Reverse Circulation with Stove

[Louis]

Hi folks
Great forum thanks

I put a stove in a while back, one that heats the water and rads.

The natural circulation is plumbed to heat the hot water cylinder. Unfortunately when the stove is left overnight and gets cold, reverse circulation starts in the system and by morning time most of the heat is gone from the hot water cylinder

Any thoughts on the best way to stop this reverse circulation??

Thanks in advance
Louis

 

[nick penfold]

hey Louis, there's a load of great time served guys on here who can help, Im only just starting to cover this subject at college so probably best that i don't advise.

it may be easier for the guys to advise if you can give some more details such as the following

Is it a pumped system?
Maybe an S plan or Y plan set up?
Installed a balancing valve?
Where is the pump in relation to the vent and cold feed pipe?

sorry i can't help anymore but im interested to hear what the big boys have to say for my own education )

goodluck

A basic drawing from you showing you're piping would also be handy.

 

[sonray ltd]

as said a basic drawing etc of your layout as reverse circulation can be caused many ways. but is useally only a simple error of pipework.

 

[tipper]

What you need is some form of non-reverse valve/clack valve to stop reverse flow as it cools down.

Go and talk to you plumbers merchant about a check valve.

 

[Louis]

Thanks for the replies guys

Ok, the stove has two inlets and two outlets.
One inlet and outlet is used for the pumped system to the rads.

The other inlet and outlet are used for the natural circulation and safety system. It comprises of a 1 inch pipe going up to the attic and down through the coil in the hot water cylinder and then straight back to the stove.
There is nothing really to stop the reverse circulation in this circuit, in my view, and this is where the problem is.

Nick, I take it your comments apply to a pumped system. Although this is also a pumped system the problem is with the natural circulation part of it which does not use the pump.

Sonray, thanks for that, I would hope that something simple could be incorporated to prevent it. Does the above description give you enough info?

Tipper, since this is the safety circuit, it is supposed to be uninturrupted with any valves etc.. At least this is my understanding, although a non-return valve would seem like the easy option, if it were safe practice.
Any thoughts?

Thanks so far, hope you have enough info above.

 

[sonray ltd]

louis i take it you have your vent pipe coming off this side ?
as you say a vent pipe must be clear off any etc plus a anti gravity valve won't open under gravity.

so what is happenening when the stove is turned off ?
cold water is going back up the return pipe.

 

[Louis]

Sonray
Yes, the vent comes off the highest point on the 1 inch pipe in the attic and goes to the expansion tank in the attic.

When the stove is turned off and cools down, the heat in the hot water cylinder then starts the reverse circulation in the coil, which circulates back through the stove and is cooled down overnight leaving a luke warm or cold hot water cylinder in the morning.

The same thing actually happens if the hot water cylinder is heated with the electric immersion heater - after a while the stove water jacket will get warm because of the reverse circulation caused by this also.

 

[sonray ltd]

the feed/return is on the right way on the stove & cylinder?

 

[Mike Jackson]

I think the problem is the pipe running up into the attic and back down again. Once the stove and pipework have cooled down the cylinder is the hottest part of the circuit. Natural convection will cause heat to rise back up the pipe and start the reverse circulation. Other than the vent the coil needs to be the highest part of the circuit to stop this happening.

Mike

 

[tipper]

Tipper, since this is the safety circuit, it is supposed to be uninturrupted with any valves etc.. At least this is my understanding, although a non-return valve would seem like the easy option, if it were safe practice.
Any thoughts?

As long as one of the pipes up to the expansion tank is clear of valves, ie either the flow or the return then the 'safety' escape route is not compromised by one of the pipes having some form of check valve. And yes I have seen a gravity check but that was a long time ago! (Actually my first house had a coal fired back boiler, a cylinder and 2 radiators, one up and one down, in 1975)

BTW could it be the heating pumped flow that is cooling the cylinder and not gravity alone?

Have a chat to a stove installer about what they normally do to stop reverse flow and cooling of the cylinder. They may have a simple ****ion.

 

[Louis]

sonray, yes it is connected up correctly and works very well apart from the reverse flow.

Mike, I think there must another way than having the coil at the highest point. It is a standard gravity loop that is fitted with most backboliers.

tipper, yes, that makes sense as long as there is an escape route incase of overheating or high pressure.

It is nto the pumped flow as we have the pumped flow valve to the hot water cylinder shut most of the time. In any case overnight the pump is always off.

