Calculating and Designing a New HVAC system

[Jeremyt]

Good evening everyone. I am new to the forum, but I have been lurking for months. After much debate I decided that this may very well be the best place for me to ask question and brain storm ideas.

We purchased a home in North Central CT last August. Its a disaster. I knew going in that it needed a lot of work, but its going to end up being a complete gut and renovation. Its 2800 sqft in total, consisting of two floors, with a bonus room over the garage. I am in the middle of constructing an addition of the garage now. I do race car things from time to time, and I would consider myself an extreme DIYer... HAHAHA

Anyways, I have been kicking around calculations on the CoolCalc website, and it seems as though Im either missing something, or its telling me that I live with my windows open 24/7 and 365 days a year. I would like to run a ductless heat pump in the bonus room, and now that Im working on the garage addition I think Im going to condition that space as well. So that would be 2 zones. Then the main house I would like to install ducts, while having an upstairs zone and a first floor zone on a single pump.

So the garage will be 24x50 with ten foot ceilings. It has three 9 foot wide insulated garage doors, two are 7 feet high and one is 8 feet high. The construction is 2x6 and will have Rockwool R23 in the walls. The ceiling joists are filled with Rockwool R30 right now, and I will be putting another R15 in the walls before sheetrock. The floor is concrete slab and un-insulated.

Here are the results I get
42,930 Heating BTU
17,603 Cooling BTU
0.912 SHR
810 CFM

Is this really a correct calculation? For a single floor garage with tight walls and new insulation and sheathing? Thank you everyone..

 

[Dana]

If that's just the heat load for a 1200' garage with 2x6/R23 walls and an R30 ceiling it seems high, even though garage doors are leaky as hell and pretty low-performance even if "insulated". How much is it showing for just the garage door & window losses? (Half? Two-thirds?)

How are you planning on adding another R15 to the 2 x 6/R23 walls?

A whole-house heat load of 42,930 BTU/hr for a 2800' house @ +5F (or thereabouts) is a ratio of ~15 BTU/hr per square foot, which would be somewhat high for 2x6/R23 rock wool construction house with low-E windows, but realistic enough for 2x4/R15 with clear-glass double panes. But the house doesn't have three 9' wide leaky garage doors.

Is the basement insulated to the current IRC 2018 R15 continuous insulation or better?

 

[Sponsor]

 

[Jeremyt]

Dana, thank you for the reply. I will be adding another R15 into the ceiling of the garage/floor of the bonus room over the garage. The floor joists are 2x12. Currently I have R30 in there which is 7.25 thick, so there is still room in the cavities to add more. The walls are 2x6 construction and will be R23 when they are full. With CoolCalc Im not sure how to calculate for only the door and window losses.

The basement has no insulation. It is just bare concrete walls.

My end "goal" would be to have two outside units with four zones. One unit running a zone for the garage, and a zone for the bonus room over the garage. Then a unit running the upstairs zone and the first floor zone. This is for a few reasons. The house is a traditional cape, then there is a first floor addition with cathedral ceiling, which has a three foot tall crawlspace underneath. Then the garage is on a slab with the bonus room over it. There really isn't any feasible and cost efficient way to connect the HVAC between the main area of the house and the bonus room/garage without some serious wall removal in the basement. Another reason is that I believe it will allow me to calculate these as separate areas, which I think might be easier for me to grasp the concepts and design of the systems. The last reason is up front costs. The garage is being renovated now, and then the bonus room will be next. I can purchase and install the system for these two zones. Then I can move on to the main floor and upstairs at a later date as funding allows.

I might add that in regards to heating the garage. It won't really be a "livable" area. More of a "keep it above freezing" area. The race cars only have water in their cooling systems so they can't freeze. There are also some fluids out in the garage in storage that can't freeze. The AC will be nice in the hottest of summer days, but in the winter the normal conditioned temperatures will probably be set around 50 unless Im out there all day working, then I may turn it up.

 

[Jeremyt]

Oh wait, you're up in Worcester??? I grew up there. Over in the Webster Square area.. My parents still live there, which is the main reason we moved back up from North Carolina after 21 years away....

