Back Boiler - run with no water

[Chris B]

Friend of mine has a leaking HW cylinder. They have a very expensive maintenance contract with BG so they have rung them up to come and have a look at it.

"Oh you have got a very funny tank - its got 2 coils in it."

"Yes we know it has 2 coils, one is for the Gas the other is for the back boiler behind the open fire in the lounge."

"It will take us ages to get hold of a cylinder of that size with 2 coils can you live without the back boiler?"

"Well yes its never been used since the gas was installed."

My question to the group, if you drain all the water out of a back boiler system, does that preclude using the open fire? IE will you melt/damage/distort the fireplace/boiler?

I don't think it will bother them if the answer to the question is no you cant use the fire - as it has not been used in 25 years - but they would like to know if it MUST not be used rather than they choose not to use it.

[AJH]

IIRC people would fill the back boiler with dry sand and the top vent left open and carry on burning.worse case then would be sand spilling into the fire as the steel oxidised.

[S Viemeister]

MUST not be used, unless made safe first.

[newshound]

The issue there was that the pipework had been capped and there was presumably still some water left in the back boiler. Very bad idea.

As the other poster said, filling with sand AND LEAVING VENTED is traditional, although TBH I am not quite sure what the sand filling does. My woodburner is just a cast iron box; no firebricks so the back panel sees direct heat. With a back boiler, it's the front face of the boiler that sees direct heat. Its back face will get up to a similar temperature by radiation, depending on how well it is cooled from behind. You are not going to melt anything in a woodburner, although in a coal stove (especially with smokeless fuel) the grate may get hot enough to distort significantly.

If it were mine, and if I was unable to confirm with high confidence that the pipework had not been capped (and perhaps bricked in) then I would probably drill a few 3/8 inch holes in the boiler through the open door. It should drill reasonably easily with a cordless drill and a sharp HSS drill bit. Come to think of it, a couple of through-wall slits with an angle grinder would have the same effect.

Also, back boilers are sometimes an optional "add-on" so it might be possible to remove it, although you would probably have to do some dismantling.

[The Natural Philosopher]

Conventional wisdom says you fill it with sand and then its OK I didn't bother and 20 years later no issues..,

It will be fine. At the worst you may melt or distort the steel boiler housing if you get it to yellow or white hot

[The Natural Philosopher]

Well making safe consists in simply disconnecting the pipes - in that case it happened the very first time she lit the fire before the water had any chance to evaporate and some numpty had left the pipes sealed

[Martin Brown]

That was what the guy who looked at ours wanted to do. You need something behind it to take way some of the heat. The metal on its own would heat up too quickly. It will run a lot hotter with dry sand in but nowhere near as hot as a thin sheet of metal with air behind it would.

[Chris B]

Thanks all for the comments (particularly the references to the explosion and HSE advice) I will be sure to pass it on. It looks like its not just them but any future house owners who need to be considered.

[The Natural Philosopher]

The key is not to seal it up with water inside

It can handle hot air but not steam

[George Miles]

I thought you fill the back boiler in a woodburner with sand so that it can be emptied and used as a water boiler later on, otherwise the tank will burst.

Might be wrong.

george