Auto bypass always open - help!

[Chris Clarke]

Hoping that someone might be able to help me.

I have a sealed heating system comprising 15 radiators and hot water cylinder. In the last 18 months all radiators were replaced and plumbed using 10mm microbore pipe back to manifolds on 22mm flow and return pipes. The system is S plan with two heating zones and hot water.

The problem I am having is that the auto bypass lets hot water through in any combination. When both upstairs and downstairs heating is on the boiler stays on and good heat is achieved, although bypassing. When upstairs is on in isolation the bypass is open and the boiler starts short cycling and also does the same with hot water only.

Initially I was concerned that the microbore pipe was causing too much resistance of flow which was resulting in the auto bypass opening. However, the bypass also opens for hot water only which is not in small bore pipe so makes me think it is not this.

I have had the auto bypass replaced, so it is not a faulty valve.

The pump was re-used from the old system and is a 15/50, I wonder if this is not man enough to overcome the resistance of the new small diameter pipes? But this would not explain the bypass being open for hot water only?

My other line of thinking is that the heat exchanger may be clogged up causing a restriction, we are in a hard water area and had a period where the boiler was kettling last year.

Overall the heating and hot water work, but not as well or efficiently as I believe they could. Really I'd like to find a heating engineer in Norfolk who really knows their stuff, a couple of people local to me have been round and I've been advised to change the boiler which I just don't believe.

 

[Rob Foster]

Hoping that someone might be able to help me.

I have a sealed heating system comprising 15 radiators and hot water cylinder. In the last 18 months all radiators were replaced and plumbed using 10mm microbore pipe back to manifolds on 22mm flow and return pipes. The system is S plan with two heating zones and hot water.

The problem I am having is that the auto bypass lets hot water through in any combination. When both upstairs and downstairs heating is on the boiler stays on and good heat is achieved, although bypassing. When upstairs is on in isolation the bypass is open and the boiler starts short cycling and also does the same with hot water only.

Initially I was concerned that the microbore pipe was causing too much resistance of flow which was resulting in the auto bypass opening. However, the bypass also opens for hot water only which is not in small bore pipe so makes me think it is not this.

I have had the auto bypass replaced, so it is not a faulty valve.

The pump was re-used from the old system and is a 15/50, I wonder if this is not man enough to overcome the resistance of the new small diameter pipes? But this would not explain the bypass being open for hot water only?

My other line of thinking is that the heat exchanger may be clogged up causing a restriction, we are in a hard water area and had a period where the boiler was kettling last year.

Overall the heating and hot water work, but not as well or efficiently as I believe they could. Really I'd like to find a heating engineer in Norfolk who really knows their stuff, a couple of people local to me have been round and I've been advised to change the boiler which I just don't believe.

Can you produce a drawing please then we can all have a go,at this just attach it
Rob Foster aka centralheatking

 

[Chris Clarke]

No problem, will do this later.

I should also point out that the auto bypass valve is adjusted to a high setting.

 

[Ben Slade]

Hi Chris, can you set this bypass valve or do they come pre set? I have had a look online and some people have had to adjust theirs. I could understand if it opened for the mass amount of micro bore but the hot water is puzzling.

I am not a plumber just a student at the minute so I don’t want to offer any advice but I am interested in your problem. Best of luck.
Ben

 

[Ben Slade]

Sorry mate you posted just as I wrote that out!

 

[Best]

Pump is wrong type.
But you also need your bypass valve adjusted to only open when system is operating with one radiator on, or when pump overrun is on.
Your heating installer should have done all that

 

[Sharp Point]

If my memory serves me correctly, the bypass should be set up for the index circuit ( furthest radiator or the circuit with most resistance) this would have to be calculated to find the flow rate to set the bypass.

 

[Chris Clarke]

Pump is wrong type.
But you also need your bypass valve adjusted to only open when system is operating with one radiator on, or when pump overrun is on.
Your heating installer should have done all that

Could you expand on the pump being the wrong type please?

 

[Best]

Could you expand on the pump being the wrong type please?

Use a 6m pump for your system.

 

[Chris Clarke]

Use a 6m pump for your system.

Ok thanks, I contacted Grundfos and they recommended even going to a 8m pump.