 

[Dana]

Dana, thank you for the reply. I will be adding another R15 into the ceiling of the garage/floor of the bonus room over the garage. The floor joists are 2x12. Currently I have R30 in there which is 7.25 thick, so there is still room in the cavities to add more.

If the bonus room and garage are both heated, the ceiling/floor insulation buys you nothing. If the garage is unheated, R30 is the current IRC code minimum for floors over unheated space. If it's partially heated (some mini-splits can be set to 50F, but not all, and none go lower than that), the additional R15 isn't buying much. Air seal and insulate the rim joist to at least R20 (current code minimum), but don't bother with raising the R-value of the floor/ceiling to R45. From an energy cost and comfort point of view you'd be better off applying that money toward rectifying uninsulated areas, such as...

The basement has no insulation. It is just bare concrete walls.

Even if the basement isn't actively heated, uninsulated concrete foundations walls are a HUGE heat loss, particularly on the above-grade section, and even if you let the basement drop to 50F. An 8" concrete wall is about R1.35, with air films about R2 (on a calm, windless day). When it's 25F outside (roughly your mean January temperature) , 50F inside is a 25F temperure difference- every square foot of exposed above grade concrete is losing 25F/R2= 12.5 BTU/hr (or more.) With 2' of exposure and a 150' perimeter that's 300 square feet x 12.5 BTU/hr = 3750 BTU/hr for just the above-grade portion(!). The below grade portion is losing heat at a high rate too, just not as high per square foot as the above grade portion. If it's warmer that that in the basement when it's 25F outside the losses are larger still. In most otherwise reasonably insulated homes that adds up to 15-25% of the total heating bill in your/our area.

Adding 3" of cheap reclaimed roofing polyiso brings it up to current code minimum and cuts the foundation losses by a full order of magnitude, even if the basement now idles at 60-65F instead of 50-55F.

My end "goal" would be to have two outside units with four zones. One unit running a zone for the garage, and a zone for the bonus room over the garage. Then a unit running the upstairs zone and the first floor zone.

Don't get too committed to the number of compressors or the zoning scheme until you have better handle on the room by room heating and cooling loads. Multi-split systems have fairly high minimum compressor speeds, and two multi-split compressors will likely have a combined minimum output that's too high to operate efficiently- they'll be cycling on/off even in relatively mild weather if oversized for the zone loads. Often the wider modulation ranges of a few single-zoned mini-splits ends up being more efficient, despite the increased number of compressors (all with much lower minimum output.)

This is for a few reasons. The house is a traditional cape, then there is a first floor addition with cathedral ceiling, which has a three foot tall crawlspace underneath. Then the garage is on a slab with the bonus room over it. There really isn't any feasible and cost efficient way to connect the HVAC between the main area of the house and the bonus room/garage without some serious wall removal in the basement.

It doesn't take a very big hole to run refrigerant lines to other zone cassettes/heads tied to a single multi-split compressor.

Another reason is that I believe it will allow me to calculate these as separate areas, which I think might be easier for me to grasp the concepts and design of the systems. The last reason is up front costs. The garage is being renovated now, and then the bonus room will be next. I can purchase and install the system for these two zones. Then I can move on to the main floor and upstairs at a later date as funding allows.

It's often (or even usually) cheaper to buy multiple right-sized single zone mini-splits.

I might add that in regards to heating the garage. It won't really be a "livable" area. More of a "keep it above freezing" area. The race cars only have water in their cooling systems so they can't freeze. There are also some fluids out in the garage in storage that can't freeze. The AC will be nice in the hottest of summer days, but in the winter the normal conditioned temperatures will probably be set around 50 unless Im out there all day working, then I may turn it up.

That sounds like a good plan, but calculate the garage load at both 50F indoors an 65-70F to be sure and size the head correctly. If you're never going to be working in that space when it's single-digits outside the load at 50F is good enough.

Even though it's not as flexible and not a fully ACCA Manual-J listed tool (like CoolCalc is), if you assume zero air leakage & ventilation and use reasonably aggressive inputs LoadCalc.net at least gives you a short list break down of the loads by assembly type, to sort out how much was window loss vs. wall loss vs. floor loss, etc. It'll probably shoot 25%-35% high, but not 2x reality. It won't let you adjust indoor & outdoor design temps though. I've not spent enough time looking at or playing with CoolCalc to point out where the pitfalls are with that tool, but I've seen multiple reports (like yours), with some truly crazy numbers when operated by relative newbies.

