Boiler Swap - Is this too small?

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Vze2hnvz

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Hello, I was going to swap out my existing Pennco cast iron boiler circa 1980, that was rated for

150000 input, 120,000 output running nat.gas. I just removed a Lennox GWB8-100E-2 boiler that

is dated to 2013 for a friend. We gutted his house for a remodel and to ditch the knob and tube.

He will be switching over to forced air to get some AC as well, now that all the wall cavities

are exposed. The Lennox is rated at 100000 input and 83000 output. I know they used to

oversize boilers so I was doing some calculations (minus a true heat loss calculation) for my

existing demands. I have 2 existing zones. Zone 1 has 30' of baseboard fin, and zone 2 is

running 7 ancient cast iron radiators. House was built in 1840 - unsure when they retrofitted

a boiler for heat. Anyway, I calculated the fin at 550 BTU/ft and came out with 16,500 BTU on

that zone. I measured the dimensions, height and number of fins on the radiators to get the

square footage and calculated the BTU's at 180 degree average water temp (I know that's a

bit high for cast radiators, but that's what I'll be setting the Aquastat to.) For the Radiators I

came out with 62,500 BTU's, for a total of 79000 BTU's. The house has always been

comfortable with this setup, and given the "new" boiler is rated for 83000 BTU output, I would

think this would be sufficient for 99% of the time (minus the odd polar vortex). Am I wrong in

my thinking figuring it this way, rather than doing a true heat loss calculation?

Thanks in advance, hydronics ain't my bag..
 

Sylvan

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A hydronics system has lots of leeway for undersizing.

If it is undersized you can always raise the temperature of the supply

Unlike scorched air or steam hot water holds the heat a lot longer
 

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Vze2hnvz

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A hydronics system has lots of leeway for undersizing.

If it is undersized you can always raise the temperature of the supply

Unlike scorched air or steam hot water holds the heat a lot longer
Thanks for the reply, I appreciate it. Just was a bit concerned with a 40000 BTU difference. Wasn't sure if there was something I didn't account for. I would hate to plumb it in and find that come winter it would be inadequate and have to bump up the aquastat to the point I start tripping the pressure relief valve.
 

Vze2hnvz

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It's tough to do a true heat loss calc on this house, being so old. Unknown what they did over the years. Not sure of wall cavity insulation/type/r value, floor type/thickness/layers. That's why I was trying to backdoor a heat loss with the calculations. I just figured if it was comfortable before with the 120000 output, and I calculated a need for 79000 btu@180 deg. (over estimated with the radiators a bit just in case) then the 83000 output should suffice. If I do the calculations at 170 deg. water temp, BTU need for the fin and radiators drops down to 71,900 BTU's. I'm just asking if there is something I'm not seeing, cause the math adds up, no? I'm not sizing/buying a new boiler to accommodate the living space, just seeing if this one would be sufficient.
 

John Gayewski

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It's tough to do a true heat loss calc on this house, being so old. Unknown what they did over the years. Not sure of wall cavity insulation/type/r value, floor type/thickness/layers. That's why I was trying to backdoor a heat loss with the calculations. I just figured if it was comfortable before with the 120000 output, and I calculated a need for 79000 btu@180 deg. (over estimated with the radiators a bit just in case) then the 83000 output should suffice. If I do the calculations at 170 deg. water temp, BTU need for the fin and radiators drops down to 71,900 BTU's. I'm just asking if there is something I'm not seeing, cause the math adds up, no? I'm not sizing/buying a new boiler to accommodate the living space, just seeing if this one would be sufficient.
Right and I gave you one way. The slantfin app is one I use to double check my heat lots calcs.

The difference between different types of flour coverings and thicknesses of insulation will only make a heat loss calc vary in a small way. If you don't know of a wall has insulation then count it as not having it. It's not hard to get a really good number and be confident that your within a small margin of error. What I do is heat to 70 or even 72 on design day. Mainly because of there's a polar vortex I want to be comfortable. Most people opt for the mod con boiler if they are spending money which will turn down if oversized a little.
 

Fitter30

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Did your existing boiler cycle at the coldest temps? Raising the water temp does nothing for capacity if boiler is short its not going to get to setpoint. What have you done to the house as for doors, windows and insulation?
 

