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Thoughts on this proposed back boiler circuit??


Biffer

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Hello all. I would appreciate some input on this diagram. I have a Squirrel stove in middle of the boat that I want to feed 3 rads and a calorifier. It is only possible to pipe up one side of the boat so a flow and return circuit must therefore, I propose, be teed into the backboiler.

In case it isn`t quite obvious, in the diagram the pump is in the return, pumping towards back boiler. The pump would be a Jabsco.

The priority would be Rad 3 as hot as possible for as much of the night as possible, then Rad 2. Rad 1 would be in the cabin nearest stove so would spend most of it's life turned off. Then the priority would be hot water, and I was thinking by morning this may need the rads to be turned off to get calorifier up to a decent heat.

Basically I'm wondering if the system looks reasonable or if the tees will cause issues.

Cheers.

20171109_152743.jpg

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You’ve basically got all the items in parallel, which is fine except that you need a means to control how much heat goes where. Fine for the radiators as they have gate / lockshield valves for the purpose. But what about the calorifier circuit? Maybe all the water will want to go that way and once the calorifier water is up to temperature, the return will be very hot and boil in the stove. So put a gate valve in the calorifier circuit so you can control how much water goes that way.

But for me, the main problem is what happens when the pump stops pumping? Batteries flat, wiring fault, pump failure, pump switched off by accident... let’s face it, pump failure is not a possibility, it’s a certainty given enough time.

if there is no convective circulation and the pump fails, the water in the stove will boil, spray out of the header tank and if you are unlucky, cause the stove back boiler to explode.

Edited by nicknorman
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3 minutes ago, nicknorman said:

But for me, the main problem is what happens when the pump stops pumping? Batteries flat, wiring fault, pump failure, pump,switched off by accident... let’s face it, pump failure is not a possibility, it’s a certainty given enough time.

if there is no convective circulation and the pump fails, the water in the stove will boil, spray out of the header tank and if you are unlucky, cause the stove back boiler to explode.

Isn't that potentially the case whenever you have a pumped circuit off a solid fuel boiler ?  It's not an unusual arrangement to have.

Some comments.......

1) Assuming the bow of the boat sits higher than the stern, if the pipe runs are parallel to gunwale, then the highest point will be at the bow, ahead of the lead radiator.  I think even if you provide a bleed point there, you will have issues with it regularly filling up with air, and hence not circulating. The system on our boat had this issue, and the previous owner had attempted to solve it by adding one of those self bleeding float type valves at that point.  (These theoretically allow air out, but as the water level comes up to the flao, should then seal off)  This didn't work in our case, because instead when the pump was on, it drew IN air at this point, and compensated for that by pushing water up into the header tank, such that it was overflowing to outside (!)  We have previously already moved the header tank to be at the highest point of the system, the theory being that any air circulating just naturally gets vented up through it.

2) I think you need some kind of simple spring loaded safety valve near the boiler to ensure that if it does boil, it gets vented out, rather than huge pressure damaging something.  These seem to cost peanuts.

3) I agree with Nick that there is a strong possibility that too muck of the flow will be through the calorifier, and not enough through the rads.  I'm sure you need to introduce some kind of ball or gate valve that allows you to restrict what percentage of the heated water flows that way, (as Nick says, if you have normal radiator controls, they can serve this function on the radiator part of the circuits).

Funnily enough, I'm currently rebuilding our system, trying to improve the crap design the last owner bequeathed to us.  Ultimately I even want to see if I can persuade it to thermosyphon with no pump, but that is perhaps too ambitious a target, and I accept I may not manage it.  Most of the replumbing is now complete, but I still have to put the stove itself back into good order.

I'll try to remember to make a thread showing what I have tried, and to say what works, and what doesn't, and to show steps I take to try and improve the first attempts at it.

Good luck

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16 minutes ago, alan_fincher said:

Isn't that potentially the case whenever you have a pumped circuit off a solid fuel boiler ?  It's not an unusual arrangement to have.

