Hot water circulates ONLY if CH is turned on, valves all OK

[i386]

Hi forum,

This problem has been scratching me for weeks; even the plumber/gas safe engineer doesn't have a clue after days of work attempting to find the root cause of, let alone a resolution to the problem.

Recently I got a qualified CH installer to upgrade our home's CH system, from a conventional open-vented system (12 years old, gravity fed) to a Megaflo system. Both the boiler and the cylinder were replaced, and the pipes modified to serve the new system.

I have surveyed and drawn a detailed schematic view of the current Megaflo system installed:

Untitled

• i386
• 25 Sep 2011
• 1

This system currently suffers from a VERY WEIRD problem: whenever the CH is off, the water in the pipe system stops circulation. Two consequences: 1) no hot water is going to the cylinder, and 2) at the point of shutting down CH, I can always hear a loud BANG in the pipes, suggesting the water flow is stopped.

One would then assume the HW Zone Valve (item [C] in the above graph) might have broken. We suspected the same and replaced the valve [C] TWICE, with different brands and models - none worked.

It is not the wiring of the zone valves either. If you listen carefully you can hear them operating normally when controlled by the programmer or the thermostat. Moreover, when I manually let the HW zone valve stuck at "Open" position, there is still no water flow if the CH zone valve is at "Close". How strange.

Then the engineer suspected airlock(s) in the pipework. He then fitted 2 air vents to the system, one before the cylinder intake and another after the cold feed. They worked effectively in venting the air out of the system, however they did absolutely nothing to relieve/resolve the strange HW problem. I knew air locks may take time to float out but it's been 3 weeks; not a little has changed.

The engineer drained the system a few times when attempting solutions - none worked and we always got no HW-only circulation right from the beginning, together with the loud BANG when CH stops (while leaving HW on).

We noticed that the INTERNAL automatic bypass is extremely hot on the boiler when HW is on alone; which means all water goes through the very small circulation around the boilder IN/OUT. When CH is turned on, the situation then reverts to normal.

He then tried to install a bypass between after the HW zone valve (section [CD] on the graph) and the cold returns (section [HJ] on the graph). This didn't work either (even when HW Zone Valve [C] is stuck open manually).

This has driven me mad as we seem to have exhausted all possible checks and found no cause whatsoever. It didn't seem that the HW pipe is jammed either; the engineer has tried blowing air into the cylinder inlet and water comes out from returns with no apparent air. Also the HW pipes flow without a problem when CH is turned on, suggesting the HW pipes are not jammed (they share most pipework with CH anyway).

To top it up, both the boiler (BAXI Megaflo 28HE) and the cylinder (HS 170L) are brand new, system was power flushed before the installation.

Now can anyone shed some light on this troubled installation? Really appreciated if you can spend a few minutes on reading the text and graph above.

Many thanks in advance.

 

[bookahandyman]

sounds like the return from cylinder is piped to the flow and not to a return.can you trace the pipe all the way back to the boiler from the cylinder?

 

[picasso]

You may have an old non return valve tucked away somewhere out of sight on the hw circuit, sometimes used to stop unwanted circulation in the rad circuit.

 

[petit_pablo]

Is the pump still running on HW only?

There has to be something stopping that circulation on the HW circs. You will need to physically trace the pipework and see where it goes.

 

[i386]

sounds like the return from cylinder is piped to the flow and not to a return.can you trace the pipe all the way back to the boiler from the cylinder?

Hi there,

Thanks for the reply - you mentioned something I've never heard of... indeed the house was recently bought by us and its piping underneath the floor is unknown to us. Your explanation sounds like quite possible though, but do you know how we can find this out or resolve this?

And how was the "return piped to flow" mechanism suppose to work in the first instance (before megaflow was fitted).

Thanks

 

[i386]

You may have an old non return valve tucked away somewhere out of sight on the hw circuit, sometimes used to stop unwanted circulation in the rad circuit.

Thanks, but if this were the case, why it works when the CH is turned on? The CH and HW return pipes join very early and the joint is visible to me in the cylinder room. after that, it's just one pipe going underneath the floor. If a valve is stuck close, then the CH return would have been stopped as well? Still not sure...

 

[Youllneverguess]

To add to the above...where is the differntial by pass?

Also check the hot water zone is the right way round. That might be the a use of your clunk.

 

[i386]

Is the pump still running on HW only?

There has to be something stopping that circulation on the HW circs. You will need to physically trace the pipework and see where it goes.

yes the pump (inside the boiler in Megaflo systems) is still working and the internal bypass joining the IN/OUT of the boiler gets very hot. The water in the other pipes is still (no flow, as can be felt by temperature).

 

[i386]

To add to the above...where is the differntial by pass?

Also check the hot water zone is the right way round. That might be the a use of your clunk.

Hi, excuse my knowledge by what do you mean by "differential bypass"?

We've checked and can be 100% sure that the zone valves are in the right direction.

 

[Youllneverguess]

Apologies, didn't check whether the boiler had its own bypass.

 

[bookahandyman]

sounds like the return from cylinder is piped to the flow and not to a return.can you trace the pipe all the way back to the boiler from the cylinder?

Hi there,

Thanks for the reply - you mentioned something I've never heard of... indeed the house was recently bought by us and its piping underneath the floor is unknown to us. Your explanation sounds like quite possible though, but do you know how we can find this out or resolve this?

And how was the "return piped to flow" mechanism suppose to work in the first instance (before megaflow was fitted).

Thanks

The pipes have poss been crossed over during the pipe modifications you mentioned. You will have to trace the pipes inch by inch between boiler and return on cylinder to confirm no crossing over has occurred. Was the old system gravity hot water and pumped heating? Some older systems I've seen have had the return pumped and not the flow, which can confuse people who presume it's the flow when piping a new cylinder in the airing cupboard.

 

[i386]

sounds like the return from cylinder is piped to the flow and not to a return.can you trace the pipe all the way back to the boiler from the cylinder?

Hi there,

Thanks for the reply - you mentioned something I've never heard of... indeed the house was recently bought by us and its piping underneath the floor is unknown to us. Your explanation sounds like quite possible though, but do you know how we can find this out or resolve this?

And how was the "return piped to flow" mechanism suppose to work in the first instance (before megaflow was fitted).

Thanks

The pipes have poss been crossed over during the pipe modifications you mentioned. You will have to trace the pipes inch by inch between boiler and return on cylinder to confirm no crossing over has occurred. Was the old system gravity hot water and pumped heating? Some older systems I've seen have had the return pumped and not the flow, which can confuse people who presume it's the flow when piping a new cylinder in the airing cupboard.

Yes, the old system was gravity HW and pumped CH. The megaflo installer didn't touch any pipes underneath the floor, but from the surface it looks OK.

but re your last concern, is there a way to identify this without stripping the floor boards? I tried, when turning the CH on, the water flows very quickly from boiler's OUT to the current 2 zone valves . It seems at least the current zone valves are installed on the current flow pipes. But when CH is shut, hey, everything stopped imminently and with a bang.

 

[bookahandyman]

With ch on dOes the return to boiler get hot.and can u see this return pipe in cylinder cupboard. If so then the return for the cylinder needs to be t'd into that pipe And i see no other reason itwont work. As you have ruled out all other things ie valve direction and pump and zone valve operation. It has to be a pipe issue. Maybe blockage in old pipe. ??