It is a Charnwood stove and I will give them a ring tomorrow.
I looked at a diagram of theirs and they have much the same as what I have except there is an additional radiator in the gravity circuit.

I have a feeling that the circuit is too free and easy for recirculation, if it had a more difficult route via bends etc. the reverse circulation might never start.
Someone told me years ago that a small tank with inlet and outlet pipe would solve this kind of problem but I can't find anything about this.

Thanks for the suggestions so far, any other thought would be appreciated.

 

[Mike Jackson]

I was thinking in your particular situation rather than as a rule. When you say the open vent is at the high point is this directly over the cylinder or elsewhere on the pipe?

Mike

 

[Louis]

Hi Mike
Yes the open vent is above the cylinder, would this make a difference?

This is a link to the stove instructions [DLMURL]http://www.charnwood.com/pdfs/Instructions/Co14B.pdf[/DLMURL] the diagram is on page 6.
I have used the same system except that I did not fit a radiator in the gravity circuit.

I have a query in with the suppliers to see if they can help also.

Thanks for helping

Was pointed to this by one of the stove people.
[DLMURL="http://www.ukplumbersforums.co.uk/redirector.php?url=http%3A%2F%2Fwww.dunsleyheat.co.uk%2Flinkupsys.htm"]dunsleyheat uk manufacturers of multifuel stoves, range cookers, boilers...[/DLMURL]

It is the unit I had heard of years ago and it seems that it might solve the problem - any thoughts or experience would be appreciated.

Thanks
Louis

 

[SmashyD]

hi all i'm new to the forum.
Mind if i put my 2 penny worth in?

It sounds as though your cylinder is not high enough - think of it as a direct cylinder as the coil may as well not be there as far as thermal centres are concerned. Is the bottom of the cylinder over say 18" above the top of the stove?
Don't make your gravity circuit difficult for convection - that will cause more problems by far than it might cure.
Water will re-gravitate very easily if the thermal centres of the two vessels are on a similar level, it doesn't matter where the pipes run, untill the water finds its own level thermally. There is no reason for hot lighter water to push cold heavier water out of the way unless there is a point where cold is found above hot and forces its way past thus displacing hot water such as heat loss in the pipe work. Make sure all the pipe work is WELL insulated as this heat loss can start circulation going. Do not put any valves anywhere in the gravity circuit. Any hot water control valve if fitted should be naturally open and you should have a heat leak radiator on the gravity circuit. This is good practice for safety reasons and also as it allows the stove to keep running without shutting itself down when the hot water is saitisfied. If you have a power cut you will either boil the water or the stove will set to run at low output. the rad smooths this out
I have a boiler stove and oil auto boiler linked together by an H2 panel. As i live in a bungalow my cylinder is in the loft but my 1200 x 600 heat leak rad is positioned so that it's thermal centre is 18" above the thermal centre of the stove. That means it's base is near the floor. It is over 10 metres of pipe away and gravitates well. (Tall towell rails are perfect for this).
Hope this helps

 

[Louis]

Thanks SmashyD for your thoughtful comments.

Actually when I checked I found the base of the cylinder is at the same level as the top of the stove. The pipe run above the cylinder to the top of the circulation bend is about 5 feet.
Originally it was plumbed for a back boiler which was lower!!

So you think if I moved the cylinder up at least 18 inches this might do the job?
The problem with this is that it is a big job and I was hoping to get away with something smaller. It is a bungalow with an attic conversion so there is very little space to do anything.

Your explanations about how re-circulation works is very valuable. I wonder at what point would re-circulation not happen in relation to the positon of the stove and the cylinder. In other words if I went to the trouble of moving the cylinder would this definately cure the problem?

I phoned Charnwood Stoves and the chap there agreed there should be no restriction in the pipework. When I suggested what Tipper said earlier, to put a check valve in the circuit, he said that would work as long as the vented side is kept free, although he wasn't recommending it, as such
I have purchased a flap type check(non-return) valve for this and hope to fit it sometime over the Christmas - the space is very restricted and my bones are not getting any younger
What do you think of this?

At a later stage I am planning to fit a heat leak radiator. Your comment about yours being 10m away interests me as I would like to fit one at the far end of the house, one of thouse vertical rads sound like a good idea.
I wondered too about the size of the piping for this as rads generally take only 1/2" pipe and the gravity circuit is 1", does this present problems?