For the raw basics, an I=B=R type spreadsheet calculation will be good enough for sizing heating equipment (won't do squat for cooling calculations) and is part of the fundamental approach underpinning Manual-J. I=B=R usually overshoots reality by (only) 15-25%, even when the U-factors have all been diligently calculated using parallel path models, and if you're doing your own it's easy to make adjustments/corrections.

 

[Jeremyt]

I see your logic regarding the R15 in the ceiling/floor. I think the R30 will be beneficial because the temperature differential between the garage and the bonus room over it may be more than 20 degrees at times once both rooms are completed, but it won't be unconditioned space.

I have kicked around the idea of some. sort of insulation in the basement. As of yet I haven't made any plans yet to do any insulating down there. Regarding the HVAC equipment, I am not committed to anything yet. I prefer ducted, and will more than likely be utilizing some sort of ducted system in the main house. I just think it will be magnitudes easier to install ductless heads in the garage and bonus room. Im sure I will be working in the garage during the coldest days of the year. For this reason I need to make sure it will be efficient at 70ish degrees indoor temp. Ive never been on LoadCalc but I will check it out, thanks for the link. Regarding HVAC, I am a complete newbie. But I like to learn, research and ask questions. And once it comes to installing I usually do pretty well for myself.

Regarding the ductless aspect of the design.
The garage will be 24'x50' with a 10 foot flat ceiling. The garage doors will be on the 50 foot wall. Will a single head placed opposite the garage doors be sufficient from an air movement standpoint? Would it be better to put a single head at the long end instead? Or will I have to run more than one head to get enough airflow to mix everything together?

Same question for the bonus room. It is 24x25 with a 12 foot cathedral ceiling. It has a 9 foot wide window on one end and a 3 foot wide window one the other. My plan is to place a single head over the three foot wide window. Will a single head have enough airflow to make the room comfortable. This room will be considered living space, and will maintain 68-72 degrees throughout the year.

 

[Jeremyt]

Using LoadCalc I have wildly different results.. I don't know if the upload with the images worked.

 

[Dana]

I see your logic regarding the R15 in the ceiling/floor. I think the R30 will be beneficial because the temperature differential between the garage and the bonus room over it may be more than 20 degrees at times once both rooms are completed, but it won't be unconditioned space.

If the garage is kept at 50F and the bonus room 70F how is that more than a 20F difference?

I have kicked around the idea of some. sort of insulation in the basement. As of yet I haven't made any plans yet to do any insulating down there. Regarding the HVAC equipment, I am not committed to anything yet. I prefer ducted, and will more than likely be utilizing some sort of ducted system in the main house. I just think it will be magnitudes easier to install ductless heads in the garage and bonus room.

You can do a lot with ducted mini-splits these days. Right-sized modulating systems are way more comfortable (and quiet!) compared to the on/off 1 or 2 stage hot air furnaces or heat pumps.

Regarding the ductless aspect of the design.
The garage will be 24'x50' with a 10 foot flat ceiling. The garage doors will be on the 50 foot wall. Will a single head placed opposite the garage doors be sufficient from an air movement standpoint? Would it be better to put a single head at the long end instead? Or will I have to run more than one head to get enough airflow to mix everything together?

A single ductless head 25' way from the doors blowing toward the doors, or sub-ceiling type cassette 15' away blowing toward the doors will usually do a decent job of mixing. At high speed the standard high-wall units can throw more than 25', but 50' would be a stretch. Blowing toward the middle zone of where the biggest heat loss is (the doors) mixes it up better. If in practice the temperatures seem uneven when letting the mini-split choose the blower speed, they can usually be programmed to always operate at a user-selected blower speed. Bumping it up a notch or two will fix that, at a very modest hit in efficiency (due to higher than optimal blower power being used.)

Same question for the bonus room. It is 24x25 with a 12 foot cathedral ceiling. It has a 9 foot wide window on one end and a 3 foot wide window one the other. My plan is to place a single head over the three foot wide window. Will a single head have enough airflow to make the room comfortable. This room will be considered living space, and will maintain 68-72 degrees throughout the year.

