Rayburn fuel change or boiler stove?

[CliffordPope]

I've found that our stove is often a bit reluctant to start a syyphonic circulation, which is why I added the circulation pump. I sometimes find that the stove itself gets very hot, but there are bumping and bubbling noises from inside the boiler, as if the water is boiling but champing at the bit to get out. That's when I switch the pump on, and in seconds the noise ceases and all three radiators are very hot. It can then support the circulation unaided, but it works much better with the pump.

I suspect that a thermosyphon works best in the traditional way with the stove downstairs running a towel rail in the bathroom immediately above, together with heating the hot water in the airing cupboard in the same room. It doesn't seem very eager to circulate if there are horizontal runs.
I wonder also if the pipe diameter is important. Ours uses 28mm copper pipes, which I read was the minimum for an efficient circulation, but I remember that the pipes in the house where I grew up were cast iron about 2" in diameter. That included the single additional radiator in the room adjoining the kitchen.


A man at the local stove installers said that for efficient running the fuel should be up to the level of the sill with burning logs or coal, and the unburnt then piled on top. They don't run very well with only two logs.

 

[fernicarry]

It doesn't seem very eager to circulate if there are horizontal runs.

There's probably guidance for the minimum gradient but our Aga with back boiler used to manage about a 4m horizontal run with probably no more than a 9" rise on that distance. That was just to the hot water cylinder though, not through any radiators. Its all plumbed in 28mm which does make a difference, greater bore equals less friction.

The radiators don't look too old or rusty, but certainly worth checking

The tappings on modern steel rads are generally only 1/2". If they've been plumbed in 15mm or worse still 10mm as it typical nowadays I could see how that would restrict the flow to the point that not much happened. I have recently acquired an old Ideal Standard cast iron radiator from a friend and its tapped for 1.25" and would originally have been on a thermosyphon system. Getting the bushes off of it to reconfigure it for bottom flow took quite a bit of doing...

A pump could be the solution but it should also allow thermo-syphon bypass in case its not running for any reason so that the heat can still go somewhere albeit more reluctantly. Or replumb your radiators in parallel. Whatever your heat source that's desirable anyway as you'll be able to control them individually.

But it could be as simple as bleeding the rads...

 

[a twig]

We had similar problems with a wood burner with back boiler that ran three rads. In theory it had a thermo-syphon to the first rad which was in the kitchen, and then a pump to reach the other two.

In reality even when we could hear the back boiler bubbling away (and the occasional hiss from the over pressure valve if I was feeling brave and really wanted to test if the system was working) nothing really circulated unless the pump was on, at which point the rads would then get very hot very quickly

The whole set up made me very nervous especially when my father used to visit and enthusiastically throw a couple more logs on when I was out the room.

The whole lot was in 15mm pipe which was teed into the 22mm which exited the back of the stove, and a plumber suggested it would need a header tank to work reliably without the pump, at which point I gave up diagnosing and we ended up ripping the whole lot lot, plumbing those rads into the gas CH, and replacing the stove for one without a back boiler

 

[CliffordPope]

I once found a useful website, long since lost, which gave the effective resistance in a network of water pipes taking into account diameter, vertical height, horizontal distance, bends, etc. Every component and section had to be individually listed, and then it came up with the answer. You could play around on paper as it were making changes gradually improving the overall performance.
Bends are much less resistant to flow than elbows, and especial care was worth taking to source threaded connectors, isolators etc that didn't needlessly impede the flow path. by restricted internal bores or sharp changes in direction of flow.

 

[a twig]

Might well be something similar on here:

The Engineering ToolBox

Free tools and information for engineering and design of technical applications

www.engineeringtoolbox.com

I’ve found lots of useful resources there over the years when designing things

 

[CliffordPope]

Many thanks. It's in there somewhere - then I've just got to understand it!
The one I remembered was about 100 times less complicated. I think it just started with a standard 20 foot drop, and then showed the effect of adding bends, choosing a wider pipe, simplifying a pipe route, etc. It demonstrated that the pressure loss at the tap in a bathroom in a distant point could be overcome by using 25mm pipework instead of 15mm, or by bending the pipes properly rather than using capilary 90 degree bends.

 

[kernowqueen]

urs uses 28mm copper pipes, which I read was the minimum for an efficient circulation, but I remember that the pipes in the house where I grew up were cast iron about 2" in diameter. That included the single additional radiator in the room adjoining the kitchen.

The tappings on modern steel rads are generally only 1/2". If they've been plumbed in 15mm or worse still 10mm as it typical nowadays I could see how that would restrict the flow to the point that not much happened. I have recently acquired an old Ideal Standard cast iron radiator from a friend and its tapped for 1.25" and would originally have been on a thermosyphon system. Getting the bushes off of it to reconfigure it for bottom flow took quite a bit of doing...

These are very interesting points! The pipe coming out of the Rayburn is very wide and the pipe feeding the rads is tiny, although I haven't measured them, I imagine that will be restricting the flow. May need to consider a pump then!

 

[fernicarry]

If you do fit a pump then make sure that the hot water can still go somewhere even if the pump is off (power cut etc.) as it will restrict the flow even more.

One of the big pipes should be an expansion pipe leading to a header tank in the loft or possibly out through the roof.

Does your Rayburn also heat a hot water cylinder? Just wondering with all the wood you are burning where the heat goes if not into the radiators?

 

[kernowqueen]

If you do fit a pump then make sure that the hot water can still go somewhere even if the pump is off (power cut etc.) as it will restrict the flow even more.

One of the big pipes should be an expansion pipe leading to a header tank in the loft or possibly out through the roof.