Thanks in advance
Louis

 

[SmashyD]

hi louis
Sorry, only just received notification of your post.
Problem i can see with a check valve is reduced performance in the gravity circuit but i'm not sure about that.
I would say your cylinder is fine at that position. My heat leak rad may be a long way away but much of that is distance is in 28mm and only reduces to 22mm for the last 2 or 3m. Also to be honest i'm not sure it is that good really as i lose some heat in the loft. ( It is fitted as a safety device rather than an effective radiator.
Building regs state that the heat leak pipe work should be 22m and reduce to 15 only for the last 300mm maximum. This is important for gravity circulation. It is a very weak motive force and any friction introduced can slow it down so much that you will lose as much heat from the pipework as you are trying to convey to the cylinder! this can bring the heat conveyance to a halt. Until i insulated everything really carefully my system wasn't very effective at all. If you do fit the heat leak a distance away you have to remember that all your fittings (including straight unions) have a significant effect on the flow resistance so you will need to bend the pipe rather than use elbows, etc. (do not use plastic anywhere on the gravity side).
I'm a bit puzzled as to why you're getting reverse circulation to the point where your cylinder is cooling right down - just goes to show how effective gravity can be for transfering heat!
Also dont rely on the foam that the cylinder comes with, it will lose a significant amount of heat over the course of the night. The cylinder i have is foam covered but without a jacket loses 2.7kw/h!(written on the label) Put a jacket over it as well. Try double insulating the 28mm pipe work in the loft, cover it over with loft insualtion aswell and insulate all the way to the stove - you will lose a lot of heat through the pipes where they come down from the ceiling to the stove . I think the pipework heatloss may be where your problem lies.
At the end of the day gravity works simply because cold water is heavier than hot water. Therefore once you have cold at the bottom and hot water at the top of the circuit it should want to stay there providing there is no great change in the temperature of the stored water.
Let us know how you get on. I take it you have already fitted that flap valve - how is that working out?
Regs SmashyD

Just been thinking about the way your plumbing is set up.
You say that you have 4 tappings on the boiler - 2 for gravity and 2 for radiators and you have a long gravity circuit.
Have you considered piping it a different way.
Another way in your situation may be incorporating an injector T as in the second diagram (fig 6) in the manual. This has the effect asssisting the gravity circuit when the pump runs therefore heating your cylinder much more effectively. C/H often steals all the heat from the circuit when the pump runs.
Use thermostatic rad valves (all bar one) and once the house is warm, if the stove is still hot the pump will run the gravity circuit pretty exclusively. Wire in a high limit pipe stat on the cylinder flow from the boiler and it acts as a safety cool down for the stove by getting rid of excess heat round the rad circuit. Obvously only when there is power - it doesn't replace a heat leak rad.
Once you've got your heat up there your half way there.
BTW: A few questions:
Could the pump be running after the boiler has cooled to below that of the cylinder? Not sure how this might effect it but it might cause some movement. The pump should be wired to stop if the return or flow temp is too low.
How long does it take for the boiler to heat the cylinder to 60c?
Are you reaching 60c, or higher - too high?
Do you have a thermometer for checking pipe work, boiler and cylinder? (if not, consider getting one - it's like a sixth sense where plumbing is concerned)
Is there any kind of temperature control for the water?
What controls do you have other than the stove thermostat control?
Does the stove ever kettle or boil?
Is your f&e tank (or for that matter, the cold water storage tank) getting hot? BTW: Does it conform to the latest building regs for solid fuel systems? (it has to be able to handle 110c including ball valve and overflow: BS4213). Check the "HSE safety alert on scalding risk from domestic hot water systems"
regs

 

[justlead1]

Hi. When this type of system was popular a valve was developed and sold, it was (if my memory has not fail) a Cytrol valve. with 1" BSP female inlet and outlet, this was screwed directly to the cylinder (indirect) return and the pipework connected via a C/Iron connector. They allowed the domestic hot water to be stored at a lower temp than the boiler flow temp. This allowed the rad temp to be greater with out making the water dangerously hot. They also checked the reverse circulation problem. although this is oftern due to pumped circuits and in sufficient hight between boiler and cylinder. Good Luck

 

[Louis]

Hi SmashyD
Apologies for the delay in replying. I was leaving it untill I had done something and it went on and on, partly because I could not buy any materials over the Christmas break.

I had intended fitting the non-return flap valve over the holiday period. But reading you last post got me thinking again(dangerous).
Instead I did manage to get into the tight attic space and insulate the 28mm gravity flow pope with a load of attic insulation.
There is still a 4ft stretch that is un-insulated going fromt he cylinder to the loft, will need to buy pipe insulation for this.