It would be more comfortable if the head were above the 9' window blowing toward the far end. That will disrupt the convective flow of cold air sinking down the cold glass of the larger window, and pull some of that colder air up toward the ductless head.

If there's room for a floor cassette rather than a high-wall unit, putting that under the 9' window (or putting a high-wall unit below the window) would be more efficient, since there's usually a pool of colder air below big windows.

^^floor console unit^^ Floor units are usually thinner profile than high-wall units eating up less floor area.

^^^ Low-mounted high-wall unit^^^ (Seems dogs always know how to find the warm spot in winter. )

In heating mode mini-splits generally run more efficiently when closer to the floor, taking in cooler air for a bigger delta-T through the coil. But it's not always practical to put it near the floor.

 

[Dana]

Using LoadCalc I have wildly different results.. I don't know if the upload with the images worked.

The images worked, but I don't see any square footage entered for windows or doors, and that's likely to be the single largest loss assembly type for the garage.

 

[Jeremyt]

I apologize for the delay. We have been hard at taking full advantage of the recent weather and getting the foundation walls poured, backfilled and compacted before it started snowing again yesterday. I guess Mother Nature had an eye on us.

In regards to the temperature differential between the garage and bonus room. Im sure there will be days (like today) where it is rather cold outside and I am on shift for 48 hours. So the wife may want to occupy the bonus room for the day, and no one will be in the garage so there is no point fighting the weather. So long as it stays above freezing it'll be fine for the temperature in the garage to be lower than normal. Or if we went out of town for a week. No point keeping the garage at 50 when its not occupied, so long as it stays above freezing. I can't say whether this will happen often or not, but I can only assume that it will happen.

I really need to actually draw up and get some visuals with the garage as well as the bonus room so you guys can see exactly what I am referring to. With a low mounted unit under the window in the bonus room there just simply isn't enough space. The window sill might be at most 18-24 inches from the floor. Also, the large window faces the front of the house. If I were to do something on that side of the house, the lines would be in excess of 80 feet. From what Ive read this isn't a huge deal, but you begin to lose efficiency with longer line sets. Or am I wrong here?

I didn't see anywhere to input square footages, but I probably just missed the area to put it in. I need more experience with these programs.

 

[Dana]

Refrigerant line lengths do matter, primarily from a capacity rather than efficiency perspective. The capacity derating for longer lengths (and sometimes up-rating for shorter lengths) are usually spelled out in the installation & engineering manuals.

 

[Jeremyt]

Ok I have had a morning to sit down and go through the garage calculations again. I think I solved the window and door square footages, as well as noticing that I had not added the ceiling sqft into the original calculations.

 

[Jeremyt]

Got laid up with a back thing, so Ive been on the couch the past week or so. I was able to do some more measuring and calculating concerning the garage. Hopefully this may help?



Garage:

24’W x 50’L x 10’H

All insulation is mineral wool


Garage Doors are Clopay 4050 at R6.5

2 doors are 9’x7’

1 door is 9’x8’

4 windows at 57”x30”

U factor 0.27

SHG 0.27

VLT 0.49

CR 61


Ceiling is 24x50 (1,200sqft)

24x25 (600sqft) is R30 insulation under the bonus room

24x25 (600sqft) is R30 under a roof


Floor is 24x50 (1,200sqft) no insulation concrete


North Wall

50x10 (500sqft)

25x10 (250sqft) is R23 to the exterior

25x10 (250sqft) is R23 to the heated interior of home


East Wall

24x10 (240sqft)

2 windows totaling 1710sqin each, 11.875sqft each or 23.75sqft total

216.25sqft minus windows at R23 insulation


South Wall

50x10 (500sqft)

3 garage doors

9x7 door is 63sqft (2 doors)

9x8 door is 72sqft (1 door)

198sqft of garage door space at R6.5

302sqft of wall with R23 insulation


West Wall

24x10 (240sqft)

2 windows totaling 1710sqin each, 11.875sqft each or 23.75sqft total

216.25sqft minus windows at R23 insulation

 

[Jeremyt]

Using the information I posted above, I went back into CoolCalc as well as LoadCalc. I still get extremely different results between the two. 18,827 heating BTU and 7,482 cooling BTU on Load Calc. 30,633 heating BTU and 9,367 cooling BTU on CoolCalc.

