Clearing airlock from hot water cylinder - upto how much water needed to force through?

[flyingspark9876]

I'm trying to sort out a "no hot water" problem when boiler is set to heat "hot water only, central heating off". Just get normal flows of cold water from all the hot water taps. The hot water flow pipe going into cylinder from boiler is very hot, the return pipe coming out of the cylinder to the boiler is luke warm where it exits cylinder. The cylinder itself (including top) feels cold everywhere. The boiler fires up but goes off after few minutes (say every 5 minutes). I can hear water swishing through the pump which is running.

Its a conventional indirect system, Potterton Fuelsaver 65F boiler, cylinder on first floor, expansion tank in loft (which is full).

I thought this was an airlock in the cylinder in the non-coil part of cylinder, so attached hose & forced water out through cylinder out of vent pipe, left it coming out for say 5 minutes & thought this would be enough time to clear any air. As this didnt fix problem,my question is how much water might need to be forced through hose,cylinder & out vent pipe to get rid of air lock. Might I need to replace the entire cylinder contents. I went onto think the problem is now in the coil boiler fed part of the cylinder but its hot going into the cylinder coil. Help... Steve

[Dave Baker]

Then clearly there isn't any flow through the coil. The water wouldn't drop from very hot to lukewarm. The pipe going in is probably just hot from conduction further down. The zone valve for hot water is probably stuck in the closed position.

How could it possibly be an airlock? If there was no water in the cylinder what would be cooling down the water in the coil pipe from very hot to lukewarm? Doh!

[PeterMcC]

[e-mail] wrote in

The hot water/central heating options are acheived by a diverter valve that controls which parts of your system gets to have the heated water flow through it. It sounds to be quite possible that your diverter valve has failed.

There are two main components to the valve - (1) the body which has the inlet/outlet ports and the valve that opens and closes the flow in the ports and (2) the actuator that moves the spindle of the valve, thus opening and closing the ports.

Most valves (£40-£80) are relatively easy to replace:

compression fittings: loosen, remove old valve, replace with new valve, tighten

wiring: remove wires from old valve and replicate layout for new valve.

Job done.

And sometimes it's even easier:

The problem is sometimes caused by the valve becoming stuck - occasionally, with the actuator removed, the spindle of the valve can be persuaded to move by encouraging it with a careful to-and-fro twist using a wrench.

Then again the problem can be the motor - this can usually be replaced separately for about £25

Examples of complete valves, valve bodies, actuators and motors here:

[Roger Mills]

One thing is certain - you *don't* have an airlock in the DHW system itself - otherwise you's get *nothing* rather than cold water out of the hot taps.

For whatever reason, you have insufficient flow through the *primary* circuit (boiler, pump, coil inside HW cylinder) to heat the water in the tank. So the boiler fires for a little while, and then turns off on its (internal) stat because the heat produced isn't being transferred to the cylinder.

Could you give us a bit more detail about your system - particularly what is supposed to control it. If the pump is running on HW-only, that suggestes that it's a fully pumped system rather than gravity HW and pumped CH. Is that your understanding? If this is so, there are likely to be one ore more motorised valves in the system - probably (though not necessarily) in the airing cupboard near the hot cylinder. How many are there - and what type? [Likely to be one 3-port valve or two 2-port valves].

Is there a cylinder thermostat (as part of the controls for the boiler/valve(s), *not* the one inside any immersion heater which may be present)? Does it click when you turn it back and forth? What temperature is it currently set at?

Is there a gate valve - or somesuch - on the output pipe from the cylinder coil (i.e. the boiler return pipe)? If so, is it open?

You say that the expansion tank is full (that's the *little* tank which acts as a header for the primary circuit, right?) Is there a tap or gate valve on the pipe coming out of the bottom of that, which feeds the primary circuit? If so, is it open?

You started by saying that this problem occurs when on HW-only with CH off. What happens if you turn the CH on - do you then get get both CH *and* HW? [I know you don't *want* the CH on this tgime of the year, but a quick test may help to isolate the problem].

[flyingspark9876]

Thanks very much gents, (I can forget the airlock blind alley & disconnect the hose).

There is only 1 motorised valve (2 port) which is on a pipe going to the rads & is not in the flow going into the Cylinder. (The MV seems OK as its cold when HW is On & CH is Off, its hot when CH is on)

BleedValve I HWCyl-- V1---------- I I I I Pump VentPipe _____I I I I I I-------------------------- I FeedfromExpTank I I I I I I I--------------------------------------------I I I I I I Boiler From Boiler Return

Water flows from pump to Cylinder & Rads. There is a thermostat on the cylinder which is connected via copper wire to the ThermostaticRadiatorValve (TRV) on the pipe flowing into Cylinder coil from Boiler. Ive set it to Max & it is working. (Switching it to Max immediately causes it to get v hot including pipe after it into cylinder. Doesnt this suggest there is NOT a full blockage in the cylinder coil otherwise there would be no where for the water to go & it wouldnt get hot immediately )

The valve (V1) on the output pipe from the cylinder coil is open. It isnt a gate valve, more like a radiator valve with a square head you need to turn with spanner to open & close. There is no valve on the pipe run from the Expansion tank bottom outlet.

When CH is set to on, it also does NOT heat the water in the cylinder to normal hot temperature. (Only lukewarm after very long time)

GateValve (GV1) is open. (Not sure why its there, this pipe run seems to short circuit the Cylinder, why would water flow through cylinder coil when it can short circuit here.

Probably not relevant but the Cylinder coil inlet pipe entry point is less than a third of Cylinder height from bottom. Seems low to me but thats how it was made.