One thing I don't understand is the pumps marked 15 50/60 that are ok for 5m or 6m systems. Surely it can't be both?

 

[Best]

Ok thanks, I contacted Grundfos and they recommended even going to a 8m pump.

One thing I don't understand is the pumps marked 15 50/60 that are ok for 5m or 6m systems. Surely it can't be both?

A hot water cylinder and then 15 radiators on 2 zones supplied from
10mm coming from 22mm and Grundfos recommend 8m pump?
What is the total kw needed?

A 6m pump will do approx 35 kw.
Normally no bother on 10mm pipes if they are piped correctly and runs are not long.

The fact that you have 22mm pipes suggests your heat requirements are fairly modest.

I would guess you only need a 6m pump and also make sure the bypass is not bypassing when most of the system is supposed to be open flow.

Your installer has clearly not fitted correct pump and should have warned you

 

[Chris Clarke]

A hot water cylinder and then 15 radiators on 2 zones supplied from
10mm coming from 22mm and Grundfos recommend 8m pump?
What is the total kw needed?

A 6m pump will do approx 35 kw.
Normally no bother on 10mm pipes if they are piped correctly and runs are not long.

The fact that you have 22mm pipes suggests your heat requirements are fairly modest.

I would guess you only need a 6m pump and also make sure the bypass is not bypassing when most of the system is supposed to be open flow.

Your installer has clearly not fitted correct pump and should have warned you

I'll get back to you on the KW but it is less than 35.

All the manifolds are located in the upstairs hallway so a lot of the 10mm runs for downstairs go across a room and then down and back again. The longest run is probably 8 or 9 m each way. But most are around 5m downstairs. Interestingly the downstairs radiators heat up better than upstairs.

Grundfos said the pump I mentioned is their recommendation for an average 4 bed!

So could an under sized pump cause the bypass to open?

I have a heating engineer coming next week but would be good to point him in the right direction.

 

[Gasmk1]

photos would be more help to us ofthe bypass and the pump set up

 

[Chris Clarke]

Here you go, this is all located about 2m from the boiler.

 

[Bogart]

Might just be me but I like intelligent pumps like the Grundfos Alpha.

 

[Gasmk1]

thanks any chance of some close ups of the bypass top and side views

 

[Chris Clarke]

Will get some more photos for you later. I've (perhaps stupidly) adjusted the bypass valve and it still passes on the highest setting.

It would definitely appear that my pump is undersized, if it cannot overcome the frictional head of the pipework could this cause the bypass to open?

Some more details for you all.

• Pipework from boiler is 28mm reducing to 22mm at zone valves.
• Boiler is a British Gas 330 which I believe has a maximum output of 31kw.
• Downstairs radiators total 28560 BTU which I think is 8.4 kWh?
• Upstairs radiators total 17445 BTU which I think is 5.1 kWh?
• Zone valves, expansion tank etc is located in the garage at about the same level as the first floor.
• Manifolds are located within the first floor hallway. So downstairs radiators have greatest runs of 10mm, up to 9m each way, probably averaging about 5m each way.
• Upstairs has shorter runs, probably ranging from 1m to 4m each way.
• Downstairs radiators seem to heat up quite nicely despite the longer runs.
• Upstairs radiators don't heat up particularly well due to boiler cycling if upstairs zone is on only. Also radiators with short runs have to be throttled right back to get heat to the radiators with longer runs.

 

[Best]

Are you turning the bypass towards closed position in clockwise direction?

A minor point, but the head of the pump should have been turned 180 degrees to keep the electrics on top. Your installer probably didn’t do that because he didn’t want to disturb the seal in a second hand pump, to be fair to him.

 

[Chris Clarke]

Are you turning the bypass towards closed position in clockwise direction?

A minor point, but the head of the pump should have been turned 180 degrees to keep the electrics on top. Your installer probably didn’t do that because he didn’t want to disturb the seal in a second hand pump, to be fair to him.

Yes clockwise.

I was starting to think that an upgraded pump might solve my problem but that wouldn't explain why its bypassing on hot water only? I would have thought this circuit has a very low resistance?

 

[Chris Clarke]

Could the output of the boiler affect things? IE if it is adjusted to put out the full 31kw? I believe its maximum output can be limited.

