De-commissioning back-boilers?

[S Viemeister]

Our out-dated, hard-to-control, inefficient heating system is due to be replaced this year. Currently, we have an oil-fired Rayburn (converted from solid fuel), _and_ a back-boiler in the livingroom fireplace. The Rayburn is to be removed, and an externally mounted condensing boiler will be installed. The back boiler concerns me - it will, of course, be disconnected from the heating system, but I wish to be able to continue to use the fireplace. How does one safely decommission a back boiler? Can this be done without wrecking the fireplace?

[PeterC]

Exactly the same here - ex-council, by any chance?

When the CH was fitted I removed the pipes etc. (as no other part of the system was still there) but have never done the last bit (dI don't use the fire). ISTR that the back boiler should be filled with sand, but never checked on that.

[Dave Liquorice]

Presumably this is an open fire with back boiler? Personally I'd keep it as a backup/auxillary heat source. Unless it is really kanckerd in which case your into replacing it anyway to keep the fireplace operational.

If you want to drain the back boiler and keep the fireplace I think you have to fill the boiler with sand to prevent (slow down?) it burning through. Without the water to take the heat away the metal will get very hot...

[Jeweller]

We simply left ours in situ and continue to use the open fire.

[js.b1]

There is H&S document on this.

backboiler behind a real fire (or box-tank behind a gas fire) works by recovering heat from flue gases, said heat then being used to pre- heat a HW tank or radiator. Decommissioning such back boilers (or either type) must be done correctly.

Immediate risk - explosion Water left in the tank which can not circulate or vent will superheat

- eventually rupturing or even exploding.

Secondary risk - flue gases If pipes are left vented the tank will eventually corrode permitting flue gases to be transferred elsewhere, posing a serious CO risk.

The proper solution is...

- Remove the pipes

- Fill holes occupied by pipes with fire cement

- a) Fill the tank with sand & leave ventilated to the flue only

- b) Remove the tank from the fireplace and refill with suitable bricks, refractory lining as necessary

They are actually quite good at heating water with a gas fire. However I suspect the decommission cost is rather high even if a simple "box tank". More substantial dismantling is properly rather expensive - anyone know?

Beware such backboilers exhibit a winter flood risk when the house is unoccupied. Cold air plunges down the chimney and over days can cause the tank &/or pipes to split - the resulting flood from the CW-HW tanks is either unlimited or still 800L if you have the CW isolated. I've known 3 fail in this manner, wrecking expensive wooden flooring and downstairs contents throughout (DG doors meant the water literally had nowhere to go with solid floors throughout).

[cynic]

Disconnect the pipes and plug the holes. Drill a couple of small vent holes at bottom front (you may get some residual water out). Use the fire as normal. At some stage it will burn through the front panel of the back boiler at which point you can use fire cement to effect a repair until such time as you wish to rip it all out and install a refractory fire back

[Onetap]

Thanks for that. I was aware of the explosion risk ; the freeze & flood hazard had never occurred to me.

A link to an HSE document on the fatal explosion is below.

[S Viemeister]

It seems that our short term solution will likely be to drain/disconnect/remove the piping, making sure that combustion products can't find their way into the house, and drill holes into the boiler body. I've seen mention of filling the boiler with sand - but am not sure how feasible this is - or why it should be done. The fireplace only gets occasional use, but eventually we'll need to remove all of the back boiler bits.

[Andy Dingley]

Heatsink. If you don't do it, the cast iron back boiler gets to about

400C, cracks and collapses, possibly bringing down some of the fireplace.

[S Viemeister]

That makes sense. Would it be a reasonable idea to make a decent-sized hole in the upper part of the water jacket (for the sand), along with one or two smaller ones along the bottom front (for water release)?

[cynic]

Never come across cast iron firebacks then? Its probably not cast iron anyway but mild steel.

[Andy Dingley]

Yes. They're bedded in mortar, which has a similar heatsinking effect. If they're not, they can crack too.

