Reverse flow from stove?

LostInLM

Flow and return pipes from living room stove rises up into the attic and runs across the living room before dropping down to the cylinder in the hotpress. They continue on down to connect into the radiator circuit from oil boiler, with a pump on the return controlled by a pipe stat near the stove. Cylinder has a dual coil with second coil connected to flow and return from the oil boiler. There's NRV's where the stove pipes and oil boiler meet, I would guess to prevent stove heating the leg out to the oil boiler and vice versa.
Bottom of cylinder is slightly higher than the top of the stove (~30cm).

In the early morning our oil boiler is timed to come on to heat rads and hot water. This heats up the cylinder and there is still a lot of hot water left after family showers etc., but I also notice that the pipes near the stove boiler are warm to the touch. This week, after the oil is turned off for about an hour, I lit the stove to keep the house warm, but I notice that as the fire heats the boiler its return pipe gets very warm as it leaves the stove and at the cylinder. Its like the water is circulating in reverse. Depending on the fire size, the return pipe gets very hot ( burning to touch) and the water is close to boiling in the stove before the flow pipe gets warm enough to trigger the pipe stat (set to ~45 degrees). Once the pump kicks in things return to normal and normal gravity circuit appears to happen, with the pump kicking in depending on the size of fire.

Not entirely sure whats going on here! My theory is that the gravity circuit is not happening or reversed, as heat leaked back up out of the cylinder making parts of the pipework in the attic hotter than the stove.

My question is :

1 - Is this safe?
2 - How can the system be aadjusted to prevent this from happening?
3 - Would this reverse flow pull air into the pipework?

I had thought about putting a second pipestat on the return pipe so that the pump would kick in when the return starts to overheat and get things moving in the right direction. Not sure if this would be just a sticking plaster on a bigger issue. Would like to hear experts thoughts.

Dtp1979

LostInLM wrote: »

Flow and return pipes from living room stove rises up into the attic and runs across the living room before dropping down to the cylinder in the hotpress. They continue on down to connect into the radiator circuit from oil boiler, with a pump on the return controlled by a pipe stat near the stove. Cylinder has a dual coil with second coil connected to flow and return from the oil boiler. There's NRV's where the stove pipes and oil boiler meet, I would guess to prevent stove heating the leg out to the oil boiler and vice versa.
Bottom of cylinder is slightly higher than the top of the stove (~30cm).

In the early morning our oil boiler is timed to come on to heat rads and hot water. This heats up the cylinder and there is still a lot of hot water left after family showers etc., but I also notice that the pipes near the stove boiler are warm to the touch. This week, after the oil is turned off for about an hour, I lit the stove to keep the house warm, but I notice that as the fire heats the boiler its return pipe gets very warm as it leaves the stove and at the cylinder. Its like the water is circulating in reverse. Depending on the fire size, the return pipe gets very hot ( burning to touch) and the water is close to boiling in the stove before the flow pipe gets warm enough to trigger the pipe stat (set to ~45 degrees). Once the pump kicks in things return to normal and normal gravity circuit appears to happen, with the pump kicking in depending on the size of fire.

Not entirely sure whats going on here! My theory is that the gravity circuit is not happening or reversed, as heat leaked back up out of the cylinder making parts of the pipework in the attic hotter than the stove.

My question is :

1 - Is this safe?
2 - How can the system be aadjusted to prevent this from happening?
3 - Would this reverse flow pull air into the pipework?

I had thought about putting a second pipestat on the return pipe so that the pump would kick in when the return starts to overheat and get things moving in the right direction. Not sure if this would be just a sticking plaster on a bigger issue. Would like to hear experts thoughts.

Can you post a few pictures

upupup

Just until you get it sorted turn on the pump every time you light the stove by turning the stat down to zero,you will here a click.Its not a problem if you forget to turn it off at bedtime,the worst it will do is rob the heat from the cylinder.
The return pipe form the stove should not be going up into the attic.It should go from the lowest connection on the stove to the lowest connection on the cylinder.

Wearb

upupup wrote: »

Just until you get it sorted turn on the pump every time you light the stove by turning the stat down to zero,you will here a click.Its not a problem if you forget to turn it off at bedtime,the worst it will do is rob the heat from the cylinder.
The return pipe form the stove should not be going up into the attic.It should go from the lowest connection on the stove to the lowest connection on the cylinder.

"The return water temperature should be maintained at not less than 40°C so as to avoid condensation on the boiler and return piping."

From Stanley manual.

LostInLM

upupup wrote: »

The return pipe form the stove should not be going up into the attic.It should go from the lowest connection on the stove to the lowest connection on the cylinder.

It does, but follows the route back up into the attic, over and back down to the cylinder as there is no other alternative. I believe that this is a fairly common way of plumbing it in this situation but not ideal.

upupup

LostInLM wrote: »

It does, but follows the route back up into the attic, over and back down to the cylinder as there is no other alternative. I believe that this is a fairly common way of plumbing it in this situation but not ideal.