 

I think a well designed system has a degree of convective circulation to a nearby radiator(s), with the pump merely augmenting the flow to the further radiators/ calorifier. 

Either that or twin pumps with independent power supplies, failure warning systems etc (all a bit complicated!).

Edited by nicknorman
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A friends boat had a similar layout which worked quite successfully BUT the outlet from the boiler went straight into the running length of the T, and then into an elbow, this then can be set up with angles allowing both outlets to run "Uphill" and thus promote a thermo-syphon through both halves of the system - it may even be sufficient to work without a pump if you can get enough rise between the stove and the furthest points

 

springy 

Edited by springy
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Personally I would tee the header/expansion tank off the highest point in the system, somewhere off the end of the red pipe. I would also try to make everything thermo syphon as well but that is because storing electricity on a boat is always a problem (Well it is for me anyway). If you did that it should be possible to balance the calorifier and rads with some sort of rad valve thingy.

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My usual thoughts are:

- Backboilers aren't really worth it unless a year round liveaboard, or doing a lot of winter boating. Or you really love fiddling with plumbing stuff.

- Unless there's some sort of gravity/thermosyphon rad, there must be a working pump in order to use the stove. No working pump, no stove heat, simple.

That said I feel the best setup for a typical narrowboat would be one or two thermosyphon rads near or next to the stove, with one or more more pumped rads and calorifier on a separate pipe run, eg:

gallery_2174_346_1611.png

It goes without saying there needs to be a wide bore pipe between backboiler and header tank vent, with NO valves in between. The pump can be one of those little 'solarproject' type 12V circulation pumps.

Usually when I post this there's a few possible reactions:

- OP feels it's too complicated and quickly gives up on the idea.

- OP say 'can I do it someotherway' instead. By all means, but if it doesn't pan out what can I do? :unsure:

- Some nitpicking from the assembled experts which may not serve any useful purpose. Usual hazard of posting here :rolleyes:

Edited by smileypete
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23 minutes ago, smileypete said:

My usual thoughts are:

- Backboilers aren't really worth it unless a year round liveaboard, or doing a lot of winter boating. Or you really love fiddling with plumbing stuff.

- Unless there's some sort of gravity/thermosyphon rad, there must be a working pump in order to use the stove. No working pump, no stove heat, simple.

That said I feel the best setup for a typical narrowboat would be one or two thermosyphon rads near or next to the stove, with one or more more pumped rads and calorifier on a separate pipe run, eg:

gallery_2174_346_1611.png

It goes without saying there needs to be a wide bore pipe between backboiler and header tank vent, with NO valves in between. The pump can be one of those little 'solarproject' type 12V circulation pumps.

Usually when I post this there's a few possible reactions:

- OP feels it's too complicated and quickly gives up on the idea.

- OP say 'can I do it someotherway' instead. By all means, but if it doesn't pan out what can I do? :unsure:

- Some nitpicking from the assembled experts which may not serve any useful purpose. Usual hazard of posting here :rolleyes:

That looks good to me. Only question is if all TRVs are closed the pump is pumping against a blocked outlet, not sure if it would like it.

Our last boat had a pumped backboiler and it was a pain, The boiler would probably almost boil before the pipe stat got hot enough to switch and then the back boiler which was probably over 90 deg C would get purged with cold water, the stat cut out and the cycle run again until it was warm all the way round

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Many thanks to you all for your comments. I will take what I am able to on board so, control flow to calorifier with perhaps a gate valve, use bends wherever possible, and fit auto air release valves at high points, including in flow near BB.

For lots of reasons pipework will be at floor level so I will have to be happy with the risk of a failed pump and let the header overflow, rather than try to induce thermosyphon. As Alan said it's not uncommon.

