Central Heating Flow Not Getting Past Non-Return Valve

[Glennington]

Hi All,

So, I am currently having an issue with my central heating provided by my combi-boiler.

Every time the central heating is called and the central heating pump turns on, the flow can't get passed the 22mm double non-return valve (labelled by red arrow in attached photo - "Top of Boiler (Labelled)"). I know this because the pipe is cold downstream of the non-return valve (labelled as yellow in same photo) and hot upstream. The green arrow in the photo indicates direction of flow.

To temporarily fix this issue and get the flow past the non-return valve, I have to open the test plug on top of the non-return valve and allow water to shoot out for about 3secs. After doing this, turning on the heating and thus the pump, it gets past the non-return valve and the heating works.

The pump is 3 speed and is at the maximum speed. It would seem it is an air lock? However, I do not hear air leaking out every time I loosen the test plug on the non-return valve.

I am guessing the pump isn't providing enough head to get past the non-return valve? I have had the pump head off and checked the impeller and it doesn't seem damaged or seized, I have even had the pump on when the head is off the housing and I can see the impeller spins.

I have also replaced the non-return valve and the same issue persists. FYI: the pressure of the central heating system is about 1.8bar and the top pump in the 2nd photo is the central heating pump, the other one is the domestic hot water pump.

I have attached more photos to help with my enquiry.

Has anyone else experienced a similar issue and know what could be causing this?

Thanks

 

[Johntheo5]

Why do you need a double or even a single, is it a combi boiler?

 

[Glennington]

Why do you need a double or even a single, is it a combi boiler?

Hi John,

Thanks for the reply.

It is a combi boiler.

The non-return valve is positioned on the heating flow circuit just before it enters the house. I believe it is to prevent back flow due to gravity from the radiators into the central heating pump?

Regarding the pump below the central heating pump (the very bottom pump) this is the domestic hot water pump. The one above is the central heating pump. Both pumps pump in opposite directions (not in series). This boiler does not have a 3 way actuated valve but instead has a pump on the central heating circuit and one on the domestic hot water circuit. I hope this makes sense!

Thanks

 

[Johntheo5]

Yes, I vaguely remember changing a pump and a NRV or two on a Grant Combi a good few years ago for a neighbour but I don't recall them being DCVs, was there allways a DCV in that position, I might have a eManual for that Grant and have a look at it.

 

[Glennington]

Yes, I vaguely remember changing a pump and a NRV or two on a Grant Combi a good few years ago for a neighbour but I don't recall them being DCVs, was there allways a DCV in that position, I might have a eManual for that Grant and have a look at it.

Hi John,

Good question if it really needs to be a double! I'm guessing a single is prone to failing?

I've been living at this property for only 2 years and my guess it has always been but I have no idea!

Do you know if it is possible to get a 22mm single non return valve with the same length?

Thanks

 

[Johntheo5]

Yeah, that was the type, below, I'm almost certain that they were normal single check valves but I can't remember if they were swing check or the poppet type, possibly the latter, no I don't know of any single that would fit in there without modification, you could allways make up a spool piece, there's also a smaller bore pipe taken off the DHW pipe thats teed in just down stream of that DCV
can you post a close up photo of that circ pump. looks as if it was renewed recently, is it a genuine Grundfos?.

 

[Glennington]

Hi John,

Cheers, thanks for this. Just to let you know the boiler is Warmflow Kabin-Pak 70 HE (Oil Fired).

Is there such thing as a fake Grundfos? To me it looks genuine and yes, it does look newer than the other pump head (DHW pump). However It hasn't been replaced while I have been here (in the last 2 years)

Please see below:

 

[Glennington]

Also, you mention about the 15mm pipe downstream of the DCV - this is not connected (just looks like it on the photo!) It's the cold water inlet and separate.

 

[Johntheo5]

I see references to a Warmflow 2219 NRV in googling, looks like a plastic poppet type (single) but don't know if that should be your type, are there any other NRVs on that boiler to compare it with
The grundfos is a 50/60 which is 5M/6M so should be capable of opening that DCV, I have seen cracking (opening) pressures as high as 3.5M for those types. Other than that, afraid I can't offer much more advice.

 

[Glennington]

Thanks for the help John, really appreciate the replies!

I do not see any other NRVs to compare with unfortunately.

Looks like there maybe two options for me here then:

- Install a single NRV and fit a spool to make it fit in the 105mm gap from the DCV

- Replace the Grundfos Pump head (perhaps the motor isn't providing the head it should).

 

[Johntheo5]

I assume the pump impeller vanes were clear when you removed it, (I use a tie wrap to push through them & prove them clear), if so, and its rotating ok then probably OK especially if the CH is heating up properly, maybe try the single check valve?.

 

[Glennington]

Ok, I will look at replacing for a single NRV (thinking about what parts I could use so I could easily replace it back to double NRV if needed).

The pump impeller looked clean however, I didn't scrutinise the vanes - perhaps I should check again.