You are right - this bit of extra insulation has made a difference.
This morning the tank was still half hot after being hot all the way to its base the evening before. This is a big improvement and I can see the logic in what you say.
I will put more insulation on the pipe and more on the cylinder.
Was thinking also of putting some sort of jacked over the stove before going to bed, as this would further reduce the heat loss and hence the reverse gravity flow.

My impression is that the reverse circulation can only be minimised rather than illiminated completely - would that be your view also??

I wanted to test the effect of the insulation before fitting the flap valve, so this is the route I am taking for the moment.
If I can get approval from the House Boss for an additional radiator in the hall, I will fit a heat leak rad there. If I do this I will also feel better about fitting the flap valve in the cylinder circuit as the heat leak rad ciruit will be unobstructed.

With regards to your other questions:
I would not go near the back of the stove again as access is restricted and the flue in place and all that fitting was such a hassle that I would leave well enough alone.
However, I guess I could probably fit an injector tee further down the line?

By kettling I presume you mean is the stove getting too hot and starting to boil. The answer is probably yes, but very slight, as soon as it starts making that gentle hiss we put the pumped system through it. It needs to be automated.

The pump is never running after the stove is shut down.

I'm thinking of upgrading the system now anyway, it is pretty basic to start with, done 24 years ago when fitting a back boiler - no controls.
Their is also a separate pump operating in the reverse direction to the oil fired pump so that, with the fitting of non-return valves the two systems can be run without the other loosing heat throught the boiler or the stove (if that makes sense)
Anyway that's all another story, which hopefully I can get advice on when I think I have a PLAN sorted

Justlead1
Thanks for the advice, I will see if I can source and Cytrol Valve, this might be a good option if I do fit a heat leak radiator - although would a motorised valve with a thermostate be the equivalent??

Thanks
Louis

 

[SmashyD]

Hi louis
I think if you put a jacket over the stove you might get some unwanted heating!
Bearing in mind you have an oil fired boiler as well, have you considered linking the two together with a Dunsley Neutraliser such as you described in an earlier post or an H2 Panel from Heating Innovations?
I have an H2 linking my oil fired and stove together and it works well. The heat from the stove is passed into the return of the oil boiler along with the returns from the rads and cylinder. If the stove is not alight the oil fired works as if alone or the system can be set to run with just the stove.
Going down this route may well get rid of the pipework that is causing you probs just by the redesign.
Check it out at [DLMURL="http://www.ukplumbersforums.co.uk/redirector.php?url=http%3A%2F%2Fwww.h2panel.co.uk%2Fh2operation.htm"]H2 system operation[/DLMURL]. It's very simple to plumb in and connect the electric.
They were very helpfull to me while i was setting up my system.
Good Luck

 

[Louis]

Hi SmashyD

Have had a look at the H2 system and it looks brilliant, it seems like it has everything I need
I have priced it and will have to wait awhile to get the reddies together but it's definately a go go.

As a point of interest, do you have your rads zoned. I ask this because I was thinking of incorporating two or three zones when I do the revamp, and was wondering the best way of controlling this - would it have to be done through two separate control systems i.e. the H2 control and the zone control, would you know?

Cheers
Louis

 

[SmashyD]

Hi louis
Exellent idea to go with the H2. Yes I have 2 zones. I have found that it is important to have one zone that the stove alone can easily handle.
Since you asked I thought I'd pass on what I've learnt, my experience with the H2 Panel so far. Bear with me, this is info I wish I had when I built my system.
First, so you get a good picture, a "quick" description of my system.

Five bed, elongated bungalow, Kitchen (with stove) at centre, with large windows and double glazed sliding doors, medium insulation.
Forty+ year old single pipe system with grant utility 26/36kw 28mm oil fired condensing boiler set at 31kw.
172ltr indirect cylinder (in loft), and the new addition: 28mm H2 panel with Grundfos 15/50 alpha+ pump (in loft) and Nordica (Brosely) Thermosovrana boiler range cooker 7kw to water, 6.5 to room.

The stove is actually a bit too small for us - it was only meant for fun but it has performed so well we're now are taking the whole idea much more seriously! I may sell it and get bigger one (BTW: VGC, almost new if you know anyone!).

The house has 2 zones, with a room stat and thermostatic valves in each. Zone 1: 4 very large rads and one normal plus 2 heat leak towell rails.
Zone 2: 3 very large rads and 4 normal, with two underfloor rad pipe runs used as returns.