Regarding Mike and Tony's comments the pump is this one https://uk.rs-online.com/mobile/p/central-heating-pumps/7066934/

Springy - could you expand on what you mean by "straight into the length of the running tee" please? I was thinking of bringing the flow straight to the wall with maybe a swept tee down to the floor level witn an air release on the non swept side and then swept side down to tee into main flow. See diagram. Ignore failed uploads see here https://ibb.co/mzqi7b

Regarding comments about header - I should point out that the calorifier is on the shelf above the swim in engine bay so that is the highest point. 

And I've just seen Pete's response - i'm afraid I fall into the "but i can do some other way", I would just keep a spare pump and plumb the other in with service valves to effect a quick change. But may I ask Pete, are you saying I need a wide bore pipe between flow and header in my proposed system? Or only in the diagram you posted.?

Thanks.

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omg, so many pitfalls and costs, is it possible just to have a  big bore pipe out of b / boiler to the bedroom, then return , the pipe could be copper, and polished every Monday.

Or fit a wee charcoal stove at the arsse end of the boat to be lit an hour before bedtime

Edited by LadyG
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14 minutes ago, Sea Dog said:

You'd probably only need to light it the once before your carbon monoxide induced 'big sleep'!

ah, wonder why more folks don't save money and stress visiting Switzerland,  signing bits of paper.

The ventilation system would not cope if just a tray of BBQ coals, but  was thinking of a flue for exhaust gases, and a proper stove, it looks cheaper and more predictable than  earlier options. 

Edited by LadyG
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9 minutes ago, LadyG said:

ah, wonder why more folks don't save money and stress visiting Switzerland,  signing bits of paper.

The ventilation system would not cope if just a tray of BBQ coals, but  was thinking of a flue for exhaust gases, and a proper stove, it looks cheaper and more predictable than  earlier options. 

Although we have a squirrel with boiler running three radiators without pump, the end rad rarely gets warm, so we ended up fitting a second squirrel.

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15 minutes ago, LadyG said:

ah, wonder why more folks don't save money and stress visiting Switzerland,  signing bits of paper.

The ventilation system would not cope if just a tray of BBQ coals, but  was thinking of a flue for exhaust gases, and a proper stove, it looks cheaper and more predictable than  earlier options. 

They do them for boats a quick Google brought this up http://www.hampshireheaters.co.uk/

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1 hour ago, ditchcrawler said:

That looks good to me. Only question is if all TRVs are closed the pump is pumping against a blocked outlet, not sure if it would like it.

Our last boat had a pumped backboiler and it was a pain, The boiler would probably almost boil before the pipe stat got hot enough to switch and then the back boiler which was probably over 90 deg C would get purged with cold water, the stat cut out and the cycle run again until it was warm all the way round

Bit late now but there's better ways of doing it... Don't think a small centrifugal pump would mind operating at zero flow.

 

1 hour ago, Biffer said:

And I've just seen Pete's response - i'm afraid I fall into the "but i can do some other way", I would just keep a spare pump and plumb the other in with service valves to effect a quick change. But may I ask Pete, are you saying I need a wide bore pipe between flow and header in my proposed system? Or only in the diagram you posted.?

Is there not room for a 2 ft by 2ft double rad somewhere near the stove? Anything would be better than nothing. As for the header tank pipe size, how much water and energy does it take to create a litre of steam? (Not much I guess otherwise steam engines would never have worked out well :))

 

13 minutes ago, rusty69 said:

Although we have a squirrel with boiler running three radiators without pump, the end rad rarely gets warm, so we ended up fitting a second squirrel.

Yeah that's the problem, those big bore tubes lose a lot of heat, unless you don't mind hiding all that nice shiny copper with pipe lagging, and ending up with even bigger tubes I guess.

Edited by smileypete
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1 hour ago, ditchcrawler said:

They do them for boats a quick Google brought this up http://www.hampshireheaters.co.uk/

That actually appears to be quite a nice thing. If it burns for 12 hours between fills it might not be as prohibitively expensive to run as lumpwood charcoal might seem to imply either. Could also be good for those who struggle with the weight of a bag of coal, although how much room a week's fuel takes up, it's cost and availability close to the cut might scupper the whole idea.

