Bypass on Worcester S-plan not installed - Overheating

[groov]

Hello,

I own a Worcester Bosh HE 28 System boiler, it was installed a few years ago using an S-plan: two zone valves next to the boiler with a room thermostat and a Cylinder thermostat - no problems at all with hot water or CH.

One problem with the installation though... say the boiler was running for a while with a flow of 75-80 then suddenly both cylinder and CH stats shut off the valves, the boiler temps will start to rise as the water is not moving anymore until around 92c when the boiler pump will start running (overrun) again to seemingly move the water around, but as it can't (because no bypass is installed and both valves are turned off) the boiler temps keep rising until the max display of 99 (I guess they rise a lot more than that)... and eventually after a LONG while when the temps decrease to about 70 the pump stops and all is well.......

Now I could have found that about this a long time ago but have not as everything works so never looked at temps an what have you.. but purely out of coincidence, basically I bought an energy saver gizmo that tells you how much electricity you are consuming, and that prompted me to try and understand why the boiler pump was on so often and thanks to reading this forum realised that it was due to the above installation problem.

I am surprised that this boiler or the pump have not exploded after nearly 7 years and even more surprised that all the engineers that serviced the boiler over the years have never mentioned this as to the original installer, he is out of business now (I wonder why!).

Now I need to fix this and was hoping that one of you guys can help me in my quest.

This is what I could do - install an automatic bypass valve before the valves on the flow pipe and connect it to say the hot water flow pipe after the zone valve - this will work well as if I manually open the valve when the overrun kicks in the temps quickly drop and the pump will then stop after no more than 10 or 15 secs (basically when temps go back to 75 or so).

I understand that the above is the most common way of doing this and this is what the installer should have done when installing the boiler in the first place.

But.... why can't I simply connect the zone valve to the overrun signal (I have no clue how but I'm theorising) so that instead of installing this bypass valve I can simply have the normal zone valve open when the pump goes into overrun mode... this basically would be ideal as it will save on both having to buy the £40 bypass valve and the piping required to accommodate for it.

Please tell me if I can do this and your thoughts about my theory.

Thanks in advance !

Regards,
M

 

[Agile]

It all depends on how your system is wired.

In many cases if the valve is open the power is applied to the orange wire which is the call for heat which caused the boiler to come on in the first place.

Tony

 

[groov]

Thanks for that.

Yes I presume if the stat opens the valve and starts the boiler then the valve would be wired in a way that if you want to open it then the boiler would fire up.

Is there a way around this ? (wire wise)

Or for the sake of simplicity just install a bypass valve and forget about it... If this is the best approach what are the best valves (easy to tune) for my requirement - and would te choice of dumping the heat in hot water or CH curcuit matter ?

Why don't they just make normal zone valves open if they sense an increase of pressure - surely this would make the concept of bypasses a very simple thing... Basically a normal 2 port valve that automatically open the same way as a bypass valve...

 

[picasso]

bite the bullet and install a bypass, honeywell du145 is easy enough to install, although its normally installