Boiler temps

coalcracker

This type of overheating is a problem with coal stokers and a cause of outfires. Temperature rise after the aquastat is satisfied can easily be 30 degrees in the summertime and not allow the stoker to refire because it's in a high limit condition. Therefore, there is always a 50, sometimes 60 degree spread between the low and high aquastat limit

Dean_24

Boiler temps rise

Is it normal for the temp of your oil boiler to rise 20 degrees after it shuts down?

coalcracker

This can happen if the boiler is overfired or there is no recirculation pipe installed, among other things.

Dean_24

Over Fired?

What does that mean and what do I do about it

Brad White

Post-Purge

Not so much an over-fire condition as the following:

You have just finished heating your boiler mass to 150 degrees or more, with some parts, the iron, nearly five times that much. Suddenly, you turn the circulator off. Boom. No flow, nothing is carried away.

That hot metal says, "Hey, look! There is water surrounding me at a lower temperature!

(In my world, boilers speak and apparently have eyes.)

That fixed and still volume then absorbs the remainder of that heat and could easily go up 20 degrees or more depending on the volume. This continues until equilibrium.

A post-purge cycle will take care of that, by operating your circulator for a fixed time period after "burner-off". An aquastat can do the same thing.

Dean_24

Aquastat

So how do I set my boiler and aquastat to do that? I know the aquastat relay starts my circulating pump, but unless one of the 4 zone valves is open, there wont be any circulation.

Brad White

That is the down-side of zone valves.

They close the door to post-purging.


(Personally I like constant circulation but that does not matter to you I suppose.)

To make it happen with zone valves, I would have a delay or make sure that the zone valves close slowly over time and keep the circulator running until the boiler cools.

The way a normal (meaning back in the 50's and 60's) heating system would work is to fire the boiler until a "make on rise" aquastat closed. This would then start the circulator to prevent thermal shock, condensation and moving cool water around for nothing. Typically this would be set for about 130 degrees. By default, this circulator would shut down when the temperature dropped below 130, just like we are discussing.

More current control systems have a built-in post-purge feature, usually time based for about three minutes, fine for most low-mass boilers. If you can adjust this for twice the through-put of a high mass boiler, that would be enough in my book. In other words, if you have a 15 gallon volume boiler and a 5 gpm circulator, six minutes of post-purge would do it.

Your anticipator settings on your thermostat should take this into account so that you do not overheat. The anticipator (if you have one, it is an older technology), will "think ahead" and shut the burner off before reaching setpoint, knowing that it will "coast" to setpoint once the burner is off and post-purge continues.