It may be an airlock in the pipework in the attic.The stat may be heated by conduction rather than convection so there may be no movement in the gravity circuit.
Did it work ok before?is this a new problem?

LostInLM

Wearb wrote: »

"The return water temperature should be maintained at not less than 40°C so as to avoid condensation on the boiler and return piping."

From Stanley manual.

Thanks for the reply. Not sure what this means for my situation. Can you explain?

LostInLM

upupup wrote: »

It may be an airlock in the pipework in the attic.The stat may be heated by conduction rather than convection so there may be no movement in the gravity circuit.
Did it work ok before?is this a new problem?

How would I check for an airlock? It worked fine before, or at least I didn't notice it before!

Wearb

LostInLM wrote: »

Thanks for the reply. Not sure what this means for my situation. Can you explain?

It is the reason why the circulating pump should not be running until the stove is up to temperature.
A post had suggested manually turning down the thermostat when you light the stove.

John.G

The problem may be the type of NRV fitted in the stove flow, if its a poppet type then this needs a certain differential pressure to open it and as gravity circulation only produces very small circulating forces then it may not open under these conditions, a oil/gas boiler fitted with a poppet type has no problems as the circ pump runs at all times. Systems that use pumped zoning as distinct from motorized valve all have these poppet type NRVs fitted to prevent gravity circulation when the zone pump is off.
Have a look and see what type of NRV is fitted to the stove, if its the poppet type I would suggest changing to a swing check type which must be installed in a horizontal section of piping.
There may be special poppet type NRVs suitable for gravity circulation but hard to beat a swing check IMO.

Of course if the cylinder coil circuit is not in this circuit then it should circulate once the stove gets hot.

upupup

LostInLM wrote: »

How would I check for an airlock? It worked fine before, or at least I didn't notice it before!

Its not an easy thing to find but you can check all the bottle vents/air vent on the system and take a look in the attic,there should be a bottle vent up there.
You probably need a plumber as some pipes may need to be opened if it is an airlock

upupup

Wearb wrote: »

It is the reason why the circulating pump should not be running until the stove is up to temperature.
A post had suggested manually turning down the thermostat when you light the stove.

I think the condensation situation that you highlighted was cold water coming back to a very hot stove like the situation on the other thread with too many rads.Turning on the pump when lighting the stove when wont cause condensation as everything is the same temperature.The water starts circulating cold and gradually heats up

Wearb

upupup wrote: »

I think the condensation situation that you highlighted was cold water coming back to a very hot stove like the situation on the other thread with too many rads.Turning on the pump when lighting the stove when wont cause condensation as everything is the same temperature.The water starts circulating cold and gradually heats up

What you are talking about there is thermal shock. Different thing.

upupup

Wearb wrote: »

What you are talking about there is thermal shock. Different thing.

Ok,but the issue you highlighted isn't a problem here.It is safer to turn on the pump when lighting the stove than waiting for high temps to turn on the stat and pump

Wearb

upupup wrote: »

Ok,but the issue you highlighted isn't a problem here.It is safer to turn on the pump when lighting the stove than waiting for high temps to turn on the stat and pump

One thing being safer than another doesn't make the safer option correct.


I don't install them and was just quoting manufactures instructions.Perhaps your experience with them allows you offer that advice without causing damage.

Stoves are not my thing.

LostInLM

John.G wrote: »

The problem may be the type of NRV fitted in the stove flow,

There isn't any NRV on the stove gravity circuit. The NRV's are where it joins up to the oil boiler circuits. There's no restrictions on the gravity circuit between stove and cylinder just the up and over route. Perhaps I should draw a diagram to clarify.:)

Its not an easy thing to find but you can check all the bottle vents/air vent on the system and take a look in the attic,there should be a bottle vent up there.
You probably need a plumber as some pipes may need to be opened if it is an airlock

No bottle vent in the attic. Vent pipe over the F&E tank joins the flow at the drop down to the cylinder and goes stright up and over the tank. Flow pipe is rising as it crosses the attic with vent joining at highest point Feed from the tank joins the return pipe where it comes back up from the cylinder before going across the attic.

Only bottle vent is on the oil boiler side of the cylinder. It tends to have some air in it if top is unscrewed.

When pump kicks in things seem to regulate themselves. Would this be consistent with an air-lock?

Thanks for the help. I might run it by a local plumber to see what they think as I appreciate its tricky to explain and fault-find in text.

upupup

LostInLM wrote: »

When pump kicks in things seem to regulate themselves. Would this be consistent with an air-lock?

Thanks for the help. I might run it by a local plumber to see what they think as I appreciate its tricky to explain and fault-find in text.

The pump can push through a small airlock but the gravity circulation strength is a lot weaker.When you have cold water trying to rise and hot water trying to sink because of the route through the attic then the gravity circulation gets weaker.

your stat should be within a meter of the stove.

John.G

If you get the circulation going by running the pump briefly, does the circulation through the cylinder coil then continue when the pump stops?.