Minimum safe hot water boiler water return temperature

BruceLundeen

During these so-called shoulder seasons sometimes spaces are overheated even with 140 degree return temperature. Much has been said about condensation ruining non-ModCon boilers when running lower water return temperature.
You know, it is very brief periods that the reset controller will hold the water temperature this low.
Anybody have any first hand experience - especially bad experiences (like a failed boiler)?

kcopp

Most all Mod/con boilers have no issue at all w/ low temp return water temps because of their construction... they love low temp water.
Now a typical cast iron boiler that's different. The low water temps can do 2 things... shock the boiler if a cold slug returns to a hot block AND prolonged temps will allow condensation to form on the fire side to become acidic. That will eat through the cast over time.

Jamie Hall

Buffer tank. Let the boiler keep the buffer tank up to temperature -- it won't have to run that often in the warmer weather -- and run your system with another pump and mixing valve using the buffer tank.

Dave H_2

It sounds like you have an outdoor weather responsive control on this boiler. I would check the reset curve. It's programming may be off a little. Depending upon the unit there are several factors needed to calculate the proper curve like:
1. Outside Design temp (different than the coldest it has ever gotten)
2. Minimum Supply water temp
3. Maximum Supply water temp
4. Warm weather shutdown temp

Dave H

STEVEusaPA

Here's what can happen when you don't protect the boiler from low water return temps.
New customer, claims it was cleaned last year. Could've been. This is what Icesailor used to call 'Kibbles & Bits'

neilc

did you salvage / clean that mess?
or replace it?

wow, just wow,

STEVEusaPA

Cleaned it, and it runs fine-2 vacuum full bags later. Inside the chamber you can see where the crud was as high as the bottom of the air tube. Have to go back to install bypass (boiler protection).

kcopp

surprised to see it that bad on a steel boiler w a tankless. The circulator would usually turn off until the boiler got back up to temp.

EBEBRATT-Ed

I agree. Install a boiler bypass which will let supply boiler water mix with cooler return water to raise the boiler return water temp.

Gordy

It's the amount of time you are sub 140 not so much being there for a short duration. I would say the boiler Steve worked has never seen anyone until him.