If cold water bad for CI, what about startup after setback?

R. Kalia

I'm probably missing something here. I understand (I think) that if the water is colder than 130, there will be condensation of flue gases and this will corrode cast-iron.

But the water is always quite cold at recovery (after setback), and in fact it is probably cooler than 130 every time the boiler starts up after having been off (due to the thermostat) for any period.

We have fat pipes from the original gravity system, so there's probably many gallons of colder water returning to the boiler before any hot water makes the round trip. But the boiler has survived ~35 years so far.

What am I missing here?

GEO_3

Continous running below 130 like you might see with radiant is harmful. Cold start is not. Gas boilers are are mostly all cold start. Any condensate generated is quickly evaporated as the temp. climbs.

R. Kalia

Oh. Well, never mind.

Oh. Well, never mind.

Sorry about the stupid Saturday Night reference. Many thanks for clarifying!

Unknown

What you are missing

is that the 35 year old boiler you have probably has flue passageways between the sections that are large enough to stick your arm down through. Thus it it not very efficient combustion wise and probably has a large water content with rather thick cast iron walls. Today's boilers are much smaller, incorporate thinner cast iron walls with heat peg design and have rather tight passageways. They are built this way to meet the criteria for today's required efficiencies.

Connect a piping system such as yours to one of the modern day boilers without any measures such as a bypass, thermostatic diverter valve, boiler loop or primary-secondary and you have yourself a possible mess. With a smaller water content system and cold start, the brief period that the boiler will be operating in condensing mode will probably not cause any concern because any small deposits of condensation that have accumulated will burn off and evaporate quickly as the boiler heats up. Hope this helps.

Glenn Stanton

Burnham Hydronics

R. Kalia

small water content

Yes, that made sense, thanks. We're probably getting a Munchkin in the fall. May not be what you wanted to hear!

Boilerpro_3

Keep the old one hooked in

the system and pipe the new boiler in parellel. This way you have built in back up when the new boiler goes down someday. It helps you sleep a little easier when your'e away in really cold weather.

Boilerpro

Earthfire

boiler pro

but wouldn't that old boiler just be a big old radiator sucking heat out of the system and sending it up the flue

Boilerpro_3

Not if piped properly

Pipe both boilers P/S to the system with each boiler having it's own pump and flowcheck to prevent flow through the old boiler when the new is running. Just like multple boiler installs.

BP