Central heating feed and vent pipe

Tom_DCU

I have an indirect vented hot water cylinder which is heated (together with radiators) by a gas boiler. The combined feed/expansion/vent pipe is connected to the upper entrance of the coil of the cylinder. There is no separate vent pipe; the feed/expansion pipe is connected to the bottom of the feed/expansion cistern in the attick and continues as a vent pipe overhanging the cistern.
There is some fitting on this feed/expansion/vent pipe close to the hot water cylinder which I do not know what is for. It cannot be a non-return valve because the the water has to be able to move in both directions in the pipe during expansion/contraction. It cannot be an isolation valve either because there should not be one on this pipe in principle. All I can think of is a safety valve. However, I do not know why you need one on this pipe; the expansion/vent pipe is a safety device in itself.
I am to replace this hot water cylinder (DIY job) and am wondering if I can remove the fitting that I do not know what it is for.

John.G

That device may be some form of deaerator to help filling the system as the water is trying to flow down the same pipe as the air is trying to escape from. I have the combined cold feed and expansion with nothing in the pipelines as expected. I have a 1/2" gate valve in the feed to the (ballcock) in the F&E Tank and on the very very rare occasions that I have to refill the system I just crack open the gate valve and have very little problem in venting any air afterwards.

I know you can't easily post a picture of the device as its your first post but I think there is some way of doing it via a link, some other poster might help.

Wearb

John.G wrote: »

I know you can't easily post a picture of the device as its your first post but I think there is some way of doing it via a link, some other poster might help.

If you pm link to pic I will post it here.

Tom_DCU

Thanks to all trying to help; unfortunately I cannot include a link to the picture in my message either.

Tom_DCU

Last time I had to refill the system I had air lock in the coil of the cylinder and had to crack open a nut near the coil to let the air out, so the device in question does not seem to work as an air vent (or it is not working).

Wearb

Tom_DCU wrote: »

Thanks to all trying to help; unfortunately I cannot include a link to the picture in my message either.

If you upload the image to a file sharing site such as dropbox, you then can PM the link to me and I will post it here.

Wearb

Here's the pic.

https://drive.google.com/file/d/1kwb-YgGuqY_B2NYzNIS07CsHg2SLctsb/view

John.G

Tom_DCU wrote: »

Last time I had to refill the system I had air lock in the coil of the cylinder and had to crack open a nut near the coil to let the air out, so the device in question does not seem to work as an air vent (or it is not working).

Unless someone can definitely identify it in the image above then I dont think it is advisable to just plumb in the new cylinder with it in place. When/if you do remove it you might section it to see what it is.

Conservatory

Looks like a non return valve

John.G

Conservatory wrote: »

Looks like a non return valve

It does!, thats what I was hoping it wasn't!.

Dtp1979

It’s an nrv. Your system is a semi sealed system. Badly done too. You should have an expansion vessel elsewhere or in your boiler. What boiler do u have?

K.Flyer

That is a non-return valve, definitely must be removed if the pipe is acting as an expansion pipe. There should be no valves of any type on any pipe that is acting as an open expansion pipe.

Wearb

Dtp1979 wrote: »

It’s an nrv. Your system is a semi sealed system. Badly done too. You should have an expansion vessel elsewhere or in your boiler. What boiler do u have?

What’s the betting on there being another boiler on the system.

Conservatory

To me it looks like it was an old open system converted to a sealed or semi sealed system. There is I’m guessing a vessel on the new boiler.

Cylinder needs upgrading an insulated one will be a huge difference.

K.Flyer

Wearb wrote: »

Here's the pic.

https://drive.google.com/file/d/1kwb-YgGuqY_B2NYzNIS07CsHg2SLctsb/view

That looks like one if those old cylinders where the immersion boss is held in place by two countersunk brass screws, beasterds for getting water tight when changing the immersion.

Dtp1979

Wearb wrote: »

What’s the betting on there being another boiler on the system.

Jesus I hope not.

Wearb

Dtp1979 wrote: »

Jesus I hope not.

+1

Tom_DCU

There is a small hexagonal-head screw in the device; if opened some air is getting out and then water in small stream (like bleeding a radiator).

Tom_DCU

When I moved in the house (some 18 years ago) there was a back boiler in an open fire; that back boiler was used to heat the house and hot water. The back boiler was shortly after replaced by a gas boiler (but the cylinder remained the same). I do not remember if this device was installed when the gas boiler was installed or it was already there (it was a long time ago). Together with the gas boiler they installed a small (red) expansion vessel and replaced the old pump with an automatic one controlled by the boiler (before the pump was manually operated).
Some two years ago the gas boiler was replaced with a newer one (condensing); they have removed the expansion vessel and the pump because the new boiler have both.

Tom_DCU

I have always assumed I had an open vented system because the feed pipe from the attick cistern continues as a vent pipe. However, in this case the expansion vessel does not make much sense to me, and the device in question cannot (should not) be a non return valve because it would not allow the expansion of water.
If it is converted into a sealed system then why is there a non return valve still there?

