Boiler bypass always opening

[robbyc0]

Hi all - hopefully I can get some answer to a maddening issue. The boiler bypass just refuses to stop letting water through.

Here is the run down. We bought the house last year. It has a Vaillant Ecotec Plus 418 which is only 3 years old. It's on a Y plan, with a hot water cylinder that looks to have been installed at the same time as the boiler - it's very new and a sealed foam type.

When we moved in we had new rads all round and all the microbore removed. The system was powerflushed thoroughly.

The 3 way valve was replaced as it had seized so that's new and works fine.

The pump is a Grundfos Alpha2 15-60 130.

The hot water tank thermostat was not working so was replaced (and works fine).

The whole heating system works well but it's very slow to heat the hot water tank - sometimes over 2 hours, and the boiler keeps going into S46 - Comfort protection, loss of flame.

There is a 15mm bypass from the exit of the pump to the return which had a tacosetter on it set to 11l/h (presumably because the boiler min flow is 10.5).

This has now been replaced with a straight auto bypass which ranges from 0.1 to 0.5 bar.

The ch system has 2 towel rails on it which are always open, and the pump is level with the highest rad. It's a 2 story house, open system with a tank in the roof about 3m above the pump.

Even with the hot water tank calling for heat and the ch circuit on (3 way valve on 'mix') the bypass is still open. It's a brand new valve. This occurs whether the pump is ramped all the way up to 45w or tricking along at 11w. On pp1, or pp2, or even auto (which gives the lowest power) of fixed at I, II or II it's the same.

It makes no sense. Even with ch and hw calling for heat, a good few trvs open and the 2 towel rails, and the pump on its minimum setting, water is still getting though the bypass even if it's ramped all the way up to 0.5 bar.

I have tried running the pump at 45w and shutting off the stopcocks to the ch and hw tank to force water though the bypass at a good rate in case it was blocked by something, but no solution.

Any ideas?

 

[robbyc0]

By the way if it's been on hot water only (roasting hot bypass pipe) and I boost the heating the pipe then goes cold immediately because the cold water from the heating system starts going around the system and straight through the bypass valve as well. This even happens with the pump on its lowest setting - I

 

[John.g]

What is the ABV (index) set to to?? If its set to max of 5 then the Alpha2 on CC1 or CC2, 1.0M & 3.0M or PP1 or PP2 at 2.0M & 3.0M cannot cause it to bypass, it may have some crap under the valve seat.
What make/model is the ABV.?

 

[robbyc0]

What is the ABV (index) set to to?? If its set to max of 5 then the Alpha2 on CC1 or CC2, 1.0M & 3.0M or PP1 or PP2 at 2.0M & 3.0M cannot cause it to bypass, it may have some crap under the valve seat.
What make/model is the ABV.?

View attachment 96036

Thanks so much for coming back - great forum! However, I have solved the issue. It was driving me so mad I drained a little off the system, split the bypass and put in an isolation valve. When I ran it all up again the whole bypass pipe got hot EXCEPT the short length between the ABV and the isolator. Heat must have been passing up the pipes from the feed and the return sides and heating the pipes. By adding the isolator I created a situation which PROVED the ABV was closed, otherwise that short section would be hot as well. As soon as I opened the isolator the short middle bit got hot - the only explanation being it is heat being carried from the return end. At least I know now!

 

[RAJAYES]

Hi all - hopefully I can get some answer to a maddening issue. The boiler bypass just refuses to stop letting water through.

Here is the run down. We bought the house last year. It has a Vaillant Ecotec Plus 418 which is only 3 years old. It's on a Y plan, with a hot water cylinder that looks to have been installed at the same time as the boiler - it's very new and a sealed foam type.

When we moved in we had new rads all round and all the microbore removed. The system was powerflushed thoroughly.

The 3 way valve was replaced as it had seized so that's new and works fine.

The pump is a Grundfos Alpha2 15-60 130.

The hot water tank thermostat was not working so was replaced (and works fine).

