wood burning stove back boiler loud noises

[digbym]

It's enough to scare the (large) dogs!
So, we had a wood burning stove with back boiler installed at the end of last heating season. We didn't use it much. Now it's getting colder we've started using it for the first time in ernest.
It's connected, indirectly to what I understand is a fortic cylinder (integrated header tank) but also runs through a towel rail on the circuit.
The cylinder is heated overnight with an immersion.
Getting to the point, it's started making some pretty serious noises. Enough to make us stop using it.
It starts with a gurgling but builds up (I assume as the fire gets hotter) to what sounds like a loud metal on metal crack.
Only other things to say are that we're in a hard water area and we've only really been running this stove for a couple of weeks since installation.
Hot water is taken off the cylinder via another coil so that it comes out at mains pressure. The cylinder is on the floor above the stove/back boiler. The back boiler circuit is not pumped. The cylinder was installed new at the same time as the stove (March this year)
Anyone got any idea what might be wrong?
Is this seriuos?

Thanks,

Digby

 

[jackthom]

You don't say how hot the towel rail gets or how hot the cylinder gets when this noise starts.

It's likely the water in the back boiler is actually boiling and if so it isn't a healthy situation.

Is there no way of closing off the fire or controlling how much heat from the flames the back boiler receives?

 

[kevplumb]

Hot water is taken off the cylinder via another coil so that it comes out at mains pressure. The cylinder is on the floor above the stove/back boiler. The back boiler circuit is not pumped. The cylinder was installed new at the same time as the stove (March this year)

so its a heat bank or thermal store

The cylinder is heated overnight with an immersion.

why

 

[Goldspoon]

Has the header tank got water in it (just checking)?

 

[TicklyT]

Is the cylinder coil suitable for use with a gravity primary circuit?

High recovery rate coils are usually designed for pumped circulation. They offer too much resistance for effective gravity circulation.

A poor pipework layout or incorrect pipe sizing can also stall gravity circulation.

 

[cider]

Was it installed by a numpty

 

[kevplumb]

no i don't do thermal stores

 

[digbym]

Thanks all for your replies.

jackthom: I haven't checked the temperature of towel rail or cylinder in this situation. I have an external thermometer attached to the cylinder so will fire it up again and check this.

There is no way of controlling the amount of heat from the flames the back boiler receives but I can shut down the air to the fire and slow it down quickly. However, we want the stove primarily for room heating and ambience so I'm not keen on having to put out the fire whenever this happens.

kevplumb: I guess it is a kind of thermal store. I've never really worked out the exact definition of a thermal store but yes, my understanding is that the water in the cylinder stays there, there are two coils in the cylinder - one for the back boiler to supply heat and the other to take off mains pressure hot water.

The overnight immersion is our primary hot water source. on't want to have to fire up the stove all day every day for hot water. But I did like the idea of getting a little 'green' hot water every time we had a fire in the living room.

Goldspoon: it's a fair point, I'll check tonight

TicklyT: I hope I am right in thinking you've got the two coils mixed up. The high recovery rate coil is main pressure, the other coil is connected to the back boiler. I assume the cylinder was correctly specified since it was installed new with the stove.

cider: I don't think so. Our builders in house plumber did it - HETAS approved etc ...

I'll post back once I've checked out the above .... Thanks again

 

[digbym]

Goldspoon: There is water in the header tank - checked last night

Stove never got veryhot last night so didn't have the problem. It's quite mild today so probably won't fire it up too hot again tonight.

Will post back when I've fired it up fully

 

[Nige F]

FWIW I can`t see any thermal store being fitted with a coil suitable for a gravity circuit Would be interested if there is a specific beast .

 

[Goldspoon]

FWIW I can`t see any thermal store being fitted with a coil suitable for a gravity circuit Would be interested if there is a specific beast .

But ordinary indirect cylinders can use gravity circuits?

 

[digbym]

So, maybe there's no coil on the stove/towel rail/cylinder circuit and it is a direct cylinder heated by the stove directly.

This still doesn't answer what's going on.

Fired it up last night and had same issue.

Towel rail was hot but I could hold it without burning my hand.
Cylinder was at 60 degrees C
Flow from stove was too hot to touch without burning hand. Return was hot but I could hold onto the pipe.

I also took a look in the header tank whilst the gurgling and intermittent metal-on-metal cracking noise was happening. There was some kind of overflow which I guess was coming up from the cylinder and fed into the header tank (not the header tank overflow to outside) that had "slugs" of water pumping into it every now and then .... Is this being forced up this pipe by air coming up to the cylinder from water that is boiling in the stove?

 

[jackthom]

It's almost certainly steam from the boiling water in the stove which is venting into the F&E tank.

The back boiler seems to be producing more heat than the cylinder/gravity feed can deal with.

For the time being is there any way you can cut down on the contact the fire has with the back boiler using firebricks, or do all the hot gases have to go through the heat exchanger to the flue.?

 

[ajstoneservices]

If the store is still hot from the immersion when you light your fire it's no wonder it's over heating.

Use the immersion only when not using your fire. you may need a bigger heat leak.

 

[digbym]

Jackthom:
I don't get why it would be boiling when the store's only at 60. I know the gravity feed is slow but surely if there's 90 degree water in the stove and only 60 degrees in the store, the water's going to start moving pretty sharpish to balance out the temperature difference?

there's no real way to block the back boiler with firebricks


ajstoneservices:
the store is only ever at 50 degrees max by the time we light the fire. Is this really too hot for the fire?
we don't light the fire for long enough of an evening to get the store up to the temperature we'd need for the use we'd have the next day. We must use the immersion overnight to top up the stove heating.
But, even if we did light the fire for long enough to get the store up to 80 degrees (that's the temp the immersion is set at) by the evening and turned the immersion off, we'd then be in the same position the next night of having a store at max 50 and the fire heating the back boiler to what everyone on this forum seems to think is boiling point.

I just don't get how a store at 60 connected - probably directly based on comments previously - to a stove that's only a few meters away can result in the stove back boiler reaching boiling point. Surely the problem is that the heat isn't getting from back boiler to store quick enough ?

Could the towel rail be the problem? Is it constricting the flow too much?

There certainly seems to be a significant delta T here with the flow pipe too hot to hold and the return hot, but cool enough to grip and not burn my hand.