12 awning, 7 casement, and 3 fixed. We selected Integrity by Marvin, and the SHGC range from .3 to .33 and U-values from .28-.3.

We keep our rental thermostat on 68 in the winter and 72 in the summer, but I'm not sure how accurate it is. The house certainly seems warmer year round than the thermostat would indicate, so that's why I used 74 and 72 as cooling and heating temps, respectively.

For the ceiling, I used "under ventilated attic" and put in R-38 insulation. We are using trusses, and we have a ridge vent. Is that the right choice?

Thanks for continuing to help me along with this. The more I tweak my manual J, the more I think I really have no idea what I'm doing. I'm really interested to see the sales rep's results, if he is able to be any more accurate than I am.

cintijen


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RE: Manual J opinion, please (x-posted in building)

The ceiling sounds right. You have zero for duct losses - are they all in conditioned space?


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RE: Manual J opinion, please (x-posted in building)

I wasn't sure what to do about the ducts. None are in the attic, but all the first floor ducts will be in the unfinished basement. I looked back at the contractor's manual J and he had a gain of 1080 and loss of 2965 on the ducts, but I have no idea how he derived those numbers. He was using HVAC-calc 4.0, too, by the way...


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RE: Manual J opinion, please (x-posted in building)

I just received the Man J run by the HVAC contractor's sales rep. Can anyone help me interpret the results? I will be speaking to the HVAC contractor later today and would like to have a better understanding of this report before I do. What size system would you recommend?

Thanks,
cintijen


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RE: Manual J opinion, please (x-posted in building)

3 tons. Be sure to spec multi-stage or variable speed compressors, and variable speed air-handler to better handle your cooling(dehumidification).

Iam surprised the tonnage is that low based on the inefficiencies of your Building Envelope. Be sure you get good Blower Door Test results if you assume you will have tight construction.


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RE: Manual J opinion, please (x-posted in building)

"What size system would you recommend?"

That all depends. Full capacity heating on your Manual J seems to indicate 3-ton. Existing duct system seems to indicate a little over 2-ton. Installing a 3-ton system could be noisy and possibly result in the heat exchanger in the fan coil unit freezing up if sufficient air cannot be moved over it. 3-ton may also result in poor dehumidification and short cycling. Duct modification may be required to handle the additional cfm of a 3-ton system.

Sensible cooling seems to indicate 2-ton. You could install a 2-ton geothermal unit and be fine based on the sensible cooling load. This would mean that you would be OK for about 95% of the time in heating mode during winter. The other 5% to be made up by supplemental resistance backup heat. Remember, geothermal heat pumps have no defrost cycles ever. Supplemental or aux heat comes on in addition to the HP not instead of the HP so that the backup is on for relatively short periods of time even during that remaining 5% of the winter.

This means that the smaller geothermal system will be OK in cooling, with better dehumidification, provide heat almost all of the winter, be a better match to your duct work and cost a lot less to install in that you will require one less borehole and smaller equipment.

On the other hand you could install a 2-1/2 ton or 3-ton 2-speed compressor geothermal HP and be assured of full capacity heating 100% of the time. You should install a variable speed fan coil unit with stage resistance backup on any system.


SR


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RE: Manual J opinion, please (x-posted in building)

I certainly wouldn't go down to 2 tons. As you can see, manual Js can be all over the place. You have a 6 ton and a 3 ton. Unbelievable really. Oh I forgot - your own (probably the best) at 4 tons.

I'm not sure if this is a custom build or what. But you might consider an independent manual J - not by some salesman. I would hope you could fine some unbiased company that would let you look it all over and check for mistakes. The only number that should have so much variability is the infiltration and you might want to size after a blower door test if that is possible (it is but you may have to dig your geo wells before that point).

A geo company's motivation is to make your house seem really efficient so that you can get by with just 3 tons. The cost of the geo is lower making it competitive with other options. Then you run aux heat a lot which drives up your operating costs.

If it were me, I'd ask for prices on different tonnages. I suspect a 4 ton system would cost a decent amount less to run but also probably not pay for itself for a really long time. For example, it might cost $5000 extra but only save you $200 a year. Now given current mortgage interest rates, that is actually not a bad deal.

Are your ducts in because that is a consideration also?


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RE: Manual J opinion, please (x-posted in building)

Update after my husband talked to HVAC contractor...

He is recommending the 3-ton Carrier 50YDV, 27 EER, dual-stage compressor. He said stage 1 runs at 60% efficiency (?). He told my husband that there is an auxiliary heat source built in to this HP and that he estimates 4% auxiliary heat usage. Thermostat is the Edge model. SR, he said a fan coil is not necessary because we are not splitting the system. (?) Can anyone provide further feedback now that we have more equipment specifics?

David, ductwork is in. I think the plan is to drill wells before drywall, or maybe concurrent with. We've considered the independent man j but at this stage aren't sure if it makes sense to pursue this any further. At some point relatively soon we will get to the point of delaying the project over this. Funny, my concern was being oversized but you suggest perhaps we are undersized. So confusing.

Cintijen


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RE: Manual J opinion, please (x-posted in building)

Stage 1 is probably 60% of capacity - not efficiency. So it will act like a 1.8 ton on single stage. I wouldn't worry much about being oversized. I mean you are on the cooling side but you are in a heating dominated climate. At your lower stage, you will get decent dehumidification.

I am curious (because I am a permanent skeptic), do they give you any guarantee on that 4% number? ... I didn't think so.. It would be nice if they did or at least some cap but really do you have any recourse if your aux is 20%?


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