Boiler is still pumping over

[Deleted member 63160]

I have never seen a motor valve on a gravity heating circuit before.

Tony

You definately can have a motorised valve on a gravity circuit tony-working along with a cylinder stat.

Ah the jolly extra white wire which confuses the sparks from time to time

 

[gas4you]

Yes I argee you can have a valve on the return, provided the copld feedand vent are connected at the boiler, as per the manufacturers instructions

If its on the branch to the cylinder then cold feed and vent can be on primaries.

 

[Softus]

It's difficult to see how this can work without a valve unless your heating is rarely on and you use the hot water a lot.

Frankly, if you're finding this difficult to understand, you're not well placed to be advising the OP.

Please go and read a book about heating systems.
_____________________

Since the OP seems unwilling to describe, or sketch, her system, perhaps we should list every permutation possible and let her pick the one that seems the best match.

Strike that - we're nearly there already.

 

[doitall]

Yes I argee you can have a valve on the return, provided the copld feedand vent are connected at the boiler, as per the manufacturers instructions

If its on the branch to the cylinder then cold feed and vent can be on primaries.

Thats how we used to do it in the good old days gas4you. I wouldn't try it on a potty profile though

 

[Agile]

I only said that I had never seen one yet, not that they could not be used.

The whole concept of the gravity hot water was simplicity and cheapness.

I have seen several of the Drayton waxstat control valves on the return and more recently the clever Drayton TRV based control with a seperate temperature sensing phial.

Tony

 

[Space cat]

Both the pipes come out of the top of the boiler spacecat.

Fair comment doitall. I was thinking of my old Potterton and I meant the bottom of the heat exchanger.

I'm now asking myself whether this problem could come about even without any valve in the cylinder return. With HW only, everything works in the sense that it heats the water. I'd really like to know which end of the cylinder coil gets hot first though. With CH only, everything works.

The problem happens only at the precise moment of switch-over from CH to HW. (jamesn6 has already told us that she can't have CH without HW so HW will be on.) What if - and I agree that it's a big if - the pump has been pushing water through the boiler one way but gravity flow requires it to go the other way? Won't it stall and boil as it tries to reverse?

There's a test for this. When CH first comes on, which boiler pipe gets hot first? When HW comes on ON IT'S OWN, which boiler pipe gets hot first?

 

[doitall]

First you have to understand the system spacecat. In all cases where you have gravity primary circuits you cannot programme the heating on its own,

With the profile all four pipes connect to the flow and return pipe at the boiler, without the injector disc you have a problem as the pump will rob the gravity circuit which is what is happening.

There is also a gravity flow switch, which tells the boiler to do things differently from a fully pump system.

 

[jamesn6]

AL

It is very interesting 'listening' to your thoughts.

My plumber - whi is an ex BG engineer - is coming on Friday as he has to flush the system through in any case.

We will remove the boxing around the boiler pipework and I can take some pictures.

The lack of an injector disc sounds promising - could it have worn out?

This morning the CH came on cos it was cold. My husband had a bath so he ran off alot of the HW. The boiler kept firing as it needed to satisfy both CH and HW I guess. House never reached thermostat temp so CH didnt click off. Husband changed the time when boiler went off from 9.30am to 8.30am (guess he was wary it might 'blow' after he had left for work). It didnt pump over when he was there (ie before 8.45am).

The boiler, though, still got v v hot.

I am thinking that the problem may not have been too apparent over the last 3 or 4 years since new boiler was fitted as I am prone to whack the thermostat up to 28ish in the colder months so the CH is always on so to speak. In the summer we dont have the CH on.

It could have got more noticeable because I do think that the coil in the old cylinder did finally go.

 

[doitall]

If as you say you have orange water in the header tank then you have a corrosion problem, possibly caused by the sludge and air in the system.

Before wasting money on a powerflush you need to sort the problem out first, whilst you still have radiators.

Your ex BG guy needs to get to grips with the system layout and re-pipe if necessary.