Tom_DCU

Here is a larger image of the device

K.Flyer

Because the new gas boiler has an expansion vessel, presumably checked to be adequate for the size of the system, the need for an open expansion vent pipe is redundant. The header tank was left as a feed for the system and the non-return valve was fitted to prevent expansion back.
Unfortunately the set up was left without any venting device at the cylinder, which explains the difficulty you had trying to remove air the last time.
My suggestion would be to do away with the tank connecting the old feed to the mains supply, fit a filling loop and pressure gauge in the hot press and fit an automatic vent valve on the feed to the cylinder coil.

John.G

Tom_DCU wrote: »

I have always assumed I had an open vented system because the feed pipe from the attick cistern continues as a vent pipe. However, in this case the expansion vessel does not make much sense to me, and the device in question cannot (should not) be a non return valve because it would not allow the expansion of water.
If it is converted into a sealed system then why is there a non return valve still there?

The reason for the NR valve is to make the expansion take place in the expansion vessel, any slight water leak will be made up by the water in the F&E tank.
I am not quite sure what you need a NR valve there at all because there will be a minimum of ~ 0.6 bar at all times, so one might ask why doesn't the system just operate this way?, my neighbour has a oil fired boiler with a integral expansion vessel and with a combined F&E tank but it is not semi sealed.

Conservatory

John.G wrote: »

The reason for the NR valve is to make the expansion take place in the expansion vessel, any slight water leak will be made up by the water in the F&E tank.
I am not quite sure what you need a NR valve there at all because there will be a minimum of ~ 0.6 bar at all times, so one might ask why doesn't the system just operate this way?, my neighbour has a oil fired boiler with a integral expansion vessel and with a combined F&E tank but it is not semi sealed.

If the tank was to overflow (ball clock in tank goes) central heating water could get sucked back into the mains supply

Dtp1979

Conservatory wrote: »

If the tank was to overflow (ball clock in tank goes) central heating water could get sucked back into the mains supply

That’s the case with every expansion tank but that can only happen if it’s installed incorrectly. The overflow should be around 50mm lower than the ballcock

Conservatory

Dtp1979 wrote: »

That’s the case with every expansion tank but that can only happen if it’s installed incorrectly. The overflow should be around 50mm lower than the ballcock

How many Celtic tiger houses have you seen with badly installed overflow pipes?

Also I’ve seen a few overflowpipes clogged with insulation and a few melted looking tanks that were left sitting on the ceiling joists with no tank stand.

John.G

Conservatory wrote: »

How many Celtic tiger houses have you seen with badly installed overflow pipes?

Also I’ve seen a few overflowpipes clogged with insulation and a few melted looking tanks that were left sitting on the ceiling joists with no tank stand.

I was thinking more around the gas boiler operating mode, I suppose having a system pressurized to 1.5/2 bar will reduce the chances of any entrained air bubbles in the heat exchanger which are very highly rated in comparison to a oil fired boiler.
Does anyone pump them up (the semi sealed system) with a test pump to give a higher filling pressure than the maximum of 0.6 Bar available from gravity alone I wonder?.

Conservatory

I think you are worrying too much about that old system. Cylinder is costing you a fortune. I’d recipe the whole hot press if I was you.

John.G

Conservatory wrote: »

I think you are worrying too much about that old system. Cylinder is costing you a fortune. I’d recipe the whole hot press if I was you.

Its the OP that is changing his cylinder, I have a oil fired system with a combined cold feed and expansion and I have a number of my original rads installed over 45 years ago, there seems to be greater corrosion problems associated with fully pressurized systems probably caused in part by these auto filling valves and little or no expansion vessel testing/servicing.
If it were my house, I would retain the gravity fed system as long as the F&E tank doesn't have to be relocated with any associated plumbing changes.

Tom_DCU

If mine is a semi-sealed system then does it mean it is always 'pressurised' to the same pressure defined by the head of water in the F&E cistern in the attick? And obviously there is never a need to re-fill the system because the F&E tank does it (in fact the F&E cistern now becomes a feed cistern only once expansion happens in the expansion vessel in side the boiler).
What is the advantage of a sealed system? I can see several disadvantages like the need to re-fill the system if there is any loss in the system (say after bleeding the radiators), and also the higher pressure might cause leaking of not well made joints. I understand that a pressurised system is filled form the mains so it fills faster and also there is less chance of trapped air. Any other advantages?

K.Flyer

Some new gas boilers will not operate on low gravity water pressure due to the water pressure safety switch in the appliance, but a few will.
Typically on new gas boilers the circulation pump is rated 5 - 6 metres. On an older system with typical separate feed and expansion pipes you can get water pitching into the F & E tank, especially with the radiators off in the warm weather. This in turn creates its own problems for the heating system as well as damp and condensation issues in the attic, hence a sealed system is more favorable as it also helps sort out poor circulation problems and also lets you know if there are any leaks on your system with the pressure drop usually shutting down the boiler.