Back boiler and a pressurised water system

gears · 06-08-2009 11:09am #1

I've tried to find this in old posts as I assumed it would have been talk about before, but no luck. So forgive me if it's a repeat post.

Any way my question is that I seen to remember being told that there is a risk in using a back boiler and a pressurised water system because there is essentially no control over the heat out put from the back boiler, just throw another log, shovel of coal or what ever and there's a sudden increase in heat which could result in an explosion (extreme I know) from a pressuriesed system.

Does any one know if there's truth to this?

sydthebeat · 06-08-2009 12:23pm

ive put similar questions to plumbers.... usually when condensing boilers are also in the mix.....
one has said that theres no problem using a pressuirised system once the back boiler can take the pressure, which the usually can (3 bar??) ... expansion vessels may be required if stoves are used....

but professional plumbers out there would have more info...

Poor Uncle Tom · 06-08-2009 12:28pm

I looked into this a few years back with the same concerns you voiced here. Basically I found that it would be a very similar concept to the boiler on a steam train and similarily would need a pressure release valve and possible back-up release valve system. Also all the pipework would have to be a pressurised system. Finally, none of the pipework could be encased in concrete because of the expansion/contraction and there would be continuous noise within and from the system. It would be a costly system to install properly.

sydthebeat · 06-08-2009 12:59pm

Poor Uncle Tom wrote: »

I looked into this a few years back with the same concerns you voiced here. Basically I found that it would be a very similar concept to the boiler on a steam train and similarily would need a pressure release valve and possible back-up release valve system. Also all the pipework would have to be a pressurised system. Finally, none of the pipework could be encased in concrete because of the expansion/contraction and there would be continuous noise within and from the system. It would be a costly system to install properly.

some plumbers ive spoken too say it would cost in the region of €1500-2000 extra for a basic system in a 2000 sq ft house...

gears · 06-08-2009 2:33pm

I'd still be concerned about the uncontrolled heat source and high water pressure. Relying on pressure release valves doesn't fill me with confidence having a second one as a back up would help.

I would be interesting to hear from any one who has such a system though... Any one out there?

Leadership · 06-08-2009 3:42pm

I am putting in a Thermal store to store the heat from the stove with back boiler. Basicaly the store (500litres) is vented and backs up the main heating system which is pressurised. Heat is transfered between systems by a heat exchanger in the store so essentially they are seperate systems.

The aditional costs are roughly €650 for the store, €300 for the pumps and gubbins that go with it oh and the labour but as mine is new build the plumber is only charging an aditional €100 for this.

The system can be seen working at Brooks heat merchants. Its a safe efficient system.

Poor Uncle Tom · 06-08-2009 4:15pm

sydthebeat wrote: »

some plumbers ive spoken too say it would cost in the region of €1500-2000 extra for a basic system in a 2000 sq ft house...

I was given a cost excess of €4,500 for a 2,500 sq ft house...

sydthebeat · 06-08-2009 4:24pm

Poor Uncle Tom wrote: »

I was given a cost excess of €4,500 for a 2,500 sq ft house...

oil boiler and rad system?

Poor Uncle Tom · 06-08-2009 4:29pm

sydthebeat wrote: »

oil boiler and rad system?

high pressure back boiler, oil boiler and rad system

sydthebeat · 06-08-2009 4:36pm

Poor Uncle Tom wrote: »

high pressure back boiler, oil boiler and rad system

are you including the back boiler in the extra?? i wouldnt be...
back boiler and associated work = circa €2000 thus our figures would add up...

im simply talking about the extra to make a system pressurised....

Poor Uncle Tom · 06-08-2009 4:40pm

sydthebeat wrote: »

are you including the back boiler in the extra?? i wouldnt be...
back boiler and associated work = circa €2000 thus our figures would add up...

im simply talking about the extra to make a system pressurised....

I'm including the cost of the new backboiler less the cost of a standard one and all associated work.

Poor Uncle Tom · 06-08-2009 7:11pm

Sorry Sydthebeat, I hadn't much time to expand on this earlier. Fundamentally our prices are the same, but my client and I were put off the system by the idea of all the noises the system would make due to the stainless expanding and contracting. The price itself wasn't off putting but it is a leap in any budget. I do think the concept is very good though and way more functional then the conventional back boiler system.

I also find the combined oil and back boiler system has a huge flaw in that the back boiler becomes another radiator heating the chimney brest when the fire is not lighted and the oil boiler is working the system.

Carlow52 · 06-08-2009 8:17pm

Just musing here:

Closed heating system with back boiler.
Fire going full bore, rads piping hot.
-
Power Cut.
-
How does the excess heat get dissipated under gravity in a closed pumped system?

sydthebeat · 06-08-2009 10:18pm

Carlow52 wrote: »

Just musing here:

Closed heating system with back boiler.
Fire going full bore, rads piping hot.
-
Power Cut.
-
How does the excess heat get dissipated under gravity in a closed pumped system?

expansion vessel...