Besides which, firebacks are cast in thicker sections than back boilers.

No, mild steel rusts through too quickly.

There's advice linked from the HSE site on a recommended process for decomissioning. I think this was linked from here recently too. Mostly (AFAIR) it involved a drilled hole moderately high up, then slowly pouring dried sand down a pipe.

[Adam Aglionby]

Any idea what the service life of a back boiler is/was expected to be?

Know someone with a back boiler still in ,er, service,haousing association , its whole house heating days are long gone, concerned that its days of safe operation may be past as well...

Thanks Adam

[Adam Aglionby]

Should have said , back biler on coal grate fire.

[cynic]

Quite the opposite to heat sinking, the refractory retains the heat so the cast iron attains a higher temperature. Solid fuel (coke) boiler firebars and fireplace grates are also made of cast iron and attain red heat (1000C) on occasion. When in a hole stop digging!

I should tell Dunsley and other companies who make wrap round high output solid fuel back boilers from 4 mm mild steel with long service life. I am sure they will be fascinated by your information.

[Roger R]

illustration of a back boiler installation shows that is not acting as structural support for any of the chimney brickwork.

Is that what I'm likely to find in this 1950's house enabling the boiler to be taken out without risk of some of the chimney liner brickwork coming down, or at least needing support ?

Roger R

[Roger R]

:From my mother's 1950s (ok 1947-1951) the chimneys are all U-shaped :self-supporting (and supporting joists of rooms above with walls & :ceilings additionally). Lintel may be angle iron, but I suspect it may :be concrete. Very small windows (a foot) use what looks like cast in :place concrete 8in high, larger windows use a smooth grey cement :lintel equally oversized into the room & span, both on slate :padstones. Note however the mortar may be utter crap so beware :cracking particularly if the lintel is above the fireplace opening :which is not unusual

All original, and with concrete lintles for window openings so probably ditto for fireplace.

Lime based weak mortar with sand and lime (white) internal bricks.

:People here are referring to cast-iron backboiler, if yours merely :heats the HW tank you may find it is a simple box made of copper. I :assume copper going by the green verdigras all over it.

Perhaps you are confusing me with someone else - I havn't posted any picture.

The back boiler is indeed only for the hot water tank, but is a cast iron type with a removable panel above a coarsly ribbed lower part. Below this ribbed tank is a gap allowing the hot gasses to pass under and around the back. The hot gasses are diverted by means of a sliding plate out of sight on top of the unit. The brand name on the front face is 'Redfyre'.

Photo available later.

Roger R

[Andy Dingley]

Rubbish. Whilst it's going to "retain the heat", all that's going to do is to keep it warmer in the morning, after the fire has gone out. Peak temperature when lit for a piece of thin cast iron is going to be higher for a plate with an air gap insulation behind it, compared to masonry in reasonable contact. It's about the dynamic equilibrium of heat flow in vs. out when burning (i.e. conduction), not about heat storage (i.e. heat capcity).

Firebars are made of two different metals: some are a simple grey cast iron, others an austenitic stainless. Parkrays and similar use a mix, with the better steel in the middle where it's hotter. If you put them in the wrong place under a coke fire, the plain grey iron doesn't last at all long.

Mostly though, firebars are just that: bars. The reason they're installed as separate bars is to stop cracking and warping like this. The bars are free to move (compared to a one piece grate) so that they don't crack (they'd warp first) and any slight warping is absorbed in the rattle space around them. Although solid one-piece grates were used for a long time, they didn't work for the higher temperatures when we switched from coal on open grates to coke in enclosed box stoves.

Also, "red heat" is around 600 to 800 C. 1000 C is a pale yellow. Although you can achieve orange heats within the coals of a domestic fire, you won't get the ironwork to 1000 C.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Andy Dingley saying something like:

Not if it's 6mm thick to start with. I have a rescued BB that's 20 years old and still plenty thick. I intend to re-use it and expect it to last for many years yet.

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Adam Aglionby saying something like:

I know of many that are 30 years or older.