The whole heating system works well but it's very slow to heat the hot water tank - sometimes over 2 hours, and the boiler keeps going into S46 - Comfort protection, loss of flame.

There is a 15mm bypass from the exit of the pump to the return which had a tacosetter on it set to 11l/h (presumably because the boiler min flow is 10.5).

This has now been replaced with a straight auto bypass which ranges from 0.1 to 0.5 bar.

The ch system has 2 towel rails on it which are always open, and the pump is level with the highest rad. It's a 2 story house, open system with a tank in the roof about 3m above the pump.

Even with the hot water tank calling for heat and the ch circuit on (3 way valve on 'mix') the bypass is still open. It's a brand new valve. This occurs whether the pump is ramped all the way up to 45w or tricking along at 11w. On pp1, or pp2, or even auto (which gives the lowest power) of fixed at I, II or II it's the same.

It makes no sense. Even with ch and hw calling for heat, a good few trvs open and the 2 towel rails, and the pump on its minimum setting, water is still getting though the bypass even if it's ramped all the way up to 0.5 bar.

I have tried running the pump at 45w and shutting off the stopcocks to the ch and hw tank to force water though the bypass at a good rate in case it was blocked by something, but no solution.

Any ideas?

I don't think this is a by-pass problem. Unless every radiator/towel rail is fitted with a TRV, a by-pass valve is not normally required as the Y Plan valve can only divert the pump flow, not shut it off. You can confirm this by temporarily isolating the bypass and seeing if the hot water recovery rate improves. I don't think it will.

If the heating works ok, then there is no problem with the pump.

The problem is that the boiler is probably going to "S46 - Comfort protection" because the hot water primary circuit is unable to dissipate enough heat into the cylinder. The boiler is effectively switching itself off. There are a number of reasons for this:

1) There is a blockage (air or sludge, etc) severely restricting the flow. (Check/fit air vents and/or blast sludge from circuit, including cylinder coils.)
2) The hot water primary regulating valve (if fitted) is practically closed. (Check and make sure it is sufficiently open.)
3) The coil inside the cylinder is badly coated with lime scale preventing an efficient heat transfer. (Try to visually confirm through the immersion heater boss and if necessary, replace hot water cylinder.)

Hope that helps.

 

[RAJAYES]

I don't think this is a by-pass problem. Unless every radiator/towel rail is fitted with a TRV, a by-pass valve is not normally required as the Y Plan valve can only divert the pump flow, not shut it off. You can confirm this by temporarily isolating the bypass and seeing if the hot water recovery rate improves. I don't think it will.

If the heating works ok, then there is no problem with the pump.

The problem is that the boiler is probably going to "S46 - Comfort protection" because the hot water primary circuit is unable to dissipate enough heat into the cylinder. The boiler is effectively switching itself off. There are a number of reasons for this:

1) There is a blockage (air or sludge, etc) severely restricting the flow. (Check/fit air vents and/or blast sludge from circuit, including cylinder coils.)
2) The hot water primary regulating valve (if fitted) is practically closed. (Check and make sure it is sufficiently open.)
3) The coil inside the cylinder is badly coated with lime scale preventing an efficient heat transfer. (Try to visually confirm through the immersion heater boss and if necessary, replace hot water cylinder.)

Hope that helps.

 

[robbyc0]

Cheers for this. Tank coil has a bleed valve so it's not that. Could be limescale, though it's not old. Hard water area though. It was all power flushed but whether he did the tank circuit or not i don't know. I know it's not the ABV though if you look at my other reply. I don't know if there is a primary regulator valve...

 

[John.g]

Thanks so much for coming back - great forum! However, I have solved the issue. It was driving me so mad I drained a little off the system, split the bypass and put in an isolation valve. When I ran it all up again the whole bypass pipe got hot EXCEPT the short length between the ABV and the isolator. Heat must have been passing up the pipes from the feed and the return sides and heating the pipes. By adding the isolator I created a situation which PROVED the ABV was closed, otherwise that short section would be hot as well. As soon as I opened the isolator the short middle bit got hot - the only explanation being it is heat being carried from the return end. At least I know now!

