Auto Bypass Not Being Favoured when zones are closed

Outkast_IRE · 06-05-2021 09:00AM #1

Have any other plumbers come across this issue.

House is a typical S plan setup with 2 zones - Heating and Hot Water.

Open System with Header Tank in Attic. - Combined 3/4 feed & Vent pipe connecting to system on Flow Side in hotpress.

Circulating pump on Return to boiler.

3/4 inch Auto bypass fitted in the hotpress before the zone valves.

I carried out a pump replacement over the weekend an old Flowson 25-60 which was working perfectly fine on speed setting 2 - no pumping over or issues. It was replaced with the Grundfos 25-60 - bigger pump than id expect but matched the existing that was working fine for a decade.

Since the pump replacement when both zones close , heating water pushes up the combined vent and feed into the expansion tank and water is sucked out of the first radiator on the circuit .

I have tried the following -
Rebalance of the whole system followed by setting the auto bypass.

Reduce Pump Speed to 1 - system heated - Auto Bypass had to be put to lowest setting but same issue occured with water pushing up the combined feed and vent pipe.

Pump speed back to 2 - Set auto bypass a little lower than required to try and make it the favoured route when the zones close.

The auto bypass is working now but i am not sure its allowing sufficent flow as we are still getting heating water pushed up the combined feed and vent when the zones close.

Anyone else come across this issue ? There are not issues with pumping over or similar when any zone is open , its just when the zones close.

Half thinking about draining down again and changing out the auto bypass or pump. My current take on it is - that when the zones close the auto bypass is not being favoured like it should be. Instead the pump is finding it easier to pump up into the header tank and the suction side pulls from the nearest rad. Which points to the auto bypass having a far higher pressure drop than what its currently attempting to do.

jimf · 06-05-2021 01:03PM

just a question more than any help

can you fit a manual bypass with a valve to do adjusting

maybe this question will confirm im not a plumber

Outkast_IRE · 06-05-2021 04:36PM

jimf wrote: »

just a question more than any help

can you fit a manual bypass with a valve to do adjusting

maybe this question will confirm im not a plumber

It's much less efficient to have a manual open bypass to the return as the return water temperature is raised which means the boiler doesn't get the full benefit from the condensing of the flue gases . It can also lead to more cycling on / off as a higher return temp is detected and boiler shuts off thinking it's satisfied the heat.

Going to drain the system there now and take out and check the auto bypass .

K.Flyer · 06-05-2021 07:23PM

Might sound like a daft question, but did you definitely fit the pump the right way around as indicated by the directional arrow on the back of the pump?
I ask, because at the moment I cannot see how you didn't have the problem before, but now you have.

Outkast_IRE · 06-05-2021 07:57PM

K.Flyer wrote: »

Might sound like a daft question, but did you definitely fit the pump the right way around as indicated by the directional arrow on the back of the pump?
I ask, because at the moment I cannot see how you didn't have the problem before, but now you have.

Pump is correct orientation alright . I think I have the problem nailed down now .

The replacement grundfos 25-60 , whilst it has 3 speed settings setting 1 still produces 4.5 m of head across most flow rates which is very high for speed setting 1 vs comparable products. Setting 2 and 3 both produce 6.5m of head.

So in essence even on the lowest setting the pump head is too high the older pump whilst a poor brand etc . Had lower head at setting 1 & 2 .

Balls of an issue as I either need to raise the header tank or change the pump . I honestly think I will just take the hit and change the pump.

John.G · 06-05-2021 08:11PM

If it a UPS2 change the setting to (proportional setting) PP3 as this will only give a head of ~ 1.8 bar at say 0.7 m3/hr which even though on the low side may do the job for you or post the pump model and we might find amore appropriate setting, it may have CP constant pressure settings as well

Outkast_IRE · 06-05-2021 08:58PM

John.G wrote: »

If it a UPS2 change the setting to (proportional setting) PP3 as this will only give a head of ~ 1.8 bar at say 0.7 m3/hr which even though on the low side may do the job for you or post the pump model and we might find amore appropriate setting, it may have CP constant pressure settings as well

Its a Alpha 1 L 25-65. Well spotted on the "Radiator" / proportional setting it does seem like the curve on that setting starts lower on the head it might save me yet. I will have to wait and try in the morning as the baby is in bed and the system gets noisy if it pumps up into the tank.

