Manual J for single zone mini split in basement

3 years ago

I'm finishing about 1000 sq. ft. of my unfinished basement for a recreation room and media room. The basement is mostly underground and well insulated, and it's comfortable year round even on the coldest and hottest days. I had someone check the capacity of the HVAC, and he said there's not enough to handle the extra space. The county said that I have to provide heating for the finished area. I was originally going to use baseboard heating, but it required 32 total feet of baseboard heaters, which would severely limit where we could put furniture, so we decided to go with a mini split. I told the person reviewing our permit, and he said that I would have to provide manual J and S calculations. I've found some websites that have free manual J calculators, but they require information for the entire house. I just need a calculation for the basement. Is there a formula I can use where I can plug in the cubic footage, insulation R value, and such, and get an accurate calculation? Thanks!

Featured Answer

mike_home

3 years ago

I told the person reviewing our permit, and he said that I would have to provide manual J and S calculations.

I am not sure a free online calculation will be acceptable for the code official. These free calculators usually don't conform to the Manual J requirements. You can get the homeowner's version of HVAC-Calc for $49. Here is the link: HVAC Calc You can treat the basement as it's own space. You don't need to calculate the whole house. The cooling load will be very small. I expect the heating load to also be small.

The basement is mostly underground and well insulated, and it's comfortable year round even on the coldest and hottest days. I had someone check the capacity of the HVAC, and he said there's not enough to handle the extra space.

I am skeptical the person who told you this information did anything to verify there is not sufficient heating capacity to heat the basement. Installing a mini split would be overkill in my opinion.

I was originally going to use baseboard heating, but it required 32 total feet of baseboard heaters.

How did you calculate the 32 feet of baseboard heating? I assume this electric heating which would be fine for your situation in my opinion. My recommendation would be to install electric baseboard wired to 240 V. You can buy an 8 foot 240 V baseboard heater for about $129 in Home Depot. The specification sheet says it will put out 8530 BTUs when wired on a 15 amp circuit. I think one of these should would be sufficient. You can buy two small units and put them on opposite sides of the space.

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Comments (6)

HU-367180668
Original Author
3 years ago
Thanks for the HVAC Calc link, Mike. I'll check it out this evening. I might also get a second opinion about extending the current HVAC to the basement. I do know that the basement wasn't included in the original calcs when the house was built in 2021. I recently had the county send me the entire permit package, and went over it thoroughly. The formula for determining the baseboard length was given to me by the guy reviewing the permit application. Here it is: width x length x height /250 = bb length.
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mike_home
3 years ago
That formula makes no sense to me.
PRO
Charles Ross Homes
3 years ago
Some folks use a simplified method to calculate the required heat load assuming modern construction and insulation methods. It's the room area (width x length) x 10 divided by 250 watts/linear foot of baseboard heater (assuming a 240 volt heater.) Substituting a value of 8 for the 10 in the calculation would get closer to the heat load for a basement depending on whether there are exterior doors, windows, and a significant portion of walls above grade. I suspect the contractor gets to that end by using a ceiling height of 8 ft. or 9 ft in the calculation.

The OP notes that the basement is comfortable year 'round so I suspect the home's existing HVAC system could handle the small incremental heating and cooling load. That said, I would consider a heat-recovery or energy-recovery ventilator to maintain indoor air quality in the basement.
mike_home
3 years ago
The adding the basement is an option but the drawback is there is the thermostat is on an upper floor. This is how it works in my house and from experience it is not ideal.

I am a proponent of doing Manual J calculations. However in this case it is overkill. If eletric baseboard heating is 99.9% energy efficient. If the baseboard is oversized, then the run time will be less and approximately the same amount of energy is used. The caculation would be helpful to determine whethen the existing HVAC system can support the additional heating load. I suppose the code offcial is not allowed to issue a permit unless a Manual J calculation is attached to the permit application.
PRO
Charles Ross Homes
3 years ago
Some 25 years ago I had a home in PA with a basement where 50% of the area was converted to conditioned living area. The incremental heating and cooling load was insignificant. I added a couple of supplies with balancing dampers and a single return. Heating and cooling for the basement was controlled by a thermostat on the 1st floor which wasn't ideal, but the air flow was adjusted down to a point where it didn't matter. An alternative that is available today is to use remote wi-fi enabled thermostats and either average their temperatures or vary which one controls based on what level is occupied.