Vaillant internal ABV question - broken?

[Waldovr]

Hi,

First post so please forgive me where I go wrong.

Wondering if anybody has some detailed knowledge about a suspected bypass valve that is not working as I expect it to.

To elaborate: I have a Vaillant Ecotec 637 boiler, with an S-plan layout. Large 5 bed, 3 bath detached, decent insulation. 14 Radiators, 210l Cylinder. House seems to be rather slow to heat up (1 degree per hour, approx). Once warm boiler copes easily.

I was investigating why the house is rather slow to warm up and I was measuring the flow and return temperatures with the system running stable and all radiators fully open, and managed to balance the radiators so they all got approximately equal flow of hot water.

Roughly doing the maths I had the impression the boiler wasn't working as hard as it could/should be with the return being over 12 degrees colder than the flow. It seems to be happy pottering along at minimal power, but maintaining flow at the set temperature.

Menu in the boiler can report return temperature and this reports some 10+ degrees higher return temperatures than I measure on the return pipe entering the boiler.

I suspect the built-in bypass valve is never fully closing, even with all radiators fully open and hot water cylinder heating up. I would expect with all these pathways fully open for the hot water to flow easily around the system and the automatic bypass valve to be fully shut.

From cold it goes to full power for a a good few minutes but reasonably quickly then settles down to minimal to lowish power.

I've asked an engineer to have a look at the boiler and he said all was working ok, he didn't find any restricted flow etcetera. I've asked him to check with his temperature sensor and he confirmed the return before pump and bypass valve is 13 degrees lower than the return just after pump and bypass valve. (All this is inside the boiler).

This with all radiators open and circuit to hot water cylinder open too.

The engineer gave Vaillant a call to ask if increasing the ABV setting would help and they said that is hardly ever required but he could try. Increasing the setting to max (350 mBar I believe) did reduce the temperature difference measuring just before and right after ABV and Pump to about 10 degrees.

Vaillant didn't mention if this large temperature difference before and after ABV is normal or not.

Can anybody confirm this is normal or abnormal behaviour for this system?

Is the ABV supposed to let by this much flow back in to the return? This way the water returning to the heat exchanger is consistently hotter than is necessary, and all my efforts to get the return flow cool enough to maximise the boiler's efficiency seem to be in vain?

Any other suggestions what might be worth investigating?

Many thanks in advance!

Waldo

 

[D_Hailsham]

Who said you needed a 37kW boiler?

What controls do you have?

What are the actual flow and return temperatures (d.40 & d.41)?

 

[Hason]

What size is flow and return pipe work, on a 37kw I'd be expecting 28mm min reducing to suite system design etc, is it range rated via d0 ? The temp diffs between flow and return should be 20oc , the auto bypass really protects the hydrolic block but also a external bypass maybe required if system is as big as you say.
Good luck

 

[Waldovr]

Thanks for looking into my post, here's some more info:

Who said you needed a 37kW boiler?

It was in the house so don't know about sizing, but as I said it's a large 5 bed 3 bath detached. Moderately insulated. 14 (several quite large) radiators + 210 l cylinder, 37 kW might be a bit too big (from what I read in the forum) but it's not short cycling, runs at near full power when reheating the cylinder or when starting up from cold, and throttles down nicely, to about 12.5 kW minimum. Correct me if I'm wrong but as long as the boiler doesn't throw a fit when starting on full it should be able to modulate and work on lower power just as well as a smaller boiler?


It is wired through an old Honeywell wiring centre and has a 240v on/off thermostat switching the heating on/off when needed. All radiators bar bathrooms and hallways have TRV's.

What are the actual flow and return temperatures (d.40 & d.41)?

For example they report 72º flow and 65º return. But I measure return on the pipes going in and out to be return of 55º, flow 72º.

But as I mentioned the return pipe going into the boiler where I measure it is almost always 13º cooler than d.41/40 report (with sensors from inside the boiler).

This happens when running stable with a warm house, but also when working full-out when, say, re-heating a cold cylinder.

As soon as the flow temperatures goes up, so does the difference between what I measure on the return pipe and what the Vaillant reports on D41/40 return temperature. Flow is always spot on.

What would cause this mismatch in temperature? If it's sludged up I could understand the flow would be restricted, but the hot water cylinder is brand new and heats up quickly enough.


Thanks

Waldo

 

[Waldovr]

What size is flow and return pipe work, on a 37kw I'd be expecting 28mm min reducing to suite system design etc, is it range rated via d0 ? The temp diffs between flow and return should be 20oc , the auto bypass really protects the hydrolic block but also a external bypass maybe required if system is as big as you say.
Good luck

It's on big copper pipes, continuing the size that Vaillant has sticking out of the boiler - I'd estimate 25mm but will measure to be exact.
Pipes go up through the house and disappear in floorspace, so no knowing but 'assume' they stay this size pipes. I can see them going up in a cupboard, somewhere underfloor they split off for radiators (smaller pipes). Not sure where without ripping up the floor.

It wasn't ranged on d0 but engineer set it to 37 on advice by vaillant over the telephone. Can't say it made any difference; from what I understood that is to make sure the boiler doesn't go too powerful for the system and overheats when the system can't transport the heat. That never happens, engineer put it in chimney sweep mode, it went on continuous full power and engineer commented that it handled it well.

