Gravity System convert with OLD condensing cast iron boiler, Help, Question

[Jackblacus]

Greetings,

I live in Queens NY and currently have a Weil Mclain Series 6 CGM-7 210,000 BTU boiler. I have owned my house for the last three years. I don't know how old the boiler is, however a service tag attached dates back to 1997. Heating bills are very high in NYC and cost 500+ a month in winter which is what got me doing research on boilers. The house is an old Victorian, however R19 insulation has been added to all exterior walls and new modern doors and windows replaced on all.

Currently I have a old Gravity system with cast iron rads which was converted with the addition of a Bell & Gossett circ pump series 100. I have cast iron rads with a total EDR of 892. The house three stories, has an unfinished basement, two living floors, and a finished attic with heat rads in all locations. Total square footage of house is 3100 sq feet. Rads get to about 100-110 degrees with FLIR.

I was doing research into new Con/Mod HE boilers when I realized that this boiler has, ALWAYS?, been condensing which is bad for corrosion. The boiler heats the house well running for about 30 minutes between cycles of 1.5 to 2 hours depending on temps in December, in colder months longer. I used a FLIR ONE recently with electrical tape (for reading accurately) on the supply and return pipes. With 30 minutes of run time the temps are 110, 100 return. With run times of an hour the temps are 120, 110. I have never seen the temps approach 140 or above, but also I haven’t checked the pipes in January/February.

There is NO temperature return mitigation on this boiler, clearly being plumed incorrectly. Even the original manual for installation talks about return mitigation.

My questions are, why hasn’t this boiler failed after so many years of usage in this configuration? And can anyone suggest a new boiler for a converted gravity system for this house.

Thank you in advance. PHOTOS attached. https://www.flickr.com/photos/197086861@N02

https://flic.kr/p/2o4kdVm

 

[Fitter30]

Edr 892 @ 170° water = 133,800 btu' s. 150 btu's per sq ft.
At the temps your stating the boiler refractory would be in pieces, flue would be rusted out and the burners. Cast boilers don't run in condensing temps for long.
Any condensing boiler is only 95% efficient below 140° return above drops to 87%. B&G series 100 pump moves a lot of water 20 - 34 gpm would pipe it primary secondary loop using a condensing boiler. First get a remote style thermometer.

https://www.amazon.com/DANOPLUS-Thermocouple-Thermometer-50-1300%C2%B0C-Temperature/dp/B09836Y2CT/ref=mp_s_a_1_5?crid=GJMIHV464F0S&keywords=k+style+thermometer&qid=1670340326&sprefix=k+style+thermometer%2Caps%2C233&sr=8-5

 

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[John Gayewski]

This can happen with cast iron boilers. Even when condensing they can last a very very long time. Some run is sub optimal conditions for thirty plus years of all conditional.

I can't recommend a boiler as I'm not familiar with your area or what's most available. I suggest you find a local installer and see what they prefer to install. There are floor mount condensing boilers that will replace yours without having to change the piping too much and those are handy. Sometimes people insist you need a wall hung boiler and what to change the piping all around, which is fine, but not necessary.

You'll want to size your new boiler based on the heat loss for your house, not the edr of your radiators. There are a few ways to do it. Using the slant fin app to double check my pencil and paper calculations is how I do it. One person on this forum swears by clocking your existing boiler. If you search sizing a boiler on this forum I'm sure you could find a link to this method.

 

[Jackblacus]

Edr 892 @ 170° water = 133,800 btu' s. 150 btu's per sq ft.
At the temps your stating the boiler refractory would be in pieces, flue would be rusted out and the burners. Cast boilers don't run in condensing temps for long.
Any condensing boiler is only 95% efficient below 140° return above drops to 87%. B&G series 100 pump moves a lot of water 20 - 34 gpm would pipe it primary secondary loop using a condensing boiler. First get a remote style thermometer.

https://www.amazon.com/DANOPLUS-Thermocouple-Thermometer-50-1300%C2%B0C-Temperature/dp/B09836Y2CT/ref=mp_s_a_1_5?crid=GJMIHV464F0S&keywords=k+style+thermometer&qid=1670340326&sprefix=k+style+thermometer%2Caps%2C233&sr=8-5

Looking at energy bills and ecobee data:

My average actual heat loss, based on january/feb of 2021 fuel burned, assuming boiler is 168k btu/hr is 42,468 btu. If less than 80% eff, due to age, this maybe more.

