Gravity Hot Water Problem - SF Rayburn

amd

Hi all,
We have just installed a second hand 355SFW Rayburn which we fire solely on wood. (Hubby a tree-surgeon). We are having problems as we've also got a backup oil boiler which we've spliced into the system. When the HW cylinder stat makes the pump kick in, the gravity flow reverses even though we have made our injector tee as per the Rayburn diagram, see previous thread.

http://forums.moneysavingexpert.com/showthread.html?t=282480

Our plumber is at a bit of a loss, and I've spent most of the last week trying to shut down lock valves to balance the system to no avail. Could it possibly be that Rayburn's own injector tee is made slightly differently than the diagram. (They're shut for hols, can't get an answer from them at the mo.) We wonder should the injector from the CH go further into the tee than the diagram shows, as it appears it's taking a short cut back around the gravity flow and cooling the cylinder down and cutting out the pump. (Oil boiler not yet connected to 'top up' the Rayburn)

We're looking at possibly having to spend several hundred on the Dunsley neutraliser, but I'm determined to get to the bottom of why the gravity is reversing when the pump fires - even at it's lowest speed. The problem is maybe exacerbated by the fact we also have a section of underfloor heating with a second pump also set at it's lowest speed although again I've shut valves down to a peep!

Any advice greatly appreciated.

Sobraon

We have a similar set-up but use a twin coil hot water tank (with an immersion for summer) as I dont think the neutraliser was available when I installed the system 23 years ago! Works well for us.

amd

Thanks for that, we actually spliced both gravity feeds together, with a motorised valve on the oil boiler section so it only opens that part when the oil boiler fires. Something we have done before with no ill effects.

With the Rayburn having only two boiler tappings, both gravity hot water and pumped central heating share the one pipe - with the pumped section via a tee on the flow and an injector tee on the return. The problem is that when the pumps kick in, the gravity feed reverses (thankfully pipes are still exposed) and we can feel the cold rise up the length of the return pipe, yet the flow remains hot as it draws heat from the cylinder on its way back down to the hot water circuit - ie taking heat out of the cylinder. Manually switch the pumps off and I can feel the hot start circulating again in the proper way. It's driving me nuts. Probably have to wait until AGA /Rayburn open up again after hols. My plumber isn't able to spend the time here investigating it fully, so it's kind of up to me to make suggestions!

Canucklehead

Hi

When you call Rayburn I suspect they will refer you to the installation guide they provide with a new cooker. This is the one from Plumb1's post...... http://www.rayburn-web.co.uk/raytech/dhw3.htm
If this is not followed faithfully then it may not work. Please note the paragraph under the diagram for the injector tee. (and as said by Sobraon) USE A TWIN COIL CYLINDER.
There is a separate diagram for the c/h.. http://www.rayburn-web.co.uk/raytech/chtg2.htm

IMO, the way you describe the set up you have had installed ,it will not work.

I note you say that you have done this before but each installation (and installer ) is different and you have also fitted an extra pump to do underfloor c/h .

Corgi Guy.

amd

Canucklehead, thanks so much for your input. I am though very familiar with Rayburn's technical pages and have followed their layout exactly to the 'tee' (pardon the pun!). I have read the tech stuff over and over incase I'm missing something, but just don't get it.

The two boilers are plumbed together before the injector and normal tee - therefore both are in the 'gravity loop'. A motorised valve shuts off the boiler section unless the boiler is activated to avoid any gravity draw. As far as the system is concerned (IMHO!) it's just as the diagram shows. My suspicions lie with the home-made injector. I think it's allowing the central heating return water to take a short-cut around the gravity return from the cylinder making it reverse! I reckon we'll have to stump up the 30 or 40 odd quid and buy one to see if it makes a difference. We made ours so the 'injector' just passes the DHW gravity return inlet, but I reckon it maybe should be longer so it can't slip back up the return pipe. I've tried slowing everything down, but surely the speed the CH water draws off from the gravity flow tee is the same as the speed the CH water returns via the injector - therefore making the number of pump irrelevant - or am I totally off whack!?! Why the heck doesn't it fully return back via the Rayburn?