Sylvan

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Did your existing boiler cycle at the coldest temps? Raising the water temp does nothing for capacity if boiler is short its not going to get to setpoint. What have you done to the house as for doors, windows and insulation?
He stated the following read up

"It's tough to do a true heat loss calc on this house, being so old. Unknown what they did over the years. Not sure of wall cavity insulation/type/r value, floor type/thickness/layers"
 

jadnashua

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If you have your energy use data from last winter and look up the degree day data for your location, you can get a very good idea of what amount of energy it took to keep the house warm. That is real information, rather than an estimate that doesn't know things like how big of air leaks, or whatever you may have that insulation values don't account for.

Using the efficiency data of your boiler, the amount of gas used, and the degree day information, you can see how many BTUs you used.
 

Vze2hnvz

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Thanks all for the replies,
I guess I'm not following. I was just trying to see if this boiler was in the ballpark. As I said, I'm not doing any heat loss calcs for getting a new boiler, just was trying to see if this one was close. If A=B, and B=C, then A=C. That was my figuring. I was thinking this shouldn't be that difficult for my case. If I calculate my existent radiant sources/btu's per hr., and the existing boiler was already sufficient to heat the living space comfortably with the output of 120000 btu's output, and I only really need a 79000 btu input, and the other boiler is rated at 83000 btu in a perfect world. Why wouldn't that work for 99% of the time? I know its not sized to x 1.4. Can someone tell me why this won't work? Am I missing something?
 

John Gayewski

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Thanks all for the replies,
I guess I'm not following. I was just trying to see if this boiler was in the ballpark. As I said, I'm not doing any heat loss calcs for getting a new boiler, just was trying to see if this one was close. If A=B, and B=C, then A=C. That was my figuring. I was thinking this shouldn't be that difficult for my case. If I calculate my existent radiant sources/btu's per hr., and the existing boiler was already sufficient to heat the living space comfortably with the output of 120000 btu's output, and I only really need a 79000 btu input, and the other boiler is rated at 83000 btu in a perfect world. Why wouldn't that work for 99% of the time? I know its not sized to x 1.4. Can someone tell me why this won't work? Am I missing something?
Your comfort doesn't mean anything. You need to be at minimum 68 degrees on design day.
 

jadnashua

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Any fired boiler has best efficiency when on the coldest design day, it needs to run constantly. That's hard to achieve. 99% of the time, it will cycle on/off. What you DO NOT want is for any one on cycle to be short. TO achieve that, you don't want a boiler that is way oversized. If it's marginal, if you did a setback, it would take longer to reheat the house, so you often want a little excess.

The best way to figure out what size is needed on an existing install, is to see how much heat is put into the house through your energy use. NG has a certain energy content, your boiler has a known efficiency, and you can find out what the load is by looking up the degree-day data.

Then, you have a radiation issue...regardless of how big the boiler is, you can't get that heat into the building unless there's enough radiation. You figured out the max heat you COULD put into the house, but not what you NEED. Depending on the boiler chosen, you may be able to lower the outlet temperature, which lowers the available heat, but also would let the boiler run longer if the outlet temperature was lower. This tends to save energy. Depending on the boiler, there will be some minimum return water temperature to prevent damaging it. That can help determine what outlet temperature is safe, and how much heat you can provide to the building.

It was not uncommon for a boiler to be 2-4x larger than needed. Back when energy was cheaper, many didn't care. That is unlikely to be the case into the future.
 

jadnashua

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The actual energy content of NG varies a little by the supplier, but somewhere around 1035 BTU/cuft is probably a good estimate. Most gas bills show how much NG you've used, but may be in 100cuft units, so you'd need to take that into account. Say your boiler was 85% efficient, ad you used 1000cuft (this is just an example)...1035*1000*.0.85=879,750 BTU. Then, you'd have to look at your degree day table you've looked up for your zip code for the time in question...that would tell you, for the outside temperature that time period, how much temperature rise you needed, and how many BTU's you needed to keep the house warm. Then, you can work out how many BTU/hour you need for your heating source. This is actual for YOUR structure, rather than an estimate for a theoretical one. It works for retrofit, but obviously, not for new.
 
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