Back boiler: friend or foe?

[Another John]

Howdy all

We have a back boiler installed behind our (60s) fireplace. However our hot water comes from the gas CH system, which heats the water just once a day for a couple of hours.

We hardly ever use the fire except in the depths of winter. (We _do_ enjoy it immensely, so we wouldn't consider _not_ having any fire at all.)

I always used to think: back-boiler: bonus! Free hot water!

However recently a different thought occurred to me: if the CH is providing a cistern full of hot water... but the back boiler system is full of cold water ... is it not going to slowly circulate the hot water down, by gravity, through the back boiler, then back up to the hot water cistern ... and slowly cool that expensive hot water?

I've a mind to take out the back boiler, in the hopes that our hot water supply will stay hotter longer. (BTW there's only two of us in the house now, and we don't have baths (we have showers instead!).)

I've been thinking about a wood stove for many a year, so now may be a good time to do it. Yes, I am aware you can get stoves with back boilers, but for the reason described above, I wouldn't fancy one.

Can anyone dispel my ignorance in this area?

Cheers John

[Phil L]

Heat has a tendency to rise, so it's unlikely that hot water will 'fall' down the pipes into the cold back boiler

[Dave]

Hot water always rises. It goes against the laws of physics to fall.

IHTH

Dave

[Dave Liquorice]

Heat moves from hot places to colder places in any direction.

Hot objects expand, thus they become less dense than colder parts of the same object. If the object can flow like in a fluid or a gas the hotter, (less dense) will be pushed up an away by coldler (denser) flowing in underneath.

So the hot water in the cylinder above the colder water in the back bolier is going to stay up in the cylinder. Well the heated water in the primary coil rather than the cylinder contents. It might be worth checking how the gas boiler is plumbed into this system. It might be that the flow (gravity or pumped) through the gas boiler also introduces an unwanted and wasteful flow through the back boiler.

[cj]

Hi You say the house is a 60's build IIR the backboilers in these houses was (as you say) behind a coal fire with a flue diverter in the chimney to suck heat around it when the fire was lit. This type of system was a direct system with the same water in the hot cylinder being routed through the back boiler .(memories of dad yelling Turn on the Bl*&^y hot water quick before the bedroom fills with steam). When central heating was fitted one way to heat the existing tank was to remove the immersion heater and replace it with a heat exchanger coil which the new gas boiler heated. Another was to replace the existing cylinder with an indirect unit and disconnect the back boiler totally leaving the boiler behind the fire empty. It may be worth checking your cylinder to verify the system.

HTH CJ.

[Jonathan]

Our wood burning stove with a back boiler has its own valves and pump and only pumps water when the thermostat attached to its pipes indicates it's hot enough. I have the thermostat set at 45C and it certainly warms the radiators.

Jonathan

[Dave Liquorice]

And what happens when you have a good fire burning and you get a power cut? Just when you need the heat from the wood burner you have to put it out to stop the boiler boiling or possibly exploding.

The circuit from a wood burner or other stove should be gravity and have a passive heat dump for when the cylinder has reached 90C...

[Jonathan]

No, two radiators are permanently attached on a separate loop, gravity fed to dissipate any excess heat and are sized for that purpose.

Jonathan

[Scion]

How come I don't get a wet ceiling when I turn on the hot tap then? :-)

[John Rumm]

Just for clarification, the CH system is separate and has its own boiler? (many back boilers actually run the whole CH system - the fire front is an added bonus, but not a fundamental part of it)

Nope, the hot water should rise and stay there since its being heated from the top. Only lack of insulation will cool the cylinder. Note that the pipes in and out of it, plus control valves etc can lose heat at

100W or so on their own.

The same would apply to a stove with a boiler. You will only get convected circulation when you are heating the water low down in the system and it then rises (causing additional cooler water to circulate into its place). Its also easy enough to prevent convected circulation (either with a valve or even just an "anti gravity loop" in the pipework, however you can only safely do that with heat sources that are properly interlocked or with fully pumped systems. Solid fuel boilers normally require convected circulation to remain safe since they can heat even in a power cut etc.

[The Natural Philosopher]

disconnect back boiler and fill with sand.

Hot water from a back boiler is such an insignificant factor its not worth it unless you have a 24x7 fire like an Aga.

[js.b1]

Seal the piping too, because otherwise that can "plumb" CO to undesired locations.

Back boiler piping sat in cold chimneys can burst during extremely cold weather, one contributory reason for their demise when combined with expensive wooden floors.

[Dave]

It's a bit like letting the smoke out of electric motor. You turn on the tap and the hot water has to fall out of a hole, just like the smoke does from the motor.

Dave