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# Sizing boiler - Heating a Condo 1000 sq ft. in NYC (natural gas)

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Member Posts: 8
edited November 2022
Hi,

I need to replace an old 60k (input) BTU boiler (natural gas, 84%) in my 1000 sq ft apt.

I had Sentry Saln Fin S-60 specs here: https://images.thdstatic.com/catalog/pdfImages/52/52185a04-3c03-41f0-b4d6-44768ce87022.pdf .

The building is from 2007 with some okeish insulation. I'm in NYC.

I believe the boiler was oversized in the 1st place (it would run for 30-40min and it would turn off, it would get hot in the apartment pretty quickly).

The contractor recommends 67k! input but he only looked at the old unit as a suggestion. The problem is the next option is 38k (nothing in between)

Here is the new Williamson 38k and 67k units specs: https://williamson-thermoflo.com/products/williamson/gwa-series-3-gas-water-atmospheric

Note that I have both radiant floors in the kitchen & 2 bathrooms + baseboards.

Can you please look at my calculation in the screenshot and critique it?

I want to go with 38k model (27k net rating) but I'm worried what if it's too low. My reasoning for going with lower one is so it will run more efficiently given my needs (less ONs & OFFs). Is this valid?

• Member Posts: 23,297
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Your calculations look OK -- so far as how much heat your radiation can absorb. And, since your radiation can only absorb about j25,000 BTUh, there doesn't seem to me to be much point in putting in a bigger boiler... does the apartment heat OK as it is?
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 8
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Your calculations look OK -- so far as how much heat your radiation can absorb. And, since your radiation can only absorb about j25,000 BTUh, there doesn't seem to me to be much point in putting in a bigger boiler... does the apartment heat OK as it is?

Jamie, thanks for your input! Yes, the heating worked fine as is. However: My ideal temp. is 71-73F during the day, 66-68F at night. What tended to happen, the boiler would run for an hour, it would get to 74-75 (per thermostat setting), and then within 1-2 hrs it would turn on again and so on.

Ideally, I would love to maintain a more stable temperature (as if the heating system could run non stop a lower pace) of 72f instead of going from 70 to 74F etc.  I know might not be achievable with baseboard heating.

I was hoping going with lower BTU would be: more environmentally and \$\$ friendly while keeping more stable temp in my apt.
• Member Posts: 1,849
edited November 2022
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Get the smaller boiler. You're right, fewer cycles will make the system more efficient. You use between 600-750 therms per year? That's about what 24kbtu would imply. If less, you're probably still oversized a tad, but you don't have any smaller cast iron boiler options, so you'll have to live with it.

The smaller boiler is actually rated at 32kBtu if the boiler is inside of the condo.
• Member Posts: 11,061
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One thing to consider is the height of the desired boiler with the in-line draft hood. (tall)
Your existing unit has a draft hood box diverter on the back. (shorter)

So would the existing b-vent flue pipe work with the taller unit.

What is the average water supply temp in the present boiler?

Any modulating boiler or combi unit, that I have seen, would require different chimney venting....perhaps not possible in a condo situation.
• Member Posts: 8
edited November 2022
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Get the smaller boiler. You're right, fewer cycles will make the system more efficient. You use between 600-750 therms per year? That's about what 24kbtu would imply. If less, you're probably still oversized a tad, but you don't have any smaller cast iron boiler options, so you'll have to live with it.

The smaller boiler is actually rated at 32kBtu if the boiler is inside of the condo.

Thank you. I actually used only 206 therms in total between Nov 1st and April 30th last year (this includes cooking)
• Member Posts: 8
edited November 2022
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JUGHNE said:

One thing to consider is the height of the desired boiler with the in-line draft hood. (tall)
Your existing unit has a draft hood box diverter on the back. (shorter)

So would the existing b-vent flue pipe work with the taller unit.

What is the average water supply temp in the present boiler?

Any modulating boiler or combi unit, that I have seen, would require different chimney venting....perhaps not possible in a condo situation.

Yes, condensing or modulating boilers are probably not an option. As for water supply, I can't really tell but the boiler room is in the top floor with other boilers and water heaters. The room temp there is probably 90-95F non-stop
• Member Posts: 1,849
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Thank you. I actually used only 206 therms in total between Nov 1st and April 30th last year (this includes cooking)

Your heat loss is probably around 10kbtu then. That's typical for a 1000sqft space that shares walls. Same solution - the smallest boiler is best. Or switch to a heat pump to get AC.
• Member Posts: 22,144
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Looks like a lot of high output baseboard, 650 btu per foot? And radiant?
Seems a 50 k mod con could be an option, especially if the fin tube could run lower temperature
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 8
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650 BTU per ft of radiant radiator it's just an assumption that I thought was relatively conservative (I saw somewhere a range of 550-700 btu) as a good estimate how much BTU is required per ft.
• Member Posts: 1,849
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650 BTU per ft of radiant radiator it's just an assumption that I thought was relatively conservative (I saw somewhere a range of 550-700 btu) as a good estimate how much BTU is required per ft
It’s a bit different - that’s how many the baseboard can output, it’s unrelated to how many BTUs are required. You need significantly less since your heat loss is ~10kbtu, hence the cycling.
• Member Posts: 8
edited January 2023
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Just an update, I ended up replacing my old gas furnace (60k BTU) with Williamson (38k BTU). That's ~37% (!) reduction in power. Clearly, my old furnace was way oversized when the building was constructed. It hurts me to think how much gas & energy was wasted over the past ~15 years.

Judging how the new system has been operating, a slightly smaller size (~30k BTU) would also be fine but the selection was limited. I'm experiencing no issues with temp. control and how quickly I can heat-up the apt.

Long story short, don't use your old boiler as the perfect anchor point. Do some research, do your own calculations, and figure things out from scratch. As mentioned, the contractor was pushing to replace the old 60k furnace with 67k which clearly made no sense in my case.
• Member Posts: 6,929
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You done good Newbie...real 👍 Good.  Williamson (Weil Mclain w a beautiful Green jacket)  is a good, simple and  reliable workhorse. If only 1/2 the installing contractors dis what you did, we'd all be out of a job and have nothing to talk 🙄 about!!!!!!  Ha ha. .  Mad Dog
• Member Posts: 8
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