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Author Topic: mystery rayburn plumbing problem!  (Read 2450 times)
j.w.reynolds.01
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« on: September 09, 2019, 09:52:26 PM »

I have a rayburn on which the thermosiphon has trouble getting going when i light up if the water in hot water tank is any hotter than 30 degrees at the bottom (i have two thermometers on the tank to see water temps). If the temperature is less than 30 then things go fine, but if the water is any hotter in the tank then the back boiler gets up to nearly boiling and then things get going with a great rush of knocks in the pipes and bubbles blowing out the top vent and then things settle down to normal silent functioning??? I've tried firing up very gently but there's no difference.

our set up is a rayburn SFW 216 run on wood (8kw total, 5kw to boiler) plumbed in to an open vented direct 300l heat-accumulater tank running 4 radiators . I light intermittently in summer, and every day in winter, I don't try to keep it in overnight as i don't like tar! Flow and return in 28mm, 5m each in length, good gradient (no flat sections on the pipework), four 90 degree bends on each. There is about 1,50m vertical height between the top of the rayburn and the bottom of the water tank. The system was installed 12 months ago by an experienced plumber and we are scartching our heads as too why this is happening!

 Not enough height in the system? Too many bends on the pipes ? Some partial pipe obstruction by debri???

any ideas welcome! I live in France so there's not many plumbers I can ask as they don't seem to know open vented systems that well over here!

thanks in advance, John
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ecogeorge
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« Reply #1 on: September 09, 2019, 10:29:10 PM »

I think you've answered your own question .............. btm of tank >30c  -not enough differential to get the thermosiphon going.
Assume its a primary circuit (coil) -where in the tank is the coil -right at the btm ?
Assume you've plumbed hot supply from rayburn into the top of the coil?
George
More cleverer folks will be along soon .
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Nickel2
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« Reply #2 on: September 10, 2019, 08:16:50 AM »

Quick look on't net shows up manual:



Page 5 lower left says a bit about it. 5 metres is quite close to the specified distance. Have you lagged the primaries?
Are the tank coil connections reversed?
As the owner, you probably have the manual, so please don't shoot me if you already know! tomatosplat

« Last Edit: September 10, 2019, 08:25:35 AM by Nickel2 » Logged

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brackwell
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« Reply #3 on: September 10, 2019, 08:53:47 AM »

Is this not normal ? -it seems to be with anyone i know.
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offthegridandy
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« Reply #4 on: September 10, 2019, 09:19:40 AM »

Hi all.

Assuming you have flushed the system and have no debris in the primary flow and return; and that the hot (top) out from the Rayburn is going to the hot (Top) of the primary coil then I'd suggest that the problem is caused by the resistance in the pipe work due to the distance involved. And Or residual air in the pipe work.  To deal with air in the primary system you can add a dose of Fernox to the header tank.

You would reduce the resistance in the primary pipework by replacing the 90' elbows with slow radius swept bends.

As a point of detail have you fitted a pressure relief valve in the primary flow near the Rayburn.  It used to be considered a necessity.

There is no reason for a back boiler, Rayburn or otherwise to create like this if fitted properly.

Final solution would be to fit a simple recirc pump on a temp controlled circuit.

Good luck,

Andy


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j.w.reynolds.01
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« Reply #5 on: September 11, 2019, 01:58:49 PM »

thanks for your replies. From the manuel:

'The cylinder should be as near to the cooker as
possible. In no case may 1in B.S.P. or 28mm copper
flow and return pipes exceed 10m each in length.'


So at 5 metres each they don't seem very long, so not up near the maximum of pipe length. I did think about doing the elbows as two 45 degrees to reduce resistence but trying to avoid this as its the most work and not sure how much difference it could make - would it reduce resistence alot to change these bends??

as specified the tank is a 300l direct accumulater tank so it has no coil in it for the rayburn - hot goes in at the top and out at the bottom, and i thought that would have less resistance than an indirect cylinder where everything goes through a coil?Huh

pipes have been flushed, but not thought about trapped air, will try Fernox

And if not we will have to put on a temp controlled recirc pump, but I'm trying to avoid pumping the system if I can!


thanks agin, John


and yes i have pressure relief valve near the rayburn, on the return pipe.
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offthegridandy
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« Reply #6 on: September 11, 2019, 09:24:52 PM »

Hi John.

Could be your problem stems from the fact that you have a direct tank/accumulator rather than indirect with a heater coil.

I would guess that the issue is that the circulation only starts when you have achieved near boiling point in the back boiler as the resistance from 300 litres (=300Kg mass)  in the accumulator is too great. The flow only works because hot water is lighter than cold. In a direct system you have to transfer a larger quantity of heat to the top of the tank for the difference to create a circulation flow.

I've always used indirect systems and have had quite a number of solid fuel heating and DHW set ups in the last 30 odd years (wood, coal and peat) and not had your problem. Have you consulted Rayburn about using the boiler in this fashion?  Might be worth asking for their advice first before going any further.