Carlow52 · 06-08-2009 10:50pm

sydthebeat wrote: »

expansion vessel...

Too risky in my view depending on pressure relief valves and expansion vessels to absorb the heat produced by say an 12kW fire with no circulation pump. Radiators at 130-150 degrees C would be profoundly dangerous

Poor Uncle Tom · 06-08-2009 11:42pm

Carlow52 wrote: »

Too risky in my view depending on pressure relief valves and expansion vessels to absorb the heat produced by say an 12kW fire with no circulation pump. Radiators at 130-150 degrees C would be profoundly dangerous

I have been told the expansion vessle can be made to work as a gravity circulation measure also, I didn't explore this though as we gave up on the idea.

gears · 10-08-2009 9:34am

Carlow52 wrote: »

Just musing here:

Closed heating system with back boiler.
Fire going full bore, rads piping hot.
-
Power Cut.
-
How does the excess heat get dissipated under gravity in a closed pumped system?

sydthebeat wrote: »

expansion vessel...

Carlow52 wrote: »

Too risky in my view depending on pressure relief valves and expansion vessels to absorb the heat produced by say an 12kW fire with no circulation pump. Radiators at 130-150 degrees C would be profoundly dangerous

This is my problem with this idea of back boilers and pressurised systems, I have a feeling that it's just too dangerous relying on an expansion vessel aNd if you were to how big should it be.... 10L, 20L 100L ???

sydthebeat · 10-08-2009 10:37am

gears wrote: »

This is my problem with this idea of back boilers and pressurised systems, I have a feeling that it's just too dangerous relying on an expansion vessel aNd if you were to how big should it be.... 10L, 20L 100L ???

www.planningportal.gov.uk/uploads/br/BR_PDF_PTL_DOMHEAT.pdf

"Solid-fuel appliances should not be fitted to sealed heating systems, with expansion vessels, except where specifically permitted by the manufacturer."

"Minimum provision for new systems...

b. Unvented hot water storage system products should
. comply with BS7206 ....
c. Unvented systems should not be use with gravity circulation."

so, you use materials designed for the purpose.... this is something your plumber should know and understand...

what do you do????... if in doubt, you put out the fire.... or dim it down so it doesnt over boil the water.....
what would you do with a solar panel on a very warm summers day if the power goes???

heat dumps are required, and thats what id look at if i was planning an open fire back boiler on a sealed system... one of course with a manual valve.... and of course an adequately sized expansion vessel... the size of the expansion vessel is directly related to the amount of pressure the system can take (higher pressure = smaller expansion vessel)

http://www.boilerstoves.co.uk/vented-or-pressurised.html

im not a plumber and certainly dont purport to know what im talking about... but i have seen pressurised systems with back boilers, generally stoves though...

sammywilsonjnr · 17-08-2009 11:28pm

Firstly - apologies Gears for jumping on your bandwagon, but your query is damn close my own.
Secondly, I'll apologise in advance for my sketchy descriptions and gaps in knowledge - this is something I was assured was under control (and may well be) but now I have some reservations.

Have just moved into a new build. Oil fired CH with condensing boiler and direct mainsflow type tank. Before a trench was dug we made it clear that we wanted some secondary heating source in the house and were looking at a multifuel stove with back boiler. Fine, no problem - the builder worked with a plumber who'd stacks of experience in integrating the 2 systems. The understanding was that the boiler would link to the rads.

Fast forward to Saturday night when, out of curiousity more than neccesity, I decide to spark up a small fire (and honestly it wasn't anything lunatic). Flow pipe to rear and side of stove begins to heat and continues to heat - grand. I'm waiting for the pump to kick in and begin circulating the water (informed by plumber of stat in attic to control flow), and I'm still waiting when the groans and thumps began in the pipe (which by now is too hot to touch). Theres a switch by the side of the chimney which I was told was an over-ride to bring the pump on in this situation. But it appears to do bugger all so I opened the stove door and let the fire burn itself out while the noise continues for a while before settling down. Had a few beverages comsumed so decided against going up into the unfloored attic for a look around there and then.

Next morning I head up to have a nosy - and here is where I'm really after some advice. The flow pipe from the back boiler comes into the attic and the first thing on it is a clip on thermostat set to 55c - theres a 3 core cable running from this down into the hotpress. Next the pipe comes to a t-junction - one tee leading to an expansion tank and the other heading towards the thermal store in the hotpress. Before getting that far though it flows through a pump which is wired up to a junction box in the hotpress beside the feed from the clip on stat. At this stage its 12 hours since the fire went out and the CH hasn't been on between times yet the flow pipe from start to finish is still very hot but not near as hot as the pump casing which is too hot to touch.