Coldest day in 2021 where I live was 10 deg on 01/16/2021 for an entire 24 hr period. Again with above assumes the heat loss was 85,859 btu per hr.

So of course with an EDR of 892 the temps won't be that hot. Temps on coldest day were probably only 140. I am just surprised that the boiler is as old as it is, while condensing probably 95% of the time has not failed, like the internet seems to think. I've read that they can fail within a few years with condensing that much.

Now that I did the analysis it seems that temps of 120-110 are normal for the EDR I have which 44,600 btu/hr which seems to line up with the fuel usage.

Is it also true that a mod/con boiler is the best for this system due to the large mass of water being heated, temps should remain low, as in the above example? Also should i put TRVs on rads? Currently it lacks trvs or a lockshield valve (also would it need a lockshield valve?).

​

 

[Fitter30]

All depends how the zones and radiators are piped.

 

[Fitter30]

Wrote : There is NO temperature return mitigation on this boiler, clearly being plumed incorrectly. Even the original manual for installation talks about return mitigation.
Are you suggesting that there should be a cross over between supply and return with a valve to open to regulate return temp?

 

[Jackblacus]

This can happen with cast iron boilers. Even when condensing they can last a very very long time. Some run is sub optimal conditions for thirty plus years of all conditional.

I can't recommend a boiler as I'm not familiar with your area or what's most available. I suggest you find a local installer and see what they prefer to install. There are floor mount condensing boilers that will replace yours without having to change the piping too much and those are handy. Sometimes people insist you need a wall hung boiler and what to change the piping all around, which is fine, but not necessary.

You'll want to size your new boiler based on the heat loss for your house, not the edr of your radiators. There are a few ways to do it. Using the slant fin app to double check my pencil and paper calculations is how I do it. One person on this forum swears by clocking your existing boiler. If you search sizing a boiler on this forum I'm sure you could find a link to this method.

Is there a way to increase water Temps to 120 (if only for the kitchen) because even with the big EDR of the rest of the house the kitchen lacks it.

The reason I ask is because my kitchen has two recessed radiator operating at the water temp range of 100-105 only outputting around 3000 btu together based on those water Temps.

I can add a wall hung rad on the one remaining wall which would output about 5000 btu at a delta T of 50. But only If I can get the water to 120 range. The btus would be a total of 9000 btu which is the heat loss calc for the kitchen.

The kitchen is a enclosed portch with bad insulation above the ceiling and no room above or below just a pitched roof.

 

[John Gayewski]

Is there a way to increase water Temps to 120 (if only for the kitchen) because even with the big EDR of the rest of the house the kitchen lacks it.

The reason I ask is because my kitchen has two recessed radiator operating at the water temp range of 100-105 only outputting around 3000 btu together based on those water Temps.

I can add a wall hung rad on the one remaining wall which would output about 5000 btu at a delta T of 50. But only If I can get the water to 120 range. The btus would be a total of 9000 btu which is the heat loss calc for the kitchen.

The kitchen is a enclosed portch with bad insulation above the ceiling and no room above or below just a pitched roof.

Yeah, but you'd be turning up your boiler temp and using a mixing valve on the rest of the zones which would require more piping and another maintenance item. There are actually several ways you could do it, but all of them require a piping rework and adding components, which you might want to add a long with an updated boiler. It just adds to the cost.

 

[Jackblacus]

Yeah, but you'd be turning up your boiler temp and using a mixing valve on the rest of the zones which would require more piping and another maintenance item. There are actually several ways you could do it, but all of them require a piping rework and adding components, which you might want to add a long with an updated boiler. It just adds to the cost.

The mixing valve piping is a good idea. I'll have to mention this when we get new boiler.

However one question, how does the boiler water temp get raised , if the thermostat is satisfied and ends the call for heat? Would a new he mod/con boiler be able to be set for say a minimum temp of 120?

 

[John Gayewski]

The mixing valve piping is a good idea. I'll have to mention this when we get new boiler.

However one question, how does the boiler water temp get raised , if the thermostat is satisfied and ends the call for heat? Would a new he mod/con boiler be able to be set for say a minimum temp of 120?

The overall water temp would go up to the highest zone's requirement. The rest of the zone's would get water that has been mixed down.

You could do like you said before and use trv's with a bypass piped in. So your radiators would shut off lower temps than your kitchen, but it depends heavily on the piping.

 

[Fitter30]

Never seen anyone design hw heating at 50° temp difference 20° normal. Run a boiler at two different temps injection method for the lower temp. How are you getting to your btu to edr ?