2 Port Valve / Pump Question

[jalexs]

Hi Guys, Hopefully someone can enlighten me here..

For the last month or so there has been an a horrible gargling / bubbling noise coming from the airing cupboard.
I have isolated this now to be only when the boiler stops calling for heat. What seems to be happening is that the boiler stops the call for heat, the 2 port valve switches to the off position, but the pump continues to run. Thus diverting the flow to the bypass / hot water cylinder (and possibly creating back pressure on the pump). I'm no plumber, but am assuming this shouldn't be happening.

If this is the case, could someone help me understand where the issue is originating from? The 2 port valve seems to be moving on and off freely when the room stat is clicked on and off, so the movement seems to be fine there. Is this more likely a faulty programmer? We have been here 2 years now and the issue has only arisen in the last month or two.

Red in the picture is the 2 port valve switching off.. sky blue is to cylinder / boiler return.

Thanks in advance!

 

[stem]

When the boiler goes off, it is usual for the pump to continue to run for a short time to dissipate the residual heat from the boiler and prevent the boiler overheat tripping. This is perfectly normal and is in fact necessary for most boilers. Your boiler manual will confirm if yours has a pump overrun.

Because your system has 2 x two port motorised valves (aka S-Plan) which close fully when heating / hot water is not required, a flow of water must be maintained elsewhere to allow the heat dissipation during the overrun. In your case it will be via the automatic by-pass valve shown in your photo.

What you describe is not an uncommon feature of the S-Plan, and I have seen (or should I say heard) it quite often, I also believe that it can be difficult to eradicate. It could be because the flow through the by-pass is insufficient. However, why it has suddenly appeared would be a bit of a mystery, unless the pump, or by-pass have been changed or adjusted. Has the boiler thermostat been turned up higher than usual?

My expertise relates to the control side of heating systems, rather than the actual plumbing, so maybe one of the heating engineers will be along shortly and able to advise you further.

 

[D_Hailsham]

Can you take a pic from further back, so we see all the pipework?

Where is the pump?

Is there only one motorized valve?

 

[jalexs]

Thanks for the replies.

When the boiler goes off, it is usual for the pump to continue to run for a short time to dissipate the residual heat from the boiler and prevent the boiler overheat tripping. This is perfectly normal and is in fact necessary for most boilers. Your boiler manual will confirm if yours has a pump overrun.

Because your system has 2 x two port motorised valves (aka S-Plan) which close fully when heating / hot water is not required, a flow of water must be maintained elsewhere to allow the heat dissipation during the overrun. In your case it will be via the automatic by-pass valve shown in your photo.

What you describe is not an uncommon feature of the S-Plan, and I have seen (or should I say heard) it quite often, I also believe that it can be difficult to eradicate. It could be because the flow through the by-pass is insufficient. However, why it has suddenly appeared would be a bit of a mystery, unless the pump, or by-pass have been changed or adjusted. Has the boiler thermostat been turned up higher than usual?

My expertise relates to the control side of heating systems, rather than the actual plumbing, so maybe one of the heating engineers will be along shortly and able to advise you further.

The pump and bypass valve were changed around a year ago, but these noises were not apparent then. I reduced the boiler thermostat recently a little, but before that it was excessive (70deg).

Can you take a pic from further back, so we see all the pipework?

Where is the pump?

Is there only one motorized valve?

The pump is directly above the valve you can see there, I'll attach another more recent picture (Sorry still isn't fully zoomed out).
There is only one valve. I think the setup is a bit antiquated as the cylinder is not separately controller.



I have been in the loft to see whether there is anything strange is going on with overflow etc but there is nothing happening up there.
If it were to be insufficient bypass flow, is there a way to test or correct this given that it's an automatic valve?

Thanks again

 

[D_Hailsham]

The pump is directly above the valve you can see there.
There is only one valve.

The pump is "upside down". This may not matter if it is pumping in the right direction (arrow on pump body).

The pump seems to be on the pipe to the right of the motorized valve. Is this correct?

What does the pipework do above the second pic? A drawing of all the pipework in the cupboard would help.

 

[jalexs]

The pump is "upside down". This may not matter if it is pumping in the right direction (arrow on pump body).

The pump seems to be on the pipe to the right of the motorized valve. Is this correct?

What does the pipework do above the second pic? A drawing of all the pipework in the cupboard would help.

The flow is down towards the radiators, so think this is correct.

After a bit of listening, this is the series of events: 1) call for heat stops 2) 2 way valve shuts 3) overrun begins 4) pumps seems to speed up quite a bit and this is where is sounds like the back pressure is created.
Even under the lowest speed, the pump periodically speeds up a great deal and the noise begins.

On these Grundfoss pumps there is the option to hold the speed selector for 5 seconds apparently to engage "proportional pressure control" (sorry for sounding like a noobie here).
I've tried this option and the pump operated at a speed lower then the lowest setting, when the call for heat goes there is no noise through the bypass and the radiators are still getting hot from what I can tell.
Could I ask, is this a) okay and advisable to use this setting? (our engineer didn't mention this setting when installing)
b) if so, what could be happening with the default fixed speed pumping and will this need to be looked at further / is the periodic increase in speed normal?

I've attached a more zoomed out diagram.
Light Green = flow through pump then zone valve then off to rads. Yellow = bypass off to boiler return. Cyan = hot water cylinder in / out. Dark Green = I'm not sure.

Appreciate time spent replying here.

 

[D_Hailsham]

pumps seems to speed up quite a bit and this is where is sounds like the back pressure is created.

Thanks for clarifying pump direction and connection to MV; the latest picture was much clearer.

When the pump is on a fixed speed it does not "speed up", it rotates at a constant rate. What you are hearing is an increase in the flow rate as the pressure across the pump has decreased. This will be due to the MV closing and ABV opening. The same thing will happen when TRVs close.

You can't use Proportional Pressure Control if you have an ABV. Grundfos say:

For condensing boiler, the operation of an automatic bypass with proportional pressure control is counter productive to maintaining condensing boiler efficiency at high load conditions. As the pump pressure increases, the bypass flow will also increase which raises the boiler return water temperature and reduces boiler efficiency.

When was the pump installed and has the problem arisen since then?

Has the system been balanced?

Is the bypass valve the correct way round (flow in direction of arrow on side)?

What is the bypass setting (make as well)?

Have you thought about installing a second motorized valve, on the connection to the HW cylinder, so you can control the time/temperature of the cylinder? At the moment the HW is heating up all the time the boiler is running, even if you don't need it.