Plumber is currently being tracked down but before I have this conversation with him, and I don't mind looking daft here, does this link-up arrangement sound right. I've only noticed today the effect, mentioned earlier in the post by Poor Uncle Tom, of hot water from the oil fired CH travelling through the back boiler flow pipe and heating the interior of the stove - should the siting of a pump on this pipe not prevent flow in that direction?

Also looking into the heatstore in more detail is it even possible to heat the rads from the back boiler or is domestic hot water the best we can get? My understanding of direct heating is the bulk volume of water in the tank is itself heated by the boiler and part of the radiator heating circuit. If this is the case how do I trigger the central heating pump and valves to run and open to circulate the water if I'm heating from the back boiler and not via the oil timer?

Like I said I'm a bit vague on actual detail but this was one of the aspects of the house I left totally to the experts. And it could turn out that the set-up I have is correct and is my only option, but I'd value any comment or feedback people may have.

Carlow52 · 18-08-2009 8:24am

Sorry to hear u are having trouble.

Just a few questions before the experts come on so as they can offer the best advice.

What do you mean by

direct mainsflow type tank

When you talk about a thermal store why do you call it that rather than a HW cylinder.
Just to confirm, the tank is an expansion tank [and not a expansion vessel] and the pipe from the Tee goes up over the tank so as if there is any blow out of hot water it goes into expansion tank.

Just on the pump: it is just a simple impeller pump and the water can pass both ways, they dont act as a valve when not running.

you talk about

If this is the case how do I trigger the central heating pump and valves

What valves and what do you expect them to do?

Lastly: is there any radiator on the bb circuit to act as a heat sink?

sammywilsonjnr · 18-08-2009 8:50pm

Carlow,

Thanks for the reply. I've just lit the thing again to answer one of your questions and ended up covering 2. Sorry the reply is out of running order .

- the boiler circuit isn't (as far as I can tell) flowing through a radiator or towel rail at any point. All the rads were bled and heat with the oil fired CH
- The expansion tank is a regular 20ish ltr tank with a cold water feed at the base, expansion overflow coming out the side (running out the soffit board) and two pipes going up and over - one from the back boiler and one from the CH system proper.

And yes this is where the hot water blows out - I know this because after the fire was lit for a while and water appeared to be going nowhereI hopped up into the attic and found hot water spraying into the expansion tank which was full of rapidly warming water.

I called it a thermal store only because the term was stuck in my head after going through the companies website - I'll pm you the make and manufacturer so you'll know what I'm talking about and because I've no issues with the tank and would hate to put anybody else off them.

The valves I was talking about were the ports that open and close to allow the flow and return of hot water from rads to tank and back - like I said could be way off the mark on this. I'll drop the details on a pm if you wouldn't mind?

Appreciate the knowledge

cmsp.plumber · 18-08-2009 9:12pm

a back boiler cannot be a closed system it must be left open, i.e have an open vent pipe. you can link 2 systems, example a gas/ oil system with a back boiler. but the back boiler must always have a open vent. sammy.. the groaning and thumping doesnt sound good and the water should not be coming out the expansion pipe in the attic when operating correctly. the switch usually operates a pump to heat the rads and the water in the cylinder(hot press) is heated on gravity, this is the way it was always done for the pass 25yrs. although now every circuit must be fully pumped due to new regulations.

doctorjohn · 18-08-2009 9:14pm

Guys,
Am at the design stage of my build. This thread has the crux of my heating dilemma.
Plan = 3 heat sources (solar panels, oil boiler in shed, back boiler on one stove) all ideally connected to a large HW tank as buffer to give heat into the house as
downstairs underfloor heating and upstairs as rads.

Is this feasible or is it a pipe dream (pun) (bad one at that)

Extra heat (if & when generated) ? where best to dump it. Suggestions gratefully recieved.
? Into bathroom towel radiators
? Into waste water

Help.!

cmsp.plumber · 18-08-2009 9:26pm

its not a dream doctorjohn! it can be done no problem. your hot cylinder can be used a heat leak. its easy to link solar and oil/gas systems. once your stove has an open vent your fine.

Carlow52 · 18-08-2009 9:48pm

cmsp.plumber wrote: »

its not a dream doctorjohn! it can be done no problem. your hot cylinder can be used a heat leak. its easy to link solar and oil/gas systems. once your stove has an open vent your fine.

Can you turn down the volume on your text as the font size is deafening me?

There are earlier post on this thread which says that u can do BB and OFCH in a closed system: I dont like it from a safety perspective but it can be done

muffler · 18-08-2009 9:55pm

cmsp.plumber would you mind using the default text settings in your posts please. I have edited your last 2 posts.

