Two pumps on a Baxi Back Boiler unit. Pumping over problem. Radiators pinholing

[buryboy]

Hello, called to a job where the bathroom radiator was leaking. Turned out to be a pinhole in botton of rad. I didn't fit this boiler so on checking the pipe runs etc., I found that there were two pumps on the system. The first pump is on the heating side. It's on the return and pushing towards the boiler. At that point OK. Then when I went into the loft to close F&E service valve I saw a second pump. This was set at its lowest speed and was on the return circ from the DHW cylinder pushing back down to the boiler.
The boiler's vent pipe was T'eed off the flow circ and the fill T was on the return. However pumping over is occuring which is the reason that she has to change radiators because of the rampant oxydysation causing pinholing.
The reason she gave for the second pump was that following many visits by the installer and then a joint visit with a gent from Baxi, the installer installed this pump.
This was done to cure the boiler only firing up for short periods, then shutting down shortly afterwards. The new pump cured this boiler cycling.
However pumping over causing more problems with radiators not lasting too long.
Has anyone come across anything similar?
In oreder to stop this pumping over, I'm thinking of moving the pump onto the flow circ pushing towards the cylinder about 5 feet away. Then re-piping the vent and fill pipes in behind the direction of the pump.
Appreciate anyones thoughts or experiences on this.

 

[Toddyplumb]

So have they put the second pump on the old gravity circuit ?
The newer Baxi BBU dont like gravity .
Can you not just change it to a fully pumped S plan ?

 

[buryboy]

Many thanks toddyplumb for your reply.

After the boiler was installed there was re-decoration over the pipe boards that looks to be of a very high standard and price. I'm hoping that the alterations I propose in the loft will solve the corrosion problem, thus minimising disruption. Why the original Installer did not opt for a fully pumped "S" plan, or another suitable plan, at that time, I do not know.
There is no Cylinder stat fitted and the Roomstat is fitted over a door!
I suppose the present setup is almost a S plan, with two pumps instead of motorised valves!(but no cylinder stat). The pumps are on the returns and both pumping to the boiler.
I would imagine that the gent from Baxi would have ensured the boiler temperature sensor is fitted right, following upon the original complaint - boiler firing up then shutting down.
I all really need to do is stop the Pumping over. The system apart from that is OK.

 

[buryboy]

Sorry, I forgot to mention that the BBU was installed 2004, the firm that installed it are no longer trading due to the proprietor's death 18 months ago.

 

[Toddyplumb]

The only thing to watch with what you propose is that the second pump from the heating does not push it over your new vent. Can you not move both pumps to the flows . I think its a suck it and see job but with what you say it sounds a reasonable solution .
I remember Grundfos did a twin pump to work like an S plan , never used or seen one though.

 

[Gray0689]

Can you not just tee the vent and feed together and increase the height on vent

 

[buryboy]

Many thanks Gray0689,.
Sadly I can't increase the height of the vent pipe as it is now touching the underside of the slates in the loft.
Regarding teeing the vent and feed pipe, not sure on this.
Some one else has said that it is very much suck it and see, I was hoping that someone might have come across a smilar situation before and could give me the benefit of their experiences. I have now changed 2 newish Purmo radiators on this system in the last 15 months and when I was there yesterday it looks as though the one in the hall is showing signs of internal problems.

 

[buryboy]

Thanks Toddyplumb for your thoughts.
Originally the circs were gravity feed. Because of the boiler shutting down, this pump was thought to be the answer. However even on its lowest setting its still strong enought to pump over.
I would be happier if there was a pump that was less powerful perhaps. Its a regular Wilo 3 speed. Also its not controlled by a cylinder stat.
There isn't enough resistance in "circs" circuit to dissipate the pump's energy.