Cheers.

Andy
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ny
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« Reply #7 on: September 12, 2019, 08:21:20 PM »

Long post short (no idea where the original disappeared to)😶
Your main symptoms, in my system, were always down to flow/return airlocks.
It might not be good practice but long pipe runs/long horizontal runs/90' bends do not necessarily stop a system performing well. If they did I would have no hot water, a cold landing and cold towels. 🙂
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Justme
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« Reply #8 on: September 13, 2019, 07:42:53 PM »

Cant really help with your case but I can tell you about my system.



Rayburn 212SFW
Flow & return less than 1.5m each.
Store base 4" LOWER than rayburn base.
Feed to store about 2 foot higher than hot outlet on rayburn
Feed to cold about 2 foot lower than raburn outlet
28mm with 4 90 deg compression bends
the return goes steeply down the feed steeply up
tank 300L ish


From cold (store & Rayburn) as soon as the fire is started you can feel heat in the pipes.

Hot store & cold rayburn, same again.

Hot tank hot rayburn & the rayburn can boil the store if left with no heat use via rads or DHW

Direct feed for rayburn & rads

If heating store via solar or elec the rayburn gets warm.

Vented with integral header (Navitron unit)
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j.w.reynolds.01
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« Reply #9 on: September 18, 2019, 07:45:29 AM »

thanks for the added replies.

Can you get airlocks in the main flow and return pipes, given that when the thermosiphon is going there is flow going through these pipes for 8 hrs at a time???

so i guess now my question boils down to were is the added resistence most likely coming from, the pipes or the direct feed accumulater tank??? (i guess someone really good at physic could work it out!!!) I guess I will have to first smooth out the 90 degree bends and see if that decreases enough resistence for better functioning. If not then its the tank and i will have to accept pumping the system.

And as for the system for JUSTME i didn't think that either the flow or return could go down at any point for a thermosiphon to work??? Is there a pump on the system?

John
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vantheman
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« Reply #10 on: September 18, 2019, 08:47:17 AM »

Hi,
I have been following this thread with interest as I have a similar setup to the OP and almost identical to Justme.The only
difference being that my store is 800 litres and is raised 18" higher than the base of the Rayburn.

My only problem has been recirculation through the rayburn when it is not fired and there is still residual heat in the store.

I did experiment with inserting a flap valve in the 28mm riser to the tank but the thermo effect was not sufficient to open the valve
and resulted in severe kettling in the boiler.

I certainly think your first step should be to introduce swept bends into the riser to eliminate any restriction

Bruce
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dimengineer
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« Reply #11 on: September 18, 2019, 06:28:41 PM »

If you are replacing the elbows with swept bends, why not go a pipe size up as well at the time - e.g 28mm copper to 35mm copper. Reduces the pressure drop significantly (whn flowing)
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Philip R
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« Reply #12 on: September 20, 2019, 09:04:16 AM »

You may find your system was plumbed with burred pipe ends that are highly flow restrictive, especially if sludge and other debris have been caught up at these hangups. Unfortunately many a pipe job is spoilt by shoddily prepared pipework,(not deburred satisfactorily)
If you replace the elbows with swept bends, dismantle the solder fittings and inspect for burring. Then you can make a judgement about the other joints.
Philip R
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Justme
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« Reply #13 on: September 23, 2019, 02:28:41 PM »

thanks for the added replies.

Can you get airlocks in the main flow and return pipes, given that when the thermosiphon is going there is flow going through these pipes for 8 hrs at a time???

so i guess now my question boils down to were is the added resistence most likely coming from, the pipes or the direct feed accumulater tank??? (i guess someone really good at physic could work it out!!!) I guess I will have to first smooth out the 90 degree bends and see if that decreases enough resistence for better functioning. If not then its the tank and i will have to accept pumping the system.

And as for the system for JUSTME i didn't think that either the flow or return could go down at any point for a thermosiphon to work??? Is there a pump on the system?

John

No pump.
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Justme
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« Reply #14 on: September 23, 2019, 02:40:45 PM »



My only problem has been recirculation through the rayburn when it is not fired and there is still residual heat in the store.



We had the same issue so the solar thermal could not heat the tank as the rayburn acted like a big heat dump.

My solution was a tap in the top pipe that can be shut off.

Yes there is a small risk of forgetting & then boiling the rayburn but it cant go bang as the lower pipe allows expansion.
(we do also have a tap in the bottom feed so we can drain the rayburn if needed but that is only turn able by me.)



When we go away for long periods we leave the tap open so the solar cant over heat the tank with no use. Even with our programmer set to dump heat tpo the rads if needed this is a very effective method.
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Navitron solar thermal system
30 x 58mm panel 259L TS
1200watts solar 120vdc
FX80 Solar controller
Victron 12v 3000w 120a
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6 x 2v cells 1550amp/h 5C
24 x 2v cells 700amp/h 5C
Total bank 4350 amp/h 5C
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