On legacy boiler replacement

GeneJames · September 2009

I'm about to replace a very old 400K btuh boiler in my home. 6000 sqft, brick veneer, no insulation, 3 stories front, 4 in back. 45 windows. Cast iron radiators, plastered walls. Perhaps someone here can give me advice.

1. I'm not sure how to size the darn thing, Current boiler is 400K BTUH input (measured). It is sized about right as it holds comfort to about -10F. It is a Bryant tube boiler, likely made in 1904. (I could post a photo, Bryant engineers have no record of their making tube boilers after about 1930). I don't know how to measure BTU out to determine efficiency, but the flame bed is about 6-8 sqft and the flue temp at the diverter is only 110F. This scares me: maybe this thing is very efficient and I'll need a 400K 95% replacement. Maybe I won't save money. OTOH, a conventional heat loss study says to size boiler at 400K.

2. No one seem concerned about mixing iron, copper pipe. Isn't there an issue with dissimalr metals that needs to be addressed? A sacrificial anode perhaps?

3. I'm replacing windows and I'm hoping to get Icynene in the walls (if I can find someone who will do it) and this will cut demand dramatically. Should I get 2 furnaces so that one will be sized for new reduced demand?

4. Someone at Munchkin factory says I need a buffer tank, but the boiler will drive a HUGE thermal mass and will suck low temp return water for a long time. Seems like that's when you wouldn't need a buffer tank.

5. Are there any intelligent residential setback thermostats? I've used several and they're all pretty dumb. EG: they fail to learn cool-down rate and adjust heating restore time properly; they'll fire the boiler 30 minutes before set-back time leaving me with 180 degree radiators for an hour after setback; they don't learn about the thermal mass of the system/building and therefore tend to overshoot the target temperature.

Your thoughts are much appreciated.

Steamhead · September 2009

I'm assuming

this is a hot-water system, rather than a steam system?

Gene_James · September 2009

GeneJames replies

Hi Seamhead... yep it's HW. Probably originally gravity, but has a 1/4 HP B&G pump now.

(My pw didn't work, so I created a new account. )

Tim McElwain · September 2009

Hi Gene

That boiler probably has an output of around 320,000 BTU's. Are you sure it is Bryant boiler and not Bryan?

I would do all the windows and insulation first and then have a heat loss done on the house. It would be best to hire a professional to do that.

Icynne is usually done on new construction with unfinished walls. Are you planning to remove drywall?

There is no real concern with iron and copper on Forced Hot Water only on Steam. What is your typical return water temperature now on the boiler. Depending on that you may want to go with outdoor reset in order to get the condensing function to work on the Mod/Con boiler.

I have put a Munchkin on an old gravity hot water system without a buffer and it has worked fine as it tends to get low return water temperatures back into the boiler.

You might look into the Ray boiler which will pretty much take care of the thermostat problem as the boiler electronics calculates boiler and system behavior and runs the system to satisfy the energy demand and not so much typical thermostat deficiencies.

Others may chime in here with thermostats that give more accurate control.

Jean-David Beyer · September 2009

you may want to go with outdoor reset

"Depending on that you may want to go with outdoor reset in order to get the condensing function to work on the Mod/Con boiler."

You might also want ot go with outdoor reset so that in warmer weather, it could put cooler water into the radiators, and thus minimize overshoot. In other words, you might get 120F water in the radiators on warmer days when the thermostat is satisfied, so setback could take effect more quickly.

gjames · September 2009

Thanks guys,

for the good input.

Furnace is Bryant. Photo of the beast is attached.

Two pros have estimated I need 400Kbtu by measuring space, insulation, windows and counting radiators. Seems to me that a better estimate could be done by heating house to 70, turn off furnace, measuring outside temp, and measuring how long it takes to drop, say, 10 degrees. They say no, but a heat transfer friend who designs nukes says that would be definitive.

Windows will be done soon so I can see if that affects their estimates,

Yes, Icynene is mostly used in new construction, but they make a mix for retrofit as well. The only Pittsburgh franchise holder no longer does retrofit... too much of a headache he says.

I have not measured return temp, can only say it's very hot in cold weather. Output temp is limited at 180.

I will ask contractor if they can supply Ray boilers and will investigate them.

Outdoor reset, I gather, will modulate boiler in accord with outdoor temp rather than return temp?

(for some reason, this site forgets my password... I have to create an acct every time I want to post.)

Jean-David Beyer · September 2009

this site forgets my password...

"for some reason, this site forgets my password... I have to create an acct every time I want to post.)"

That is not correct behavior. It does not forget mine. And I have seen no other complaimts.

Could it be that your browser forgets your password? I suggest you write it down carefully and re-enter it. These are CaSe SeNsItIve, so you must get that right.

On legacy boiler replacement

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