Did a Heat Loss calc and I'm surprised at the result. What do you think?

MSmithHH

So I did the heat loss calc, as suggested, for my 3500 sq ft of heated space (natural gas boiler with baseboards). I have many large double paned windows, multiple rooms with high ceilings, and a bunch of skylights in various rooms. I was very surprised that it appears to be reporting I need a 65k btu boiler (that's what the calc'ed numbers mean, correct? Says Heat Loss BTU/HR = 64718). I'm surprised because I got 3 quotes and 2 said I needed a 100k btu mod/con with indirect water tank and 1 said a 110k btu cast iron boiler. Only one appeared to be gathering detailed measurements for a heat loss calc.

A few things that could affect my numbers:

1) I was surprised that I wasn't entering my actual baseboard lengths in each room rather than just seeing the suggested. Am I supposed to enter that somewhere?

2) I have insulation but I don't know what type. What's a good assumption for a large house built in 1989? I chose 2" insulation for walls and ceilings along the roof

3) I do have some large open entry ways between rooms that are hard to combine into 1 room. Like a living room with 7.5' ceilings that opens up to a foyer with 16.5' ceilings, so I left them separate.

4) I did outdoor temp of 20 degrees (Massachusetts and looking at temp history seems like it's typically 22 in the coldest months) and an indoor temp of 72 degrees, which is what we often use with kids. It never asked me but it said design water temp of 180 degrees. Are these good numbers?

5) If it's an indirect water tank, it never discusses water demands, which can be high for us with a family of 5.

6) I did not record interior doors to heated spaces or entryways.

So what do you think? Does a 65k btu boiler seem inline? Also, does this tell me whether or not I'd be better off with a mod/con vs a cast iron?

THANKS!!!!

SlamDunk

Are the 100k and 110k input input ratings are out put?

MSmithHH

Hmmmm...Not entirely sure, but they were phrased like this on the quotes:
"Weil Mclain 110k btu WM97+110CT", or
"Burnham Boiler 100K BTU system"

STEVEusaPA

Which city in Massachusetts? Boston uses 9 degrees for design day.
1989 house if 2x6 walls should have R19 in walls and R30 in ceilings.
You’re probably close with your numbers, closer than 110k from the others.
I guess a better question may be why didn’t either bidding contractor do a heat loss, or even look at/review yours?

MSmithHH

"I guess a better question may be why didn’t either bidding contractor do a heat loss, or even look at/review yours?"

>> I did mine after I got the quotes. I don't think most want to because probably only 20% turn into actual jobs but doing the calc takes a long time. I guess I can understand. Everyone is so busy, it takes weeks just to get the estimate after they've been to the house.

"Which city in Massachusetts? Boston uses 9 degrees for design day."

>> I'm in Worcester county. I can try 5 degrees though and 74 degrees inside to be more aggressive though. Of course if we push it for the 1% case, we end up over-sizing, don't we? Which I thought was bad.

"1989 house if 2x6 walls"

>> Most framing I've seen in unfinished areas were 2x4 for the walls. But possible it's 2x6 in areas I can't see.

Thanks for the response!

Zman

The existing baseboards have nothing to do with heat loss unless you assume the original designer did math. They probably did not.

Your insulation is likely fiberglass the same thickness as the framing cavity. A 2x6 wall would have had R-19. A 2x12 roof R-38 ect....

You do need to adjust your design day lower. There are published design day guidelines for you area.

The indirect likely won't need to be considered because the load is intermittent.

If you multiply the input BTU by the % efficiency you will get the output efficiency. The boilers usually have an output rating as well.

Also look at the turndown ratio. Installing a 110 boiler with a 10-1 turndown in your case would not be huge deal because it will run at< 10k if needed. A single stage 110 boiler would be grossly oversized and cycle constantly.

MSmithHH

Ahh ok. Makes more sense now. Thanks. 99.6% for me was 1.6 and 99% was 6.2. The app crashed if I put in a decimal so I went a little aggressive and put in 1 degree for the outdoor temp and that brought it to 89k. At least close enough to ease my fears about the quotes. Good to know about the turndown too.

Any thing I can use this info for in order to tell me if a mod con would be a good fit for me or if it would make more sense to go cast iron? I understand mod cons aren't always super efficient depending on the situation

Thanks again!

HVACNUT

You should be able to see the R value of the ceiling insulation. Just pull back a batt and check.
In 1989 with 2x4 construction, I wouldn't go higher than R13 for the exterior walls. My 1972 home had R7 before the rip out.

hot_rod

MSmithHH said:

Ahh ok. Makes more sense now. Thanks. 99.6% for me was 1.6 and 99% was 6.2. The app crashed if I put in a decimal so I went a little aggressive and put in 1 degree for the outdoor temp and that brought it to 89k. At least close enough to ease my fears about the quotes. Good to know about the turndown too.



Any thing I can use this info for in order to tell me if a mod con would be a good fit for me or if it would make more sense to go cast iron? I understand mod cons aren't always super efficient depending on the situation



Thanks again!

Once you have a load calc that is accurate, measure how many feet of fin tube.
Use the manufacturers output data to see what is can supply.

If there is adequate or excessive baseboard you should be able to run lower SWT most of the heating season, and that may be a way to optimize a mod con.

The other part of the mod con is the modulation which will eliminate short cycling on load load days and reduce fuel consumption somewhat.

The steps to determine if you system is a match for low SWT operation are in this journal.

https://www.caleffi.com/sites/default/files/file/idronics_25_na.pdf

SlamDunk

I sized a furnace for a home in NYC. The contractor wanted to install a 225kbtu unit (180koutput) but I stopped him.

I did the slantfin thing for an 1800sqft, 100yr old house, with no insulation and came up with 60Kbtuh heat loss. I couldnt stomach anything that small. I was afraid I missed something so we settled on an 83Kbtu output furnace.
I used 5 degree outside temp and 72 deg inside temp.

It worked. The above numbers ought to be used as a gut check comparison.

MSmithHH

So I called one of the contractors on the phone and mentioned the heat loss. They said that with a 45 gallon indirect tank that dilutes to 60 gallons, anything lower than 100 would never be able to handle the demand of the indirect water tank. They said that since it modulates, I'd always be getting the optimal number of btu's anyway since it falls within range of min-max btu's.

hot_rod

it’s true that in low load homes the DHW can be the larger load to keep in mind An indirect could work with virtually and size boiler, most manufacturers suggest a minimum size boiler input to assure you have recovery and performance they present in their capacity charts

Zman

What is presently heating your DHW? How well does it keep up?

MSmithHH

It's a self contained gas hot water heater. 50 gallon with a diluter to bring it closer to 60. Does it keep up? I think so? In winter though, if kids take a bath and then we fill the Whirlpool, it runs out of hot water before filled. But that might have nothing to do with it not keeping up and instead just the number of gallons available. Otherwise I see no issues.

Zman

What you have is likely a 40,000 btu, 75% efficient water heater It will put out around 30,000 Btu continuously.

If you install the MC 110 boiler you will have more than 3x that.

Installing a 110 will give you around 98,000 Btu.
If you go with an 85, you will get aroung 67,000 Btu.

The misconception on the indirect sizing matching the boiler is pretty common. In reality the opposite is true. The oversized indirect will pull down the boiler return water temp and the boiler will run in the more efficient condensing mode.

Remember that energy cannot be created or destroyed. If it isn't going into the hot water, where is it going?

MSmithHH

Really interesting! Thanks!