I had wrongly assumed that the radiator / proportional setting starting point would be speed 1 and it would modulate between that and 3 itself.

Thanks for the suggestion.

https://product-selection.grundfos.com/ie/products/alpha/alpha1-l/alpha1-l-25-65-130-99199582?tab=variant-curves

John.G · 06-05-2021 09:11PM

Or the 3M CP mode if the PP setting too low.

whizbang · 07-05-2021 12:20AM

There's only 1 pipe tank to system ?

If its pushing back up to the tank, then it pulling from somewhere else; which means theres a separate fill, or an air leak into system.
Pump wont push water up into tank if there's no makeup into the system.

Or maybe there's just too much air in the system.

Outkast_IRE · 07-05-2021 09:01AM

whizbang wrote: »

There's only 1 pipe tank to system ?

If its pushing back up to the tank, then it pulling from somewhere else; which means theres a separate fill, or an air leak into system.
Pump wont push water up into tank if there's no makeup into the system.

Or maybe there's just too much air in the system.

Its a combined feed and vent - 1 pipe - Wouldnt be the way i would install it but it works ok. Its acceptable within the regs but is the preference by very few in open systems. I perfer the 1/2 inch feed and seperate 3/4 inch vent piped into the system within 150mm of each other myself. But we work with what we have got in existing systems.

In an open system if the pump head is greater than the tank height it can push water up to the tank in the attic and alot of the system will be under a negative pressurisation. If you went to vent the rads just after the zones closed it will suck air in rather than vent out.

There is an auto air vent on the system that is used for venting after filling and when the zones closed there was sufficient negative pressure on it to pull a small bit of air back in even when you manually close down the cap.

The radiator mode setting on the pump seems to have done the trick , i just need to test it out a bit more later today when im on a break from work.

Outkast_IRE · 07-05-2021 09:02AM

Any reccomendations on filters for the heating systems ?
Just wondering what one most are installing these days.

John.G · 07-05-2021 10:45AM

Extraordinary that its pumping back through a combined feed and expansion (like my own). I assume the vent carries on up over the F&E tank and that there is no water coming from this vent?

"Since the pump replacement when both zones close , heating water pushes up the combined vent and feed into the expansion tank and water is sucked out of the first radiator on the circuit ." Is this only happening while both zone valves close? and is there pump overrun fitted?

Outkast_IRE · 07-05-2021 01:37PM

John.G wrote: »

Extraordinary that its pumping back through a combined feed and expansion (like my own). I assume the vent carries on up over the F&E tank and that there is no water coming from this vent?

"Since the pump replacement when both zones close , heating water pushes up the combined vent and feed into the expansion tank and water is sucked out of the first radiator on the circuit ." Is this only happening while both zone valves close? and is there pump overrun fitted?

It's an unusual one alright . No water from the vent as I said it raises the water level in the tank from the feed pipe . Yeah the vent is over the top of the tank and bends down into it.

Yeah it only happens when both zones close you can feel the hot water travelling up the combined vent and feed.

It's a case where the head on the pump at speed 1 is just enough to lift it to the tank but the vent pipe may be slightly too high for it .

Changing the pump to radiator / PP mode seems of of done the trick , I will be keeping a close eye over the weekend .

Yeah the gas boiler overuns the pump

K.Flyer · 07-05-2021 04:27PM

Outkast_IRE wrote: »

Any reccomendations on filters for the heating systems ?
Just wondering what one most are installing these days.

I much prefer the Adey filters.
If the space is available and it's the typical semi or larger I like to fit the "Adey Pro 2", it's big enough to get plenty of inhibitor into it if you are after draining down for any reason and you want to top up the inhibitor level.
Not a fan of those little micro filters, and other generic types can be very fiddly to empty and clean and useless to use for topping up.

Auto Bypass Not Being Favoured when zones are closed

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