Sizing a new boiler

dvpgh

I'm trying to size a new boiler for my house.

I currently have a (very old) boiler that's over 300k BTU and it is constantly short cycling and seems to be vastly oversized for our house. House is about 3000 sq ft, three stories with a partially conditioned basement, built between 1910 and 1920 with minimal insulation. I was trying to determine how big we actually need, so I did the following:
- Added up all of the gas we used in the last year and subtracted off the amount used for cooking/hot water
- Total gas usage was ~199 Mcf on heating, or ~207,000,000 BTUs of gas burned.

Heat was being called for ~950 hours over the course of the year.

When I divide the BTUs burned by the number of hours of operation, I get ~217,000 BTU per hour of burned gas when the boiler was calling for heat.

Is this methodology going to produce accurate results when it comes to getting an appropriately sized boiler? Given that the old boiler is not operating efficiently, I assume the 217K BTU of input is probably only operating in the 75%-80% efficiency range, so I was going to look for a boiler that yields 165K-175K of output BTU.

Thanks!!

Steamhead

@dvpgh , is this a steam or hot-water system?

Hot_water_fan

Is this methodology going to produce accurate results when it comes to getting an appropriately sized boiler?

No, it will not. 

Instead, use this: 

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

dvpgh

Steamhead said:

@dvpgh , is this a steam or hot-water system?

Hot water

dvpgh

Hot_water_fan said:

Is this methodology going to produce accurate results when it comes to getting an appropriately sized boiler?

No, it will not. 

Instead, use this: 

https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler

Thank you this is very helpful! I'll run these numbers.

Hot_water_fan

Excited for the results. I think your number comes in at 100,000 Btu/h before any oversizing factors are applied. 

dvpgh

Hot_water_fan said:

Excited for the results. I think your number comes in at 100,000 Btu/h before any oversizing factors are applied. 

Yep, almost exactly.

Looks like I need somewhere around ~130K-160K of output, or ~150K-190K of input at 84% AFUE.

My previous calculations definitely got me into the right ballpark but it seems like I would have still been oversizing by around ~10-20% vs these calculations

Thanks for sharing this, it was really helpful! There's definitely a part of me that's still kind of nervous to install a boiler that's half the size of what we currently have, but I guess the math is the math.

Hot_water_fan

The 1.4x isn’t required by any means, but yes you’ll be looking at a much smaller boiler. A good thing! 

PRR

dvpgh said:

...kind of nervous to install a boiler that's half the size of what we currently have, but I guess the math is the math.

Up top you said "...constantly short cycling and seems to be vastly oversized for our house." Was that wrong?
Your initial figures gave NO hint of your duty-cycle. Not how many hours per year, but how many hours per Day?? If (like my old burner) it runs 15 minutes twice an hour on a max-cold night, it is literally twice as big as needed. You want some excess in case the house goes cold (vacation, power loss), but not twice.

dvpgh

PRR said:

dvpgh said:

...kind of nervous to install a boiler that's half the size of what we currently have, but I guess the math is the math.

Up top you said "...constantly short cycling and seems to be vastly oversized for our house." Was that wrong?
Your initial figures gave NO hint of your duty-cycle. Not how many hours per year, but how many hours per Day?? If (like my old burner) it runs 15 minutes twice an hour on a max-cold night, it is literally twice as big as needed. You want some excess in case the house goes cold (vacation, power loss), but not twice.

Ya, makes sense and I'll definitely get over it and get something properly sized! I'm honestly not sure how often the boiler fires, but just from listening it seemed like it would fire on the coldest days for only a few minutes at a time (once the system is heated up). On the coldest days this winter, the system was calling for heat almost continuously, but the boiler was not actually firing continuously. Some of this is some insulation issues (which I've been correcting), but clearly the boiler isn't sized to actually fire almost continuously when it's calling for heat continuously.

Would it be correct to assume that ideally the boiler would be firing any time heat is being called for, or close to it?

Hot_water_fan

Would it be correct to assume that ideally the boiler would be firing any time heat is being called for, or close to it?

It’s more important to have a boiler running nearly continuously on the coldest days of the year. Your question would depend on stored heat, which is a different topic. For example: a small zone (maybe a towel rack) shouldn’t cause the boiler to fire when it’s calling for heat, hopefully there’d be enough heat available in the boiler than the circulator could run without turning on the boiler. A buffer tank is often used to prevent a boiler from firing on every heat call. Same amount of heat per day, just in longer cycles. 

Intplm.

Another thing you should do is check your work by doing a heat loss calculation. Your house may thank you for it?