 

[doitmyself]

I can't help wondering if the system has been balanced for the correct temperature differential.

The British Gas 330 boiler has a maximum output of 30kW but is set, at the factory, to give 18kW. As there are only 13.5kW of rads, I would assume the output has not been changed. The boiler is designed to have a flow-return differential of about 20°C. With a 20°C drop the head loss through the boiler, at 18kW, is about 0.9m. This would leave about 3m available from the 15-50 on speed 3 for the head loss in the radiator circuit.

If the system has been balanced for a differential of about 10°C, the flow rate will double and the boiler head loss alone will be about 3.6m (four times greater), which is more than a 15-50 can deliver. This would not someone trying to balance for an 10°C, even if they could only achieve something higher.

 

[doitmyself]

Could the output of the boiler affect things? IE if it is adjusted to put out the full 31kw? I believe its maximum output can be limited.

If it has, it should be indicated on the data label attached to the inner door.

 

[Chris Clarke]

I can't help wondering if the system has been balanced for the correct temperature differential.

The British Gas 330 boiler has a maximum output of 30kW but is set, at the factory, to give 18kW. As there are only 13.5kW of rads, I would assume the output has not been changed. The boiler is designed to have a flow-return differential of about 20°C. With a 20°C drop the head loss through the boiler, at 18kW, is about 0.9m. This would leave about 3m available from the 15-50 on speed 3 for the head loss in the radiator circuit.

If the system has been balanced for a differential of about 10°C, the flow rate will double and the boiler head loss alone will be about 3.6m (four times greater), which is more than a 15-50 can deliver. This would not someone trying to balance for an 10°C, even if they could only achieve something higher.

Thanks, I don't really know what you mean by your last paragraph? What differential should be looked for at the radiators to achieve 20C at the boiler?

I know that the system was not balanced following the change of radiators / pipework. From everyone's input I'm thinking I should ask for the following to be investigated.

1. Change pump to a higher rated one, probably 6m.
2. Check that boiler output has not been adjusted from 18kw, if it has get it changed back.
3. Carry out full balance of system.
4. Re-set bypass valve, change if required.

 

[doitmyself]

Thanks, I don't really know what you mean by your last paragraph? What differential should be looked for at the radiators to achieve 20C at the boiler?

I'm sorry for confusing you. The last sentence should read:

"This would not stop someone trying to balance for an 10°C, even if they could only achieve something higher."

In the past, before condensing boilers, the standard was to balance the system - that's boiler and rads - for an 11°C (actually 20°F, which shows how old it is) differential. This has changed to 20°C for two reasons: to reduce the head loss due to friction, particularly due to the heat exchanger which tend to have very narrow pathways; and so the return temperature is closer to the dew-point of natural gas (55°C), which is the temperature at which condensing begins (It increases as return temperature drops.).

A condensing boiler should be balanced for a drop, across boiler and rads, as close to 20°C as possible, but not any higher. You may not be able to achieve 20°C due to the way the pump interacts with the system.

You may find that balancing the system will remove the need to replace the pump. There is over 3m of head available for the index circuit.

 

[doitmyself]

I based the info above on the boiler's output of 18kW. You have only 13.5kW of rads so the boiler will modulate to that output. This means a flow rate of 580 litre/hr giving a heat exchanger head of 0.5m and an available pump head of 3.5m at speed 3. This should be more than enough.

 

[Chris Clarke]

Thanks, makes sense to me. Although is 3.5m head sufficient for the longest microbore to manifold circuit of 18m? My rough calculations would suggest not.

And it seems like that was too long in the first place but that's a different story.

 

[Ric2013]

Too late for me to read all this. Basically, a pump that is too weak for the job will NOT be opeing the bypass. My experience with differential pressure auto bypasses is that they can very easily be jammed very slightly open with a bit of crud from the system and then they let by, even when the head is not sufficient to overcome the spring pressure.

If, as I suspect, your system is old and has a lining of black stuff on the pipe bores, I wonder if a bit of this could have got in between the valve seat and seal on your ABV when it was last forced to open and this has lodged there. I once tried an experiment in my own house with an ABV going through a bypass radiator, but, after the valve got jammed slightly open a second time in the same number of weeks, I gave it up.