There's another (automatic) bypass in the system, set at 3.5 (Honeywell type). Might be a leftover from previous boiler. Doesn't let any water through until I go very low, say around the 1 mark, then you feel the bypass pipe getting hot whereas otherwise it's just lukewarm. Some heat coming through the copper I expect. But obviously opens up when turned lower than about 1, you can feel the bypass pipe heat up when setting low enough.

There might be other issues but I'm very curious to hear if you think the internal bypass (or any bypass for that matter) should be open at all when all radiators and hot water cylinder circuit are fully open?


Thanks


Waldo

 

[Bunnyman]

Wont make any difference, look elsewhere

 

[bernardgreen]

But as I mentioned the return pipe going into the boiler where I measure it is almost always 13º cooler than d.41/40 report (with sensors from inside the boiler).

Which could mean that there is hot water from the flow being added to the return between your sensor on the return pipe and the boiler's sensor on the return pipe inside the boiler. The most probable reason for this is that the ABV inside the boiler is letting through.

What exactly does the ABV do ? and why does it do it ?

Why does a boiler need a bypass to alllow water to circulate through the heat exchanger and pump without going through radiators and / or the coil in the hot water cylinder(s).

What causes the ABV to open ? It seems that an external ABV will open when the pressure difference between flow and return exceeds the setting on the ABV.

Perhaps an informed professional can provide the answers, It is not gas related so the " we cannot give gas advice" escape clause does not apply.

 

[MrTherm]

@ Waldovr, your are right to suspect the bypass setting.

Re your quote:- "There's another (automatic) bypass in the system"

You cannot have two valves as they will counteract each other.

Disable, isolate or remove the existing system bypass valve and let the boiler function on its own controls.

Your boiler guy should have known this.

 

[Waldovr]

@MrTherm - for my understanding, how would they counteract? If one is on a higher setting wouldn't it just not open as the other one opens first? Or would one open, closing the other, and then close again, opening the other or something like that?

And if there are no isolation valves, would just setting it to as high as possible be enough to disable?

Thanks

Waldo

 

[Waldovr]

Which could mean that there is hot water from the flow being added to the return between your sensor on the return pipe and the boiler's sensor on the return pipe inside the boiler. The most probable reason for this is that the ABV inside the boiler is letting through.

What exactly does the ABV do ? and why does it do it ?

Agree with the impression that the ABV is letting hot water through to the return flow. From what I understand, the system needs an automatic bypass valve in order for the water to be able to flow around and cool down the heat exchanger if

a) TRV's are closing down most of the radiators so the boiler is just heating the little bit of hot water in the pipes

or

b) Thermostat in hallway switches boiler off and the heat exchanger is very hot and needs to cool down or the hot gasses still in there might damage the heat exchanger. So the pump runs on a bit but the 2-way valves have closed and the water would have nowhere to go.


So as far as I understand, the ABV is supposed to open, but only in these situations, not when the system is simply heating the radiators.

My main question is: if the ABV lets water through in normal operation (all radiators even fully open) would that be normal behaviour, and if not, what would be the cause?

 

[Waldovr]

Wont make any difference, look elsewhere

@Bunnyman you mean don't bother about the ABV?

It looks like the boiler is not pushing enough hot water around the radiators and is mostly idling at minimum even though house still cool. Radiators do get hot after some time, and eventually I get a return around 55 as I mentioned. No cold spots, bled radiators, lock-shields checked, radiators do get hot.

Any suggestions where else I could look?


Thanks

Waldo

 

[bernardgreen]

My main question is: if the ABV lets water through in normal operation (all radiators even fully open) would that be normal behaviour, and if not, what would be the cause?

The cause would be that the ABV opens at a pressure differential that is much lower than the pressure differential created by the pump when pumping into a closed off circuit ( no flow to radiators or coils ). It should open at a differential that is just lower than teh differential created by a dead ended pump and not before.

Simple.... Only with modulating pumps and pumps whose output pressure will vary with water temperature it is not a simple task to get the ABV to open when it should and to remain closed when it should be closed.

Maybe using a zone valve as a bypass valve that is opened electrically when the boiler's heat exchanger needs to be cooled would remove all the un-certainty about when the bypass is opened and closed.

Also having the (automatic) bypass valve some pipe distance from the boiler would circulate a larger volume of water to cool the heat exchanger and the heat loss from the pipe work external to the boiler would be more effective in cooling the heat exchanger.

 

[Agile]

Most boiler makers, where there is an internal ABV, say that it is only there to protect the boiler and an external one is required as well.

Vaillant say that their's is all that is needed.

I don't see that any other ABV in the system will do any harm as long as it is set high enough to not pass under normal operation.

I don't know what setting your internal ABV is at and because it is a room sealed case will not be advising you to touch it. Or giving you any detailed advice about it's setting.

It does seem to be passing somewhat and could well need further adjustment.

Tony

 

[Waldovr]

@Agile, agree it's for the engineers as it's inside the sealed case. I've had an engineer and he adjusted the ABV to maximum (350 mBar from 250 mBar) and though it reduced how much the ABV let through the return after the ABV/Pump was still about 10 degrees hotter than the return temperature before ABV/Pump (where it was 13/14 on default settings of 250).

So it was still passing a fair amount of hot water.

My puzzle remains: Is the ABV broken? Or are there other reasons the ABV is letting hot water through, and if so, how to investigate?

 

[AGAS]

well change the bl@@dy ABV then , crack on