Automatic Bypass Valve Query

[Harvest Fields]

Air? Valve not quite open enough?

 

[Chuck]

Hi folks me again. Something is not right since the bypass has been fitted. Nearly every night at 6.40pm there a crackling /gushing water sound through the lounge radiator valve. This rad is nearest to the boiler. I have the hot water cylinder programmed on my evohome system to turn off at 6.40pm so I think it’s linked to this. Any ideas why this is happening?

At 1839 the boiler is blasting away and the pump is pumping lots of hot water around the DHW loop though the cylinder.

At 1840 and 0 seconds the HW zone valve slams shut but the pump and boiler don't instantly respond so the water flows (fast) round the path of least resistance, which is either the radiator circuit or the ABV depending on how it is set.

By 1840 and 30 seconds the boiler power has been cut back and the pump has adapted to the new conditions and order is restored again.

The problem with ABVs is that they detect and act on pressure differences, which are only indirectly related to the flow rate in a system with TRV's and/or a smart pump. ABVs work pretty well with 'dumb' (constant displacement rate) pumps but not so well with 'smart' (constant pressure) pumps.

To further complicate the story, the purpose of an external ABV has changed over the years. Originally it was an energy-saving refinement to a traditional 'bypass radiator' that was always on ensure a minimum flow rate through the boiler. These days this is not usually needed but ABVs are often still used to to ensure that there is sufficient heat capacity in the circulating circulating water to avoid heat-stress damaging the heat exchanger. Some boilers, typically ones with robust stainless steel heat exchangers don't require an external ABV. Others, particularly ones with aluminium heat exchangers and low water volumes, require not just an ABV but also a minimum loop volume connecting it to the boiler.

As always, the installer needs to read the manufacturer's installation instructions carefully and follow them to the letter.

 

[Tigermad]

At 1839 the boiler is blasting away and the pump is pumping lots of hot water around the DHW loop though the cylinder.
At 1840 and 0 seconds the HW zone valve slams shut but the pump and boiler don't instantly respond so the water flows (fast) round the path of least resistance, which is either the radiator circuit or the ABV depending on how it is set.

The problem with ABVs is that they detect and act on pressure differences which are only indirectly related to the flow rate. They work pretty well with 'dumb' (constant displacement rate) pumps but not so well with 'smart' (constant pressure) pumps.

To further complicate the story, the purpose of an external ABV has changed over the years. Originally it was to ensure a minimum flow rate throuh the boiler but these days it is often to to ensure that there is sufficient heat capacity in the circulating circulating water to avoid heat-stress damaging the heat exchanger. Some boilers, typically ones with robust stainless steel heat exchangers don't require an external ABV. Others, particularly ones with aluminium heat exchangers and low water volumes, require not just an ABV but also a minimum loop volume connecting it to the boiler.

As always, the installer needs to read the manufacturer's installation instructions carefully and follow them to the letter.

Thanks. The radiator noise was better before the bypass was fitted. Can I disable it by fully opening it?

 

[Tigermad]

Air? Valve not quite open enough?

No air in the system because I have tried to bleed all rads a few times now. Although the towel rail upstairs needs constant bleeding and i guess it’s the last rad on the circuit and something weird happens with it. But this thing with the towel rail was happening before the bypass was fitted.

Do you mean the valve on the rad or bypass? The noise is mainly on the left hand side of the radiator as the water goes in.

 

[John.g]

Thanks. The radiator noise was better before the bypass was fitted. Can I disable it by fully opening it?

You disable it by fully closing it, did you ever establish if it is in fact fitted correctly by opening it fully while the boiler is firing normally, you should then feel both flow and return pipes very hot.

 

[Harvest Fields]

Yes on the rad, also you may have mentioned this already but what setting is the pump on?

 

[Tigermad]

You disable it by fully closing it, did you ever establish if it is in fact fitted correctly by opening it fully while the boiler is firing normally, you should then feel both flow and return pipes very hot.

Yes. The pipes eventually got hot. Think I was doing something wrong when checking it. So if I completely close it I can see if the noise gets better without it?

 

[Tigermad]

Yes on the rad, also you may have mentioned this already but what setting is the pump on?

I have tried opening the lock shield valve a bit and closing it but made no difference whilst the noise was happening. The pump is built in to the boiler and it can run on either 70 percent or 100 percent. It’s on the default of 70 percent.

 

[Chuck]

Thanks. The radiator noise was better before the bypass was fitted. Can I disable it by fully opening it?

No. Don't "fully open" it. It should be set to operate at the highest pressure difference that allows it to achieve its intended purpose.

If you are determined to play around open it by small amounts, say quarter turns until it reduces the 1840 noise. But, if you open it enough to completely eliminate the 1840 noise you'll probably have gone too far and have replaced one problem with another.

 

[Tigermad]

No. Don't "fully open" it. It should be set to operate at the highest pressure difference that allows it to achieve its intended purpose.

If you are determined to play around open it by small amounts, say quarter turns until it reduces the 1840 noise. But, if you open it enough to completely eliminate the 1840 noise you'll probably have gone too far and have replaced one problem with another.

Thanks. Sorry I meant fully close it to disable it completely.