Boiler circulation query

[handyjohns]

Hello, a quick query I'd like to ask for help with if I may, please don't laugh as it is probably simple but I just cannot work it out = If a boiler's heat exchanger (normal boiler not combi) is blocked with sludge etc why would the flow get really hot and the return stay cool? I know the heat wouldn't get transfered away and circulated due to the blockage, but why up the flow?

Thanks for any replies

John

 

[Agile]

If the pump is failing or the system restricted with sludge then the heat input from the flame heats up the water to a higher temperature because the water is not flowing at the normal faster speed.

Correctly a non condensing boiler is designed so that the water returns to the boiler at say 70 C and is heated up to say 81 C.

This depends on the water flow rate and the heat imput. The heat input is varied according to the temperature measured at the output compared with the temperature set on the controls.

Tony

 

[handyjohns]

Thanks for your reply

I know it will bet hotter due to circvulation problems and hence the flow increasing in temperature.

The reason I ask is I have an old boiler, Glowworm Fuelsaver, and the pilot goes out after a while when the boiler is on, the interrupter kicks in (overheat interrupter) and trips the pilot as well as main burner.

Should I check the pump first?

When the boiler has colled, and the overheat resert, the pilot stays lit

 

[Space cat]

why would the flow get really hot and the return stay cool?

You can use the same amount of heat from the burner to heat a lot of water a little bit or a little water a lot. In normal operation, a lot of water flows through the heat exchanger. It comes in hot and goes out a little hotter - maybe 10 degrees hotter.

If you restrict the flow, two things happen:

1) The small amount of water you send out has longer to cool down before it gets back.

2) The burner can easily push its temperature all the way back up again.

If the flow is really bad, it won't be able to carry away all the heat. In this case, the boiler will sense the excessively hot water coming out and protect itself by turning the heat down.

Edit: Agile beat me to it.

Yes, check the pump. It might not even be running, leaving you with gravity circulation only.

 

[Agile]

Regardless of the water flow rate, the boiler thermostat controls the output temperature of the boiler. Try turning it down!

It could be the boiler stat or the overheat stat are out of calibration. You should not be accessing the O/H stat as thats inside the combustion area.

Tony

 

[handyjohns]

why would the flow get really hot and the return stay cool?

You can use the same amount of heat from the burner to heat a lot of water a little bit or a little water a lot. In normal operation, a lot of water flows through the heat exchanger. It comes in hot and goes out a little hotter - maybe 10 degrees hotter.

If you restrict the flow, two things happen:

1) The small amount of water you send out has longer to cool down before it gets back.

2) The burner can easily push its temperature all the way back up again.

If the flow is really bad, it won't be able to carry away all the heat. In this case, the boiler will sense the excessively hot water coming out and protect itself by turning the heat down.

Edit: Agile beat me to it.

Yes, check the pump. It might not even be running, leaving you with gravity circulation only.

when you say turning the heat down, do you mean on the new condensing typer boilers? When my one gets hot it just turns off and not down

 

[handyjohns]

Regardless of the water flow rate, the boiler thermostat controls the output temperature of the boiler. Try turning it down!

It could be the boiler stat or the overheat stat are out of calibration. You should not be accessing the O/H stat as thats inside the combustion area.

Tony

I've tried the boiler stat on all different temperature settings but the temeprature must get so hot due to a circulation problem that it still trips the overheat

 

[Space cat]

when you say turning the heat down, do you mean on the new condensing typer boilers? When my one gets hot it just turns off and not down

Yes, my old (1997) Potterton does the same - and so did the one before that. Its gas valve is either On or Off, there is nothing in between. But from what I've read on here, eg

The heat input is varied according to the temperature measured at the output compared with the temperature set on the controls.

some boilers are smarter than this.

Edit: I seem to remember that my current Potterton also tripped its overheat stat when the pump failed many years ago.

 

[Agile]

I've tried the boiler stat on all different temperature settings but the temeprature must get so hot due to a circulation problem that it still trips the overheat

If your boiler stat does not adjust the flow temperature properly then its failed and needs to be replaced.

That can be a safety issue.

Thats as well as the flow problem!

Boiler engineer time!

 

[handyjohns]

the boiler stat does adjust the temeprature but the oheat still trips!! the pump seems very stiff, I freed it with a screwdriver, and tested the resistance of the windings which were 167 ohms. he system is full of water and I checked for a blocked cold feed and all ok. I'll see how this goes, seems ok so far. Pump speed is on 3

 

[Space cat]

the pump seems very stiff, I freed it with a screwdriver,

I think you've found the cause of all your problems. Perhaps the recent heat wave had something to do with it. Pumps don't like sitting idle for prolonged periods.

 

[handyjohns]

the pump seems very stiff, I freed it with a screwdriver,

I think you've found the cause of all your problems. Perhaps the recent heat wave had something to do with it. Pumps don't like sitting idle for prolonged periods.

yep, all seems ok now! although I have been using the pump when h/w was on. Funny it seized/stiffened all of a sudden