 

[Billy Bob Bob]

had this problem one on the same boiler as said they dont like gravity,sounds like the pump is knackered and you have a vent height issue baxi did make a o/h stat kit for this boiler converting it to pressurised if desired but this would cause problems with her knackered rads

 

[Gray0689]

Many thanks Gray0689,.
Sadly I can't increase the height of the vent pipe as it is now touching the underside of the slates in the loft.
Regarding teeing the vent and feed pipe, not sure on this.
Some one else has said that it is very much suck it and see, I was hoping that someone might have come across a smilar situation before and could give me the benefit of their experiences. I have now changed 2 newish Purmo radiators on this system in the last 15 months and when I was there yesterday it looks as though the one in the hall is showing signs of internal problems.

If you tee together will give more resistance

 

[Billy Bob Bob]

If you tee together will give more resistance

and its against mi on most bermudas

 

[Gray0689]

and its against mi on most bermudas

Give another solution then

 

[tamz]

Do as Gray says. It WILL work believe us, whether you understand the reasons or not.

 

[Billy Bob Bob]

Give another solution then

i already have ,having been in this situation with a bermuda

 

[Gray0689]

i already have ,having been in this situation with a bermuda

Not a good one then

 

[buryboy]

Many thanks to Gas man and Gray0689 for your replies.
My thoughts are to move the pump onto the flow circ pumping towards the DHW cylinder and bringing the vent and feed pipes in behind the pump at what should be the neural point.
I'm told there is a bronze pump that has a much reduced pressure and flow rate that can also be tried. Unfortunately its expensive at around £125 plus VAT.

 

[Gray0689]

Many thanks to Gas man and Gray0689 for your replies.
My thoughts are to move the pump onto the flow circ pumping towards the DHW cylinder and bringing the vent and feed pipes in behind the pump at what should be the neural point.
I'm told there is a bronze pump that has a much reduced pressure and flow rate that can also be tried. Unfortunately its expensive at around £125 plus VAT.

If that's what you want to do it's upto you
But I think your going to a lot of expence and trouble without trying easy options to fix first

 

[Gray0689]

Bronze pump is generaly used for a secondery return on DWH

 

[Billy Bob Bob]

bronze pump not suitable for ch

 

[buryboy]

Bronze pump is generaly used for a secondery return on DWH

Yes I would agree with you on that.

 

[buryboy]

bronze pump not suitable for ch

This would really be only done in (extreme) desperation. I've only ever installed them on DHW circuits. Thanks for your info.

 

[buryboy]

If you tee together will give more resistance

Thanks for reply. If vent and cold feed tee together would you say or agree with 22mm equal tee from of the flow circ with 22mm venting and then 22 x 22 x15 Tee, within 100mm of the equal T up the vent pipe. The 15mm tee'd from the vent pipe then conects to the f&e tank in the usually way.

Tamz agrees with you but gas man points out its against MI.

 

[tamz]

I don't think baxi actually state it is not to be done as a combined F&E on bermudas, rather they recommend a close coupled. Reference is usually made to BS5449 in which it is acceptable. It can be used for static heads down to 200mm.
Changing to a combined F&E will solve the problem. Some say they don't like this set up as if the f&e chokes the system may pressurise. Theoretically it could do that an any system.

The only way apart from doing this to cure it is to change it to a S or Y plan.

Diagram of how it is done from Potterton Housewarmer MI's albeit on a fully pumped

 

[buryboy]

I don't think baxi actually state it is not to be done as a combined F&E on bermudas, rather they recommend a close coupled. Reference is usually made to BS5449 in which it is acceptable. It can be used for static heads down to 200mm.
Changing to a combined F&E will solve the problem. Some say they don't like this set up as if the f&e chokes the system may pressurise. Theoretically it could do that an any system.

The only way apart from doing this to cure it is to change it to a S or Y plan.

Diagram of how it is done from Potterton Housewarmer MI's albeit on a fully pumped

View attachment 1241

Again thanks for that, I'll check it tomorrow, busy like all of us at times. Got to do a quote that's got to be in tomorrow thanks again. Back to this tomorrow. Cheers.