Hope you can draw some conclusions from this lot! thanks again Steve

[flyingspark9876]

RESEND (diagram realignment only difference)

Thanks very much gents, (I can forget the airlock blind alley & disconnect the hose).

There is only 1 motorised valve (2 port) which is on a pipe going to the rads & is not in the flow going into the Cylinder. (The MV seems OK as its cold when HW is On & CH is Off, its hot when CH is on)

BleedValve I HWCyl-- V1----- I I I I Pump VentPipe _____I I I I I I-------------------------- I FeedfromExpTank I I I I I I I--------------------------------------------I I I I I I Boiler From Boiler Return

Water flows from pump to Cylinder & Rads. There is a thermostat on the cylinder which is connected via copper wire to the ThermostaticRadiatorValve (TRV) on the pipe flowing into Cylinder coil from Boiler. Ive set it to Max & it is working. (Switching it to Max immediately causes it to get v hot including pipe after it into cylinder. Doesnt this suggest there is NOT a full blockage in the cylinder coil otherwise there would be no where for the water to go & it wouldnt get hot immediately )

The valve (V1) on the output pipe from the cylinder coil is open. It isnt a gate valve, more like a radiator valve with a square head you need to turn with spanner to open & close. There is no valve on the pipe run from the Expansion tank bottom outlet.

When CH is set to on, it also does NOT heat the water in the cylinder to normal hot temperature. (Only lukewarm after very long time)

GateValve (GV1) is open. (Not sure why its there, this pipe run seems to short circuit the Cylinder, why would water flow through cylinder coil when it can short circuit here.

Probably not relevant but the Cylinder coil inlet pipe entry point is less than a third of Cylinder height from bottom. Seems low to me but thats how it was made.

Hope you can draw some conclusions from this lot! thanks again Steve

Thanks very much gents, (I can forget the airlock blind alley & disconnect the hose).

There is only 1 motorised valve (2 port) which is on a pipe going to the rads & is not in the flow going into the Cylinder. (The MV seems OK as its cold when HW is On & CH is Off, its hot when CH is on)

BleedValve I HWCyl-- V1---------- I I I I Pump VentPipe ___I I I I I I----------------------I FeedfromExpTank I I I I I I I------------------------------------I I I I I I Boiler From Boiler Return

Water flows from pump to Cylinder & Rads. There is a thermostat on the cylinder which is connected via copper wire to the ThermostaticRadiatorValve (TRV) on the pipe flowing into Cylinder coil from Boiler. Ive set it to Max & it is working. (Switching it to Max immediately causes it to get v hot including pipe after it into cylinder. Doesnt this suggest there is NOT a full blockage in the cylinder coil otherwise there would be no where for the water to go & it wouldnt get hot immediately )

The valve (V1) on the output pipe from the cylinder coil is open. It isnt a gate valve, more like a radiator valve with a square head you need to turn with spanner to open & close. There is no valve on the pipe run from the Expansion tank bottom outlet.

When CH is set to on, it also does NOT heat the water in the cylinder to normal hot temperature. (Only lukewarm after very long time)

GateValve (GV1) is open. (Not sure why its there, this pipe run seems to short circuit the Cylinder, why would water flow through cylinder coil when it can short circuit here.

Probably not relevant but the Cylinder coil inlet pipe entry point is less than a third of Cylinder height from bottom. Seems low to me but thats how it was made.

Hope you can draw some conclusions from this lot! thanks again Steve

[Pete Verdon]

Sorry, still no good. You need to make sure you're using a fixed-width (not proprtional) font when you create such diagrams. I think "Courier New" is the traditional one on Windows?

Pete

[Roger Mills]

As someone else has said, even your MkII ASCII art isn't very clear - it needs to use a fixed width font.

Meanwhile, you said that when setting the controls to HW+CH, the HW *still* doesn't get hot. But you didn't say whether the radiators get hot. Do they? If so, that would eliminate the pump. If not, the pump has to be the prime suspect - its impellor could have become detached from the shaft so that, although the motor is running, it ain't doing any pumping.

The pipe and gate valve which is in parallel with the cylinder coil is probably a by-pass circuit - to provide a flow path even when then TRV is closed. Have you tried partially closing it, to give a bit more urge to the water going through the coil? Could someone have inadvertently fiddled with it - causing your problem?

Now would be a good time to overhaul your CH system, and install some proper controls which provide a boiler interlock. You could save a *lot* of gas. When was the system installed - as far as I know, the setup you describe hasn't been common in new installs for a good many years?

[flyingspark9876]

Thanks again, (sorry about the diagram-it still might not be clear but hopefully lines up and clearer than trying to put into words)

BleedValve I HWCylToRads gV1 I I I I I---------- HWCyl->-- V1-------------I I I I I Pump VentPipe _____I I I I I I-----------------I I I I I I I FeedfromExpTank I I I I I I I------------------------I I I I I I Boiler From Return Boiler

The system is about 20 years old (just bought the house). The rads do get hot when CH (& CH+HW) set to on. The Gatevalve (gV1) is fully open. I did actually close it fully in trepidation (but reasoned the boiler shouldnt explode fingers crossed. Still the same- luke warm at HWCyl -> V1 , the boiler forced up hot water through the vent pipe and stopped, so at least that bits working!) I am wondering could there be air in this boiler feed / boiler return / cylinder coil circuit that would cause this problem so that the pump is unable to pump effectively to & through coil. How probable is it that valve (V1) is partially blocked, or God forbid the cylinder coil itself. Or could the pump itself not be pumping at a high enough pressure. What does it mean if there is still no hot water coming out of coil even when the Gatevalve (gV1) is fully closed.

The CH overhaul sounds like a good idea, I just wish it was an easy (& quick) job. Thanks Steve