 

[chris watkins]

I know that the system was not balanced following the change of radiators / pipework. From everyone's input I'm thinking I should ask for the following to be investigated.

1. Change pump to a higher rated one, probably 6m.
2. Check that boiler output has not been adjusted from 18kw, if it has get it changed back.
3. Carry out full balance of system.
4. Re-set bypass valve, change if required.

Changing the pump to a 6m head one will only make the by-pass opening worse however you my need a higher head pump to over come the resistants of the micro bore system, so what is to be done.

First thing to understand is why we need a by-pass in the first place.
The main reason (& in this case) is to maintain a minimum flow rate through the boiler at all times, this allows the boiler controls to adjust the burn of fuel to try to achieve a 20deg C difference between the flow & returning water. The system is pressure dynamic, that is to say zone & TRV valves are opening & closing all the time so the work the pump has to do keeps changing. With all the valves open the pump needs move a lot of water (heat) but with only enough force to overcome the resistance in the whole circuit. As the valves close the flow rate around the circuits will decrease but the pressure generated by the pump will rise (they are directly related, see pump graphs).
You have fixed speed pump installed so the pressure will climb, this rise in pressure was used to open a auto by-pass valve at the point were the minimum flow rate required by the boiler manufacturers was not going to be achieved. This sudden opening between the flow & return should send hot water straight back to the boiler raising the return temperature which would cause the boiler to turn off on the boiler stat.
What are the options then?

 

[Chris Clarke]

Changing the pump to a 6m head one will only make the by-pass opening worse however you my need a higher head pump to over come the resistants of the micro bore system, so what is to be done.

First thing to understand is why we need a by-pass in the first place.
The main reason (& in this case) is to maintain a minimum flow rate through the boiler at all times, this allows the boiler controls to adjust the burn of fuel to try to achieve a 20deg C difference between the flow & returning water. The system is pressure dynamic, that is to say zone & TRV valves are opening & closing all the time so the work the pump has to do keeps changing. With all the valves open the pump needs move a lot of water (heat) but with only enough force to overcome the resistance in the whole circuit. As the valves close the flow rate around the circuits will decrease but the pressure generated by the pump will rise (they are directly related, see pump graphs).
You have fixed speed pump installed so the pressure will climb, this rise in pressure was used to open a auto by-pass valve at the point were the minimum flow rate required by the boiler manufacturers was not going to be achieved. This sudden opening between the flow & return should send hot water straight back to the boiler raising the return temperature which would cause the boiler to turn off on the boiler stat.
What are the options then?

Fit a Grundfos pump with proportional pressure function? God knows, that's why I'm asking! Someone else must have come across this before?

 

[Chris Clarke]

Low loss header?

 

[chris watkins]

If I am looking at your boilers M.Is then 4.3 - "a variable duty pump should be set to give a temp diff of no greater than 20deg C"
"Important F&R pipe sizes - up to 18kw = 22mm, >18kw =28mm"
The flow rate required to provide 18kw @ 20 degC would main that your pump would loose approx 1M head just pushing that amount of water through the heat exchanger if you needed it full 30kw you would loose over 2.5Metres.

Your existing pump is a 5M head but that is the max it can push with no or little flow. Moving approx 0.215L/S it would only develop a pressure of approx 3.7M head, less the boiler 1M, would leave approx 2.4 M for the rest of the system. Not likely to be enough to overcome the resistance you have put in by re-piping in 10mm.

 

[Chris Clarke]

Yes so pump size needs to go up but this will play further havoc with the auto bypass? Although it does say variable duty?

 

[chris watkins]

Fit a Grundfos pump with proportional pressure function? God knows, that's why I'm asking! Someone else must have come across this before?

Yes Chris thats why they were invented.
If you fit one would you need a by-pass ??
I don't think you do & have had my one at home closed for couple of years now with an Alpha 2 installed.
But if you do want to provide the minimum flow then why not use close coupled tees instead of LLH (to expensive) to provide hydraulic seperation. You are going to buy a new pump anyway so give it a go!!

 

[Chris Clarke]

Don't you need the bypass to prevent the pump getting dead headed on the pump overrun?

 

[chris watkins]

Effectively that is what your by-pass is providing at the moment you just don't have a pump on the other side of it to push water around the heat emitters.