Personally I think you should bite the bullet and have the system converted to fully pumped with an Yplan motorized valve and cylinder stat

 

[Space cat]

With the profile all four pipes connect to the flow and return pipe at the boiler, without the injector disc you have a problem as the pump will rob the gravity circuit which is what is happening.

Thanks for the info doitall. I'm getting a clearer picture now. My old Potterton had two outlets at the top and a single inlet at the bottom. The returns from the pumped CH and gravity HW combined in something I always imagined to be a venturi so that the pumped flow dragged the gravity flow along with it into the boiler. There was a motorized valve on the gravity return to stop the cylinder from getting too hot and to allow CW only.

My new(ish) wall mounted Potterton has only two pipes and the system is fully pumped.

 

[jamesn6]

Thought my problems were solved - but we had an almighty bang last night and water came out of the overflow.

Plumber suggested converting to fully pumped as suggested on this forum too.

Problem is boiler cannot be near the cylinder. Cylinder is in a cupboard in a corner of the bathroom that sticks out from the back of the house. Boiler is not directly under it - I think this could have been the route cause of the problem as there is horizontal pipework going from boiler under floorboard to cylinder (all tiled in now of course).

Plumber suggest doing something(?) from pipework from radiator in bedroom which is next to the bathroom but not next to the cylinder as this is in effect on an outside wall that juts out further than the bedroom.

There is no electricity in the cylinder cupboard but we may be able to get this from the loft.

Is it do-able?

 

[Space cat]

Plumber suggest doing something(?) from pipework from radiator in bedroom which is next to the bathroom

It sounds like your plumber wants to connect the cylinder heating coil as if it was another radiator. It will work but think carefully. You will only be able to get hot water if the heating is on. Do you really want to do this in the middle of a heat wave?

I see no reason why you shouldn't have a fully pumped system with the pipes you already have. The only difficult bit will be getting a cable to the cylinder thermostat which you say you don't have but will need. Also the feed/expansion pipe might need re-routing.

Meanwhile, back to the original problem. You can get HW to work on its own. You can get CH to work too but every time it switches off the boiler erupts. Here's a few tests you can do that might help. You will need to identify the heating coil pipes on your cylinder. They will be low down, one above the other on the same side (I hope).

1) Starting with a cold boiler, switch on HW only. Which boiler pipe gets hot first? Which heating coil pipe gets hot first. Please tell us it's the top one!

2) Starting again with a cold boiler, switch on HW and CH together. Make sure your room thermostat is turned up so that CH comes on. The pump should start. Which boiler pipe gets hot first. Which cylinder heating pipe? If my theory is right they will not be the same ones!

PS: It's not ideal to have a gravity system with long runs of horizontal pipe but it can work. We inherited one when we moved house. It was infernally noisy when HW started up but it worked for years.

 

[jamesn6]

Gosh, I dont want the heating on - might it just mean only the bedroom radiator would be on when hot water is called for - if it is pumped that way?
I will do what you suggest first.

 

[Space cat]

If the cylinder coil gets plumbed into the radiator circuit ALL your radiators will come on when you need hot water! The only exceptions will be those with thermostatic valves. Any others will have to be turned off by hand.

Your plumber is trying to take an easy way out. Don't be fobbed off with a bodge job. You already have pipes leading from the boiler to the cylinder which can be used for a fully pumped system as long as you take care over the positioning of the feed and vent pipes. There MUST be a clear UPHILL route all the way from the top of the boiler's heat exchanger to the open end of the vent. (When you do test one, the hot pipe will be the one from the top of the heat exchanger.)

Also, in a fully pumped system, the feed and vent pipes must join the rest of the pipework very close together. The feed can tap into the side of the vent if that is the only option, though I would prefer to have it dip down then come back up to the connection point; a sort of U bend in reverse.

If your vent comes off the top of the cylinder coil (a common arrangement in old gravity systems) then, UNLESS you re-route the vent, you CANNOT have a three port valve in that pipe. You CAN however have a pair of zone valves in the return pipes: one for HW and one for CH.

PS: Thermostatic valves are useful things to have but you can't put one on a radiator in the same room as the main thermostat.