Does your Rayburn also heat a hot water cylinder? Just wondering with all the wood you are burning where the heat goes if not into the radiators?

Thanks for this. Yes there is a header tank in the loft. The Rayburn goes directly into the immersion heater, then up to the rads from there. Possibly a bit too much work for it?

 

[fernicarry]

Possibly a bit too much work for it?

Depends on the demand of the radiators and hot water and how much heat the Rayburn is capable of putting into water. As someone else suggested you need to find out a bit more about your Rayburn to answer that. Its also possible its not in the best condition internally or been mucked about with so doesn't do what it says on the tin.

When we were converting our heating over to the thermal store one of the options was to remove the back boiler from the Aga as we were removing the hot water cylinder upstairs. As part of the research for that I read that you can move the boiler closer or further away from the burner to adjust the heat going into water. In the end we left it as is and plumbed it into the store instead. Its only an extra 2kW or so, but it gives us a back-up in case gas or electricity are off for any extended period of time. Its also very nice having a big lump of warm iron in the kitchen in the winter.

You can get boiler stoves designed for sealed systems i.e. no header tank and can be pumped. That takes all the thermosyphon guesswork out of the equation and lets you put the radiators where you want them. The installation isn't the simplest though since you need to consider the safety devices. A new stove will undoubtedly be more efficient at turning wood into heat so may pay for itself eventually.

You should also keep an eye on this country's energy policy. Its all changed rather rapidly over the last few years and electricity is being touted as the only way forward, however unrealistic that is. Scottish building regs will prohibit point of use combustion in residential buildings from April this year (there will be exceptions....).

 

[Flyfisher]

However, we burn through an astronomical amount of wood. I've never weighed it but on average at least a full sack every evening, and if we leave it for more than an hour, it's likely to be just a pile of ash. We light the fire when we get in from work, and have to load it up again every 30-60 mins until we go to bed. By morning the house is freezing again. We had to buy bags of hardwood to supplement our supply last year but we burned through it too quickly (£350 lasted 4 weeks).

We never wanted to use coal for environmental reasons, but we may have to start using smokeless (or whatever we are allowed to burn these days) because we're getting through the wood quicker than we can season it!

The alternative option is to replace the Rayburn with a boiler stove. That might be more 'fit for purpose' but we could only afford to do so if it saved us in the long run by not having to pay for wood or run the electric heaters as much. If we replaced the Rayburn and found we were getting through the same amount of wood, that would be very costly!

Any advice on boiler stoves vs sf Rayburns, or advice on getting more efficiency from the existing Rayburn would be great. Thanks in advance

I can't comment on Rayburns but do have experience of a boiler stove, in my case a Stovax 11HB. It is connected via 28mm pipework to a 'heatstore' on the same level so a pump is used, controlled by a flue thermostat. It is also connected to a radiator in the bedroom immediately above and this works very well by thermosyphon. However, the bedroom radiator is smaller than the 50% boiler capacity recommended for safe operation in case of a power cut (when all the heat would thermosyphon) so we also have two radiators in an attic room, but to prevent these being heated except when necessary (ie during a power cut) there is a normally-open motorised valve in the circuit, which is also controlled by the boiler stove flue thermostat. Basically, when the boiler stove is fired up, the valve closes and no heat flows to the attic radiators. In the case of a power cut, the valve opens and excess heat goes to the attic radiators to prevent the water boiling. It has all worked very well for the past 10 years or so. We also have a mains gas boiler and this feeds into the heaat store, but that's another story.

But. Even with a modern and (presumably) efficient boiler stove, it does burn a lot of wood. We typically re-fuel the stove every 45 minutes or so and this means a large basket of logs every evening. That's fine if it's the only stove alight but we also have multifuel stoves in some other rooms and when the house is busy (eg Christmas etc) and we have three stoves alight, burning wood means it's almost a full-time job re-fuelling them all, so I usually use smokeless fuel in these other stoves because it lasts longer between re-fuelling.

Fortunately, I've never had to buy firewood but from what I know about local prices it would work out more expensive to burn wood than coal on a cost-per-heat-output basis. Burning well seasoned wood gives about 4.5kWh/kg whereas anthracite gives about 9kWh/kg, ie about TWICE as much as wood. Smokeless coal is also denser than wood so a stove-full of coal will last a lot longer than wood before needing refuelling.

As far as 'efficiency' is concerned, some people think of this as how much, or how little, wood is burned. But while it is certainly possible to close down a stove so that it burns slower, this is not really an 'efficiency' measure because it also reduces the amount of heat released. When designing a 'conventional' central heating system, the heat requirements of each room can be calculated to give a total heat requirement for the central heating boiler. Imagine a system that requires a 24kW boiler (a typical small gas boiler) for heating and DHW that operates for 4 hours a day, ie a total of 96kWh per day. At about 8p/kWh for mains gas, that would cost £7.68. Now imagine generating that amount of heat using wood; it would require about 21kg of wood!

OK, probably an extreme example as I'm guessing that few of us use only wood to heat the whole house, but I think the basic principle is sound - a kilogram of wood will release about 4.5kW when fully burned and that's the basic physics we have to work with.

 

[casttolast]

I am planning on a small 70W circulation pump system using a 45L header tank in the roof space.
1" flow, 3/4" return in a parallel circuit. Clamp on pipe thermostat.
What radiator heat (kW or size of radiator) do people here feel a classic Rayburn Royal DHW could achieve? I would not be running any DHW of course.
Planning on 3 or 4 radiators but expecting to run only 2 at the one time.
Rayburn have always rated their cast iron vitreous lined boiler at a very modest 1.9kw but I feel that's way too conservative. I have seen other manufacturers boilers of similar size rated at 4/5kw.