 

[Dana]

Using the information I posted above, I went back into CoolCalc as well as LoadCalc. I still get extremely different results between the two. 18,827 heating BTU and 7,482 cooling BTU on Load Calc. 30,633 heating BTU and 9,367 cooling BTU on CoolCalc.

If that's for the bonus room only (not the whole house) the CoolCalc result for heating load is simply insane. The LoadCalc numbers are a bit high (as I would expect from that tool), but not by 2x.

The CoolCalc heat load pie chart shows an enormous amount of loss through the floors as well as very high infiltration losses. You may want to revisit any inputs that would affect those numbers.

 

[Jeremyt]

I have abandoned the bonus room for now, and am focusing on the garage. Those are for the garage. Uninsulated concrete floor, 3 insulated garage doors with R6.5 insulation. I am hoping to nail down one of these two rooms with some concrete numbers, and then work on the other. I would really like to run a single condenser with two zones. One for the garage and one for the bonus room above it. But if I need 30k plus just for the garage I don't know if its feasible. That seems nuts still...

 

[Dana]

I simply don't trust/believe CoolCalc's loss numbers for the garage slab, which appears to be about a third of the total. What did the table that breaks down the load by component in LoadCalc come up with for slab losses?

Is there any slab edge insulation (or can you add some)? (Continuous R10 down to 2' below grade is current IRC code-min for slab edge insulation in zone 5.)

 

[Jeremyt]

Yea I am not understanding the disparity between the two. I have attached a screenshot of the chart at the bottom of LoadCalc. There is zero slab edge insulation, and really no way to get any down there. The driveway pretty much covers most of the exterior concrete on that side of the house.

 

[Dana]

LoadCalc's table is showing 4210 BTU/hr of slab loss out of a total 18,827 BTU/hr, or about 22% of the total.

CoolCalc shows a bit more than a third of the total 30,633 BTU/hr heating load as slab loss, which would be over 10,000 BTU/hr (and pretty ridiculous), and about 20% of the 30,6K (about 6000 BTU/hr) as infiltration/ventilation loss, which is probably more than twice reality unless that garage door has the world's worst weatherstripping. Without knowing all the inputs that were entered into the tool it's hard to say why those slices of the CoolCalc pie are so large, but they don't seem very realistic.

Knocking CoolCalc's slab loss & infiltration loss numbers back by half the results are within a stone's throw of each other.

How much above grade exterior exposure is there on the stemwalls of the foundation?

Insulating even part of the foundation makes a difference.

 

[Taylorjm]

I'm sorry, but personally I think the calculations are crap. First, your talking about a garage, and I assume you are going to be heating it low until you go out there and want to turn it up. There's no way in CT winters, you can heat a garage from 45 degrees up to 65 degrees with just 43,000 btu. I'm in Michigan with a single car garage shop that is well insulated and about 12x30 and I have a 80,000 btu gas furnace. I keep it at 45 and turn it up to 65 when I go out there and that will heat it up within 10 minutes. Yours is 24x50. That's 4x larger, and your wanting to use half of the heating btu? It will never work. I'd be going with 120,000 btu and that would be a gas furnace at a minimum. As for your bonus room above the garage. Whatever your cooling calculations tell you, double it. With all the heat from the roof beating down directly in that room, you will need much more cooling and airflow in that room to compensate. I would spray foam the ceiling in that room if I had to do it again. This is from experience in our bonus room above the garage.

As for calculations. When we purchased our 2 story, 1800 sq ft house, there was a 3 ton central air system. No matter what the house wouldn't stay cool. We replace the furnace and central air and the calculations said the 3 ton would work. I told the hvac contractor no way, but he insisted. After it wouldn't cool the upstairs, he actually told me it would help if we changed our shingles on the house to a lighter color. Uh huh, yeah, right. They came back and put in a 5 ton unit. That helped, but what really helped was installing thermostats in each bedroom and a zoning board and dampers. I also added hydroponics blowers to the upstairs runs. The hvac guy said there's no way that would work. Well, I cool the upstairs in the middle of summer to a meat locker if I want without affecting the main level. So, sorry, but the calculations are crap. Get someone with real world experience in there to tell you what you need.