Mini-split sizing and selection help

[270win]

Hello Everyone! First time posting.

I would like to install a DIY mini-split in my house and am looking for some input on equipment selection.

So far it appears that the MrCool DIY line of equipment is the most user-friendly package. I want to use a two-zone system with the ceiling cassettes in the main living level of my home

Air conditioning is the primary reason for doing the installation, but the heat will also be used. The math shows me that at current electric rates, it is even more economical than my woodstove, provided I’m not cutting my own wood!

Up until now, I have been cooling my house in the summer with 1 or 2 5000 BTU window units and one 5000 BTU portable A/C. They are sufficient and keep the temps and humidity down, but I’m sick and tired of the noise. I used to get away with just two 5000 BTU units, but summers are a bit warmer now and I believe the blown in insulation in my attic has compressed and I’m getting more heat through the ceiling.

My house is ranch style, so I was thinking that I’d put one cassette in the main living area which is open concept, and one cassette down at the bedroom end. At first I thought that the 27,000 BTU 2 zone system with a 9,000 and 18,000 ceiling cassette should do the job. The18,000 cassette would be for the main area, 9000 for the back bedroom area. However, after looking at the Heat Calc output it seems that I should use a smaller unit. The way my house was built, I don’t think the “Rule of Thumb” calculations are going to work. So maybe a smaller unit is appropriate.

Location is in Southern New Hampshire

Other than watching several install videos, I don’t have much more than that to start.



I did a heat load calc on my house when I built it 17 years ago and managed to find a copy. I have included the report. For this installation the basement is not included, only the upstairs. So I would reduce the total by that amount, correct?

My math shows these numbers for the upstairs:

Total Heat Gain 19,225 – Basement Heat Gain 8,367 = Upstairs Heat Gain 10,858 BTUH

Total Heat Loss 30,376 – Basement Heat Loss 12,912 = Upstairs Heat Loss 17,464 BTUH

What do you all think about the sizing?

Comments, questions or advice?

Thanks

 

[Fitter30]

Call your electric company see if they offer a energy audit that includes a blower door test if they do they are very reasonably. Plus there could be some incentives. Your biggest load is heating not cooling. If you could cool with 10k btu 2 window shakers and were kind of happy except for the noise might want to look into their small 2 zone hyper heat equipment. Size a unit for the heat would be oversized for cooling. Call a contractor out get their input.

 

[Sponsor]

 

[270win]

Call your electric company see if they offer a energy audit that includes a blower door test if they do they are very reasonably. Plus there could be some incentives. Your biggest load is heating not cooling. If you could cool with 10k btu 2 window shakers and were kind of happy except for the noise might want to look into their small 2 zone hyper heat equipment. Size a unit for the heat would be oversized for cooling. Call a contractor out get their input.

I just read up on the Blower Door test. It seems that the purpose is to determine air leakage.

How does that relate to the Heat Load calculation? Does it effectively create a multiplier to the Heat Load Calc? i.e. - more leakage adds to the heat load?

Thanks

 

[wwhitney]

A few comments, without attempting to provide a comprehensive overview:

1) It doesn't make sense to ignore the basement heat gain/loss when you are sizing equipment, even if the basement is unconditioned, as that heat is still gained or lost from the thermal envelope and so will need dealing with. I guess if you consider the R-value and area between the conditioned space and an unconditioned basement, and the R-value and area between the unconditioned basement and the outside, you can determine the equilibrium temperature of the unconditioned basement, and then use that to scale the heat loss/gain calculation from a conditioned basement to the case that the basement is unconditioned.

I.e. if the design heating delta T is 60F, and the basement comes out to a heat loss of 300BTU/hr/F, and the heat exchange from the conditioned space to the basement is 600 BTU/hr/F, then the equilibrium temperature of the basement will be 20F below the conditioned space (as 300 * 40F delta T = 600 *20F delta T). So you could 40F * 300 BTU/hr/F basement contribution to your conditioned space heat loss, rather than the 60 F * 300 BTU/hr/F contribution you'd have if the basement is conditioned.

2) I briefly looked at your manual J, and IIRC the outside summer air temp was 87, with an inside temp of 75, so a 12 degree delta. If it ever gets hotter than 87, and that manual J is accurate, then if you size the equipment to it, and it gets to 92 out, it will get to 80F inside, etc. So just wondering if it regularly gets hotter than 87 in which case those may not be the best cooling design temperatures to use. On the other hand, if you have been comfortable using just 15000 BTUs/hr of cooling capacity, maybe some of the inputs to your manual J are unreasonably pessimistic.

3) As to air leakage, total heat/loss gain will be the sum of the conductive heat loss/gain and the heat loss/gain from air exchange. So it's an effect that you add to the conductive portion of the manual J. And it's also an effect that you can measure directly with a blower door, at least for any given level of wind-induced pressure difference between inside and outside. Conversely, if you have an effective heating or cooling system that can maintain a fairly even temperature, and you can accurately document the run time and outputs of that system for a given indoor and outdoor temperature conditions and wind-induced pressure difference between inside and outside, you can also measure the total heat loss/gain directly and compare that with your manual J computation.

Cheers, Wayne

 

[faucetfreak]

What do you all think about the sizing?

• Main Living Area (Ceiling Cassette): An 18,000 BTU unit seems appropriate given the larger space and potential heat gain.
• Bedroom Area (Ceiling Cassette): A 9,000 BTU unit would likely suffice for the bedroom if it is not excessively large.

 

[270win]

Wayne,
Thanks for your reply. I had to read it more than once to digest it

1. Basement. Very good point here. My basement is about 75% finished space with an in-law apt. As it is a walkout there is quite a bit of window space in the living area. R-19 in the floor between up and down. It is heated in the winter and we use dehumidifier in the summer to keep it dry. Unfinished space stays cool in summer and finished space does have some solar gain for sure. Exterior foundation is wrapped in 2" foam board. (Back when it was cheap!)

2. Manual J numbers. I don't think that they are super pessimistic as much as I don't really like lots of AC and I freeze in most restaurants and stores in the summer. So when I say 15,000 has been working it means the temps inside on a 90 degree day may very well be 77 but its dry and comfortable. Taking into consideration my AC units are getting on in age, who knows what they really generate.

3. I'd certainly need a blower test for accurate numbers on leakage. I built the house pretty tight and actually open windows in the winter for some fresh air. I really would have no way to perform the test that you mention at the end, although that seems like the only true way to measure ones heat loss.

With all of these factors, how could anyone have come up with a "rule of thumb" for sizing HVAC???

I guess I should concentrate on sizing this unit just for cooling as that was the main reason for considering mini-splits. The heat function will just be a bonus.