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I have a back boiler stove and have 3 rads on gravity circulation. My rads are fitted between the top flow and bottom return. My flow pipe rises 8 inches from stove vertically and then rises all the way back to the rear bedroom in 28 and 22mm copper. The rise would be around 15 inches in a 50 ft trad boat.

The return runs downhill and back to the stoves return connection. My rad valves are full bore lever valves. The cold feed and vent are piped into the highest point at far end and feed lower pipe. I have also fitted a 15mm circuit to be pumped to heat a calorifier. I have also connected in at the end a webasto with separate feed and vent pipes. The webasto heats all 4 rads.

The backboiler isn't that powerfull at 8,000btu and gets the bathroom and bedroom rads reasonably warm enough to make it comfortable.

I too would advocate at least one gravity rad!

James:cheers:

Edited by canals are us?
  • Greenie 1
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As you can see I used 300mm high radiators to enable me to get the pipe layout correct.

The second picture was originally at the end of the gravity loop and still is accept I added in a webasto using speedfit plastic plumbing which isn't as neat as copper. The 15mm pipe going down is the flow to the calorifier.

I use the lever valves on the rads to balance the rads heat when using the webasto.

I can use either the fire or webasto and don't need to turn any valves as both fitted at each end as a continual loop with separate vents and cold feeds.

James:cheers:

Gravity rad bathroom.jpg

Gravity rad bedroom..jpg

Edited by canals are us?
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7 hours ago, Biffer said:

<snip>

Springy - could you expand on what you mean by "straight into the length of the running tee" please? I was thinking of bringing the flow straight to the wall with maybe a swept tee down to the floor level witn an air release on the non swept side and then swept side down to tee into main flow. See diagram. Ignore failed uploads see here https://ibb.co/mzqi7b <snip.

It relates to a high level pipe and thermo-syphon system but allows both outlets to be running uphill, if you connect the branch of the T to the outlet of the boiler and the two pipes out to radiators to either side of the main length of the T, then one will run uphill and one downhill except when the trim is just right when they will be level, fitting the one end of the running length to the boiler outlet allows the branch to be set at an angle and thus ensure an uphill run, adding an elbow to the other end of the running length of the T allows that outlet to also run uphill. It gets a bit bulky but achieves the desired result

 

springy 

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On 10/11/2017 at 18:17, springy said:

It relates to a high level pipe and thermo-syphon system but allows both outlets to be running uphill, if you connect the branch of the T to the outlet of the boiler and the two pipes out to radiators to either side of the main length of the T, then one will run uphill and one downhill except when the trim is just right when they will be level, fitting the one end of the running length to the boiler outlet allows the branch to be set at an angle and thus ensure an uphill run, adding an elbow to the other end of the running length of the T allows that outlet to also run uphill. It gets a bit bulky but achieves the desired result

 

springy 

Crystal clear now thanks Springy.

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11 minutes ago, redwing said:

Does the return on a non-pumped circuit need to flow downhill back to the stove ?

Well to me the simple science says that for most likely chance of success the feed pipe would run uphill all the way from the stove, and the return would be downhill all the way back. (So on most "nose high" narrow boats, stove at the back end of the boat cabin will be best).

With compromises to that it may still work, but the more the above can be adhered to, (and the greater the slope of the pipes, the better I would expect it to be.

It does seem a bit of a black art, and when I eventually get ours to the point I might see if it will work on gravity alone, I am expecting a bit of "suck it and see", with no guarantee that it will.

I have run out of time this yera, (too cold now to be on the boat with no heat), but my plan is to try having a large bore circuit throughout, but to have a pump attached to it on a route that bypasses the main pipe for a short distance.  I will introduce a full bore valve in the bit of pipe the pump is bypassing, allowing it to function as a pumped circuit with that valve closed, but hopefully maybe working under gravity with the valve open, and the pump turned off.  Will that work in my particular case?  We shall see, but not until next year now, when I can take it all apart and make those modifications.

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