Can you just draw a simple schematic including that ABV and the isolating valve you have installed.

 

[robbyc0]

Can you just draw a simple schematic including that ABV and the isolating valve you have installed.

Easier than that - I had a pic. Anyway I am, solved. One question given you clearly have a good understanding, now it's summer and it's pretty much hot water only for the next few months, would you go pp1 or pp2 given my setup?

 

[robbyc0]

Can you just draw a simple schematic including that ABV and the isolating valve you have installed.

Easier than that - I had a pic. Anyway I am, solved. One question given you clearly have a good understanding, now it's summer and it's pretty much hot water only for the next few months, would you go pp1 or pp2 given my setup? PS That Isolator is open now - it was only closed while it helped me diagnose the issue. Not that it should make any difference as I always have 2 towel rails open.

 

[John.g]

Easier than that - I had a pic. Anyway I am, solved. One question given you clearly have a good understanding, now it's summer and it's pretty much hot water only for the next few months, would you go pp1 or pp2 given my setup? PS That Isolator is open now - it was only closed while it helped me diagnose the issue. Not that it should make any difference as I always have 2 towel rails open.

What was the issue?? and what is the ABV setting on its index?

That pump should show the power watts, W, with HW only requested and calling for heat, note whatever mode/setting its on just now and the power in watts, W, then change the setting to CP1 (constant pressure) which is/should be a 3M head and again note the power in watts, W.

 

[robbyc0]

What was the issue?? and what is the ABV setting on its index?

That pump should show the power watts, W, with HW only requested and calling for heat, note whatever mode/setting its on just now and the power in watts, W, then change the setting to CP1 (constant pressure) which is/should be a 3M head and again note the power in watts, W.

I suspect there may be some scaling on the HW tank coil, which accounts for the boiler going into S46. It's not dissipating enough heat which Vaillant have confirmed. Grundfos recommend one of the 2 proportional pressure settings (higher or lower) for my setup as the CP ones are designed for underfloor heating. On PP1 it drops right down to about 5W which I believe is ideal to let the tank draw as much heat as poss from the coil. It's no good in winter as takes too long to get round all the rads - but S46 is not an issue in winter anyway as there is plenty of temp difference at the boiler. I think I'll go with PP1 with a target flow of 75 degrees in summer and PP2 in winter with a TF of 65 - and have the HW on all the time the heating is.

 

[John.g]

I suspect there may be some scaling on the HW tank coil, which accounts for the boiler going into S46. It's not dissipating enough heat which Vaillant have confirmed. Grundfos recommend one of the 2 proportional pressure settings (higher or lower) for my setup as the CP ones are designed for underfloor heating. On PP1 it drops right down to about 5W which I believe is ideal to let the tank draw as much heat as poss from the coil. It's no good in winter as takes too long to get round all the rads - but S46 is not an issue in winter anyway as there is plenty of temp difference at the boiler. I think I'll go with PP1 with a target flow of 75 degrees in summer and PP2 in winter with a TF of 65 - and have the HW on all the time the heating is.

5W on PP1 means that there is practically no flow through the boiler or the coil, its no wonder the boiler is bringing up s46.
Also forget about this frankly untrue idea that a low flow will draw as much heat as possible from the coil, it will draw the MINIMUM amount of heat, the higher the coil flowrate, the lower the coil dT so the average or mean coil temperature will be greater and will emit more heat and help to keep the boiler running constantly if the coil can emit > 3.8kW which is the minimum (418) boiler output.
So, as suggested, set the pump mode to CP1, note the power in watts, W, we can then derive the flowrate from the pump curves, after 5 minutes or so note parameters D.40 & D.41, (flow&return temps) we can then calculate exactly how much heat the coil is absorbing/boiler output, and probably suggest more optimum settings.