Zone 2 is controlled by its own zone valve and room stat and only operates when ch is activated from programmer. Zone 1 has no valve between it and the H2 (which has its own 3 port mid pos valve). Thermostats connected in parallel. Zone 1 stat set slightly lower as this end is always warm and run by it's thermostatic valves.

Zone 1 is not the smaller of the zones by rad size, however that part of the house has smaller rooms, is newer, better insulated and for some reason plumbed totally in 15mm pipe as opposed to the other end which has a long 1"flow run in the loft, right to the end and 3 3/4"returns, 2 running across the loft and 2 under the floor.(one is both). Lots of heat loss and so very slow acceleration.

Ive tried swapping round the zones and have found that zone 1 heats up much more quickly and this is connected to the ch flow before the zone 2 valve.

In practice the H2 works like this:

If the stove is out, the oil fired will work as normal.

As soon as the stove is lit, gravity circ begins in the flow and return of the stove. When the flow pipe passes 58c (sensed by a low limit anti corrosion pipe stat adjacent to the stove) the pump starts and pumps the water round the primaries.

If the central heating is on, the solid fuel (sf) flow is combined with the ch returns and fed into the return of the oil fired (auto) boiler.

If only the hot water is on, the stove will heat the cyl without asking for help from the auto boiler unless or until the sf flow cools too far and switches off the pump, say when the stove is warming up, in which case the auto will cut in instead (with the pump obviously) until the sf flow passes 58 again.

When the hot water cylinder is satisfied the stove shares the heat with the rads in zone 1, overriding the room stat, as soon as the flow pipe temp passes 58c and switches on the pump. This is usefull as the stove then shares its heat around one zone in the house all through the day/night without the auto boiler firing as the heating is not programmed on.

When only heating the cylinder, the stove will obviously get a lot hotter than 58c and this is where the bias stat comes in. It has a range between 60c and 70c. (70c is the max efficient temp - if your stove boiler is outputting higher than 70, the load is too small, or the pump speed is too low and too much heat is actually going up the chimney, or it's probably set too high or over-firing and so equally undesirable).

The bias stat will allow the primaries to reach its set temp before it shares with the rads in zone 1 and so acts as a high limit stat. The higher the stat the hotter the primaries and the quicker the cylinder will heat.

When in heating mode, the lower it's set the earlier it tells the auto boiler that the stove can handle the job alone and vise versa.

One drawback i have found - with my system - is that when the auto boiler cuts in, the flow slows down through the solid fuel stove and so the flow temp rises and swithes off the auto boiler again, where upon the sf flow speeds up, cools down, and switches the oil boiler back on again and so on. The auto boiler cycles rather more than i think it should.

I thought that fitting the Alpha+ pump would do it but it obviously doesn't work in the way i understood it to and it hasn't made any difference. I've tried many different pump settings to no avail and will be calling Mike at Heating Innovations to see if he has a solution. Its most probably bad pipe work layout/sizing on my part.
If the auto boiler was only 22mm it probably wouldn't be so bad. It's like the auto boiler return should have some sort of flow control valve on it to stop it stealing the flow - but I'm not sure it would be effective, advisable or even legal to do that. I don't think it's that big a problem.

Overall I'm very happy with the H2 and would definitely suggest one for a customer. I don't know how it compares with a Dunsley Neutraliser except that the flow speed wouldn't be a problem.
Does that give you any ideas?
Regs

 

[Louis]

One drawback i have found - with my system - is that when the auto boiler cuts in, the flow slows down through the solid fuel stove and so the flow temp rises and swithes off the auto boiler again, where upon the sf flow speeds up, cools down, and switches the oil boiler back on again and so on. The auto boiler cycles rather more than i think it should.

Hi SmashD
I don't get this!!, why does the flow change?
As I understand it the pump is sending the water around the circuit in the same way whether the auto boiler is on or off - or am I missing something?

Your system sounds cool - in a hot sort of way
Yes, that's the sort of thing I was thinking about to have a zone so that the stove can heat is comfortably on its own.

All this zoning and controls is new to me and was wondering what you use in addition to the H2 control unit to control the zones etc.
I would guess you use the H2 controls and have a separate control for the zones, is that correct, or is there something more sophisticated?

Really appreciate your help with all of this, it will be invaluable when I get round to doing the job.
Louis

 

[SmashyD]

Hi Louis
There seems to be something wrong with this forum. I get told of your reply but when i go to the link / web page it doesn't show any new replies. Another email notification has just come through today and your reply is now there. Wierd. Otherwise i would have got back to you earlier on each post, sorry about this but it seems to be permanent.