Also, I may be missing something again, but I just can't see how a twin coil cylinder would affect things differently - the central heating circuit would have to come from 2 gravity flows instead of just the 1 that we have at the moment. Perhaps you could explain? Just read the Rayburn para again - the twin coil cylinder bit reads as if the second boiler is only contributing to the DHW and not the central heating too. Ours does both - we can have either on, or both - just a shame it doesn't work!:mad:

Canucklehead

Hi

Just re-read your OP . 'The cylinder stat kicks in the pump'
Both the Rayburn diagrams show the HW circuit as gravity not pumped. Doesn't that make the pump in the wrong place.? That's where the twin coil cylinder comes in .
Difficult to work out what you have without actually being in the house!
You may well be right with the injector tee.
I'm getting the picture slowly!
My first thought was how do you electrically switch two boilers , but the Rayburn doesn't have electrics ( in that the Rayburn is not switched on/off like gas /oil.)so I can see how that works.
My other question is .. have you got a blending valve on the underfloor bit so you don't roast the dog/cat?:D
Interesting.

Corgi Guy.

amd

Hi Canucklehead, thanks for the reply.

The pump only circulates the CH circuit - it is located after the tee off from the gravity flow to the HW cylinder. (Just as the Rayburn diags) The underfloor circuit also comes off the CH circuit flow part way round and has its own manifold / pump / temp valve with electronic (?) actuators operating the zone valves controlled eventually by room stats - but at the moment manually opened/closed a little by me with a radiator key (until the electrician comes back to finish his stuff connecting oil boiler and room stats.)

Taking the oil boiler totally out of the equation, the basic set up of gravity HW and pumped CH doesn't work when the pump kicks in. Still don't get the twin coil thing if I want CH from the oil too - I understand the two gravity loops both heat the one cylinder, but how would you take off a CH circuit?

We did have this set up in a different house using a wood stove with back boiler and a gas boiler, but had no issues that we remember (or noticed!).

Appreciate your thoughts.

Canucklehead

Hi

Ok for the time being lets forget the twin coil cylinder.

Things have got a little contradictory and complicated.
Your OP states your CYL stat. kicks in the pump. According to the Rayburn diagram the cylinder h/w circuit is purely gravity, no pump assistance.
Now you have said that the pump is positioned in the pipe after the tee of the gravity FLOW. It should be in the RETURN of the c/h before the injector.
Therefore it's not piped up as per the Rayburn instructions.

Corgi Guy.

amd

Hi.
Apologies for not making myself clear. To clarify -
The HW is defo gravity. The CH pump kicks in only when the DHW is up to temp to pump only the CH circuit, however as there are only 2 boiler tappings (1 flow, 1 return), therefore they share a stretch of gravity pipework - flow and return. This in turn should partially assist the gravity when the pump is on. It should not reverse it - however in my case it does as I reckon it's slipping back via the dodgy injector tee. It's all exactly as the rayburn diags. (except for the extra oil boiler spliced in - however, ignore that for now:o)

My original rayburn installation instructions show the pump can be placed in either flow or return - see Page 6, Fig 5 I can't see how the placement of the pump makes a difference. As I said, surely it's the same speed at both ends?

http://www.rayburn-web.co.uk/Prodin/Rayburn%20obsolete%20cookers/Rayburn%20Nouvelle%20solid%20fuel/Rayburn%20Nouvelle%20Solid%20fuel.pdf

I do appreciate your help, however I managed to finally get someome at Rayburn today. The chap in their Tech Dept had never heard of this prob and is sending me a detailed diag of the injector with dimensions. (Supposedly not the one from the web pages!) We'll probably have another tinker with the tee as it's slowly dawning on me that it's the obvious problem.

Unless anyone can point me in the direction of any other likely fault, we'll leave it at that for the time being until I've played with my improved injector for a few days!
Thanks all.

maid1

Have just installed rayburn 355 I am having similar problems, did you solve yours?

moneysavingplumber

Amd it would help if you could possibly draw and post a picture of the schematic of how your system is designed, excatly where the pumps, zone valve(s) and cold feed and vent are in relation to the Rayburn and boiler, to see where the problem is. As Canucklehead said, it's very hard to visualise from a description.

My personal opinion would have been to fit the Dunsley Baker neutraliser from the start, as they will usually design the control system needed and work very well indeed in my experience.