 

[robbyc0]

5W on PP1 means that there is practically no flow through the boiler or the coil, its no wonder the boiler is bringing up s46.
Also forget about this frankly untrue idea that a low flow will draw as much heat as possible from the coil, it will draw the MINIMUM amount of heat, the higher the coil flowrate, the lower the coil dT so the average or mean coil temperature will be greater and will emit more heat and help to keep the boiler running constantly if the coil can emit > 3.8kW which is the minimum (418) boiler output.
So, as suggested, set the pump mode to CP1, note the power in watts, W, we can then derive the flowrate from the pump curves, after 5 minutes or so note parameters D.40 & D.41, (flow&return temps) we can then calculate exactly how much heat the coil is absorbing/boiler output, and probably suggest more optimum settings.

OK cool, thanks. CP1 it is and I will have a look at the performance when the hot water next fires

 

[John.g]

OK cool, thanks. CP1 it is and I will have a look at the performance when the hot water next fires

Also see if the pump is like this (45W) one.

 

[robbyc0]

Also see if the pump is like this (45W) one.

Yep that's it. Here is mine

 

[John.g]

OK, the 4 LEDs on the right are respectively, PP1,PP2,CP1&CP2.
The pump is currently running with the second LED illuminated which is PP2.
It is running at 14W which on PP2 is a flowrate of 13.3LPM (0.8m3/hr) & at a 2.0M head.
You don't say what's selected, HW only or CH only or both?
If its running on HW only then IF the boiler is now running at a minimum of 3.8kW (continuously), and assuming the flowtemp is 75C then the coil outlet (and boiler return temp) will be 70.9C which is very inefficient, however can you say what system is currently on and we can take it from there?

 

[robbyc0]

Ok here you go, on cp1. Hot water only. System has just started after doing one shower yesterday eve.

 

[John.g]

Did you notice the pump power on CP1?, I would expect ~ 22W = 16.3LPM at a 3M head, based on that the boiler output/coil output is 16.3*60*(75-69)/860, 6.82kW, not bad IMO depending on coil size, and obviously also dependent on the cylinder temperature but shouldn't have a problem even when the clinder nears its cut out of 60C.
Keep a eye (sometime) and see what the flow/return temps are just as the cylinder is reaching its SP temperature, might then be able to reduce the target temp which will give a lower return temperature and greater boiler efficiency. I've mine set to 68C with a cylinder SP 60C.
What is the cylinder make/model/volume.

That S.46 alarm is a combustion related alarm which seems to be flagged at minimum boiler output so should be looked at some time

 

[John.g]

Here is some data from my cylinder, you can see where the coil output falls as the cylinder heats up, in my case from a stone cold cylinder at 13C to 56C, coil output from 7.8kW to 2.2kW

 

[robbyc0]

Here is some data from my cylinder, you can see where the coil output falls as the cylinder heats up, in my case from a stone cold cylinder at 13C to 56C, coil output from 7.8kW to 2.2kW


View attachment 96052

My plumber said that I should run the target flow as high as poss on hot water only because he said what I would lose in boiler efficiency I would more than gain back in reduced pump duty electric and loss via pipes. He suggested running at 80 degrees. But you are suggesting getting it as low as poss while still getting the tank up to temp in the required time. Your thoughts? Ps minimum the Vaillant mods down to is 5kw.

 

[John.g]

I won't go into the dreary calcs but making a few assumptions...... 100L of HW at 60C required/day. Gas cost is capped (UK) at 6.99 P/kWh & elect at 27.03 P/kWH, the gas saving in reducing the return temp by say 10C is ~ 0.45% or ~ 0.3 P/day! (for HW heating) but will be more or less balanced out by the increased pump (at 25watts) running time, if running for say 2hrs vs 1.5 hrs to reheat the cylinder then really no saving, same will apply if increasing the flowtemp to reduce the running time, lower elec cost but higher gas cost, interesting, to say the least.

Where there are very big savings are where you use priority domestic hot water, PDHW, where the boiler automatically increases the boiler target temp to 80/85C and shuts off the CH every time there is a DHW demand, once the HW demand finishes, the boiler then returns to CH only with a reduced target temperature which can be as low as 40C if weather compensation is installed, the boiler will then be in condensing mode where the relatively big savings can be made.