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