Cheers.

Carlow52 · 19-08-2009 10:43am

sammywilsonjnr wrote: »

Carlow,

Thanks for the reply. I've just lit the thing again to answer one of your questions and ended up covering 2. Sorry the reply is out of running order .

- the boiler circuit isn't (as far as I can tell) flowing through a radiator or towel rail at any point. All the rads were bled and heat with the oil fired CH
- The expansion tank is a regular 20ish ltr tank with a cold water feed at the base, expansion overflow coming out the side (running out the soffit board) and two pipes going up and over - one from the back boiler and one from the CH system proper.

And yes this is where the hot water blows out - I know this because after the fire was lit for a while and water appeared to be going nowhereI hopped up into the attic and found hot water spraying into the expansion tank which was full of rapidly warming water.

I called it a thermal store only because the term was stuck in my head after going through the companies website - I'll pm you the make and manufacturer so you'll know what I'm talking about and because I've no issues with the tank and would hate to put anybody else off them.

The valves I was talking about were the ports that open and close to allow the flow and return of hot water from rads to tank and back - like I said could be way off the mark on this. I'll drop the details on a pm if you wouldn't mind?

Appreciate the knowledge

Thanks for this and the PM.

It looks to me that what u have is a single coil heating the Thermal store There is a heat exchanger in the thermal store for providing hot water by passing the rising mains through it.

The single coil has both the OFCH and BB connected to it, hence the BB being heated by the OFCH.

What also seems to be the case is that the heating coil in the tank is not on a separate feed from the OFCH so, and I hasten to add MAYBE what u have is one feed/return loop with the OFCH, BB,heating coil in the TS and all the rads on one 2 pipe circuit.

Do u have a 2 zone time clock, one circuit for HW and the other for the rads.

You may well have a 2 zone clock with the zoning achieved by the valves.

What may be happening is that the stat on the tank is already at 55 so it says no the the BB.

I would turn off the OFCH and the elec immersion and let the hot water in tank go down below 55 : big bubble bath or 2:D

Then light the stove.

The purpose of this is to see if my theory with the 55 degree stat is correct or something similar.
Hope these ramblings help

sammywilsonjnr · 19-08-2009 9:00pm

Evening Carlow,

PM'd you on the other question. Again I appreciate your time on this - feel this is becoming a regular evening event.

Yes the timer unit has two switches - one for central and one for water heating. Theres also 2 room stats one down and one upstairs.

I'm just after giving your idea a go. Have had the ofch off since this morning and what heat remained I ran off the taps this evening. Sparked up the beast and waited. Checked the attic between times and at that stage the pipes towards the expansion tank were cold but, and this may be my problem, the pump was hot to the touch - is it possible the impellers jammed and this is the issue? To be fair the pump in the hotpress is whisper quiet, almost silent when it runs.

The other difference this time round is that the tank heated (and is still heating, but I'm letting the fire burn down) without any noise or venting to the expansion tank. But still no sign of the heat travelling to the rads. Am I right in thinking that the timer switch is mainly concerned with controlling the on and off function of the oil boiler?

Leading on from this, I'd value your opinion on a conversation I had with the plumber this afternoon that I'm still trying to get my head around. As far as hes concerned I should be lighting the fire and then turning on the ofch in order to get water temp up and pumping round the rads. At the point where the tank reaches temp. the tank stat will cut out the oil boiler and the rads circuit stay open and pumping - being heated by the BB. I haven't even considered attempting this - is this not back to where I started?

Cheers

Carlow52 · 19-08-2009 9:33pm

Interesting.

I think the tank stat when closed will shut down the boiler but not the main circulation pump.

I think the 2 room stats will do the same, u can test this by turning them down to the lowest

It seems to me that to simplify the wiring, the boiler will always need to be on so as the main circulation pump will run so as to circulate the hot water from the BB through the rads This means that the BB and the boiler are essentially like rads on the circuit.

Its a common enough problem which can be resolved by using a multi-pin relay which will allow the main cir pump to click in when the 55 degree pipe stat closes, without the CH time clock calling for heat.

To test this theory u should be able to turn the stat on the boiler down to its lowest, may only be a dial and then light your fire and fire the boiler, it will fire only until the water get to the lowest temp which should then increase from the BB, u should feel the temp rise in the pipes passing through the boiler

The BB pump with clip on stat and override switch seems knackered if its hot and the pipes are cold.

sammywilsonjnr · 22-08-2009 12:15am

Cheers for all the advice and help on this one folks. CMSP and Carlow - I think the answer is somewhere there in your feedback

Carlow52 - well on the road to a solution thanks to your time and patience. Sorry to read about the mod issue. Again, many thanks and hopefully catch you on boards in the future.

Back boiler and a pressurised water system

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