Water always finds the easiest route, with least friction and resistance, to the detriment of other, more resistant pipe runs. Thats why flow control valves are necesary on single pipe zones.

Back to my system again: When the pump is running through just the sf boiler, the liquid flows from the boiler, through the load(s) and back to the boiler via the return pipe which has 4 rad returns, 1 heat leak return and the cylinder return all plumbed in to it including the return TO the H2. The H2 return is not used in this situation - there is no flow through it.

However, when the auto boiler fires up in link up mode, the liquid is split between the H2 panel return and the solid fuel boiler return so there is effectively a pull at both sides of the sf boiler as the sf boiler flow also goes to the same place as the return in the H2 panel - both of them go into the return of the auto boiler at the H2 in 'link up' mode.

The only reason water goes through the stove at all is because it wants to find all routes. Its slower as a result. As soon as the bias stat rises as a result of the higher temp due to the slower flow, the auto boiler is told to switch off and the circuit goes back to solid only, the loop is disconnected, thus flow increases, bias stat drops and switches on the auto again! DOH! Very frustrating!

In my system the return to the solid fuel is one long 28mm pipe with the solid fuel at one end then 3 returns (2 rads returns and the cyl) plumbed into it , then the H2 return about 3/4 of the way along, then the heat leak return and 2 rad returns at the end, the other side of the h2 panel return.

Pipe size may need to be increased to 35mm or something like that.
I have installed a flow control valve on the return to the H2 but it needs to be virtually closed to make any difference and this is not possible as the auto boiler relies the H2 return as its return from the system when in auto only mode. I am thinking through the possibility of fitting a control valve here that will only open in auto only mode but i have to run it by heating innovations first.

Having some of the returns prefering to go straight to he H2 and into the auto boiler return helps the sf boiler when the system is heating up from cold by diverting some of the cold water straight to the auto boiler rather than cooling the stove so far that it shuts down the system and runs in auto only mode (anti corrosion pipe stat by sf boiler). So it is necesary in my system to smooth things out as the sf boiler is too small for the system by a long way. The slowing down is not that big a problem, however i can see that the system is being modulated by this temp fluctuation more than by correct function of the various thermostats in the system, such as the auto boiler stat and room stat.

Using a Dunsley Baker Neutraliser instead of the H2 will erradicate this problem by design. However, with the H2 the heat from the stove is used first, ie: the auto boiler wont fire if the stove is above a certain temp and the solid fuel energy is sent through the auto boiler thus reducing the amount of energy required to heat the water to full temp.

I'm not very familiar with the neutraliser but I think the water is pumped by two pumps, one for the sf and one for the oil and they are not able to effect each others hydraulics. I'm not sure how it is set up and whether the same amount of control can be used to minimise the auto boiler use but I went for the H2 because i heard that neutralisers could be tricky to get right ( and as I live in a bungalow and for various design limitations, it would have been more awkward to plumb in, at least I thought so at the time ) Best thing to do is to learn everything you can about both systems. Once i get around to calling heating innovations I'm sure that all will become clear.

The zone that the solid fuel can run on its own (smaller zone) is controlled by the mid pos valve on the H2. When the programmer and thermostats call for heat it opens as normal. There is a Tee off the rad flow pipe to that zone.

The other zone valve is plumbed in after the tee and it will only open when the programmer and its thermostat asks for it. (The H2 will often use the rad circuit to cool the water down if the cylinder is satisfied to make use of the heat and this is the first, smaller zone when set to normal operation, and both zones when manually switched to 'solid fuel only' mode with the ch on).

I have my thermostats wired in parallel so that both can call for heat. Only down side is that when the second zone is calling for heat and the 'sf only' zone is already up to temp, the rads still get hot as it is plumbed in before the second zone valve. Thats where thermostatic rad valves come in very handy. They close down and do the thermostats job for it. In practice, that end of the house is always warm which is fine as that is where my old mothers bedroom is and she needs the heat.
There is a way to set up a mid pos valve instead of the tee and 2 port valve and I may well modify this at a later date. Once I think it through.
Am i making sense so far?
Regs

 

[pedder63]

I'm new to this but a cold fill dead loop seems to be the simplest answer. Check out the plumbing diagrams on the woodwarm pdf booklet.

 

[kenneth11]

Hi there put a non return valve on the flow of the gravity circuit to the cylinder ,between the vent pipe and the cylinder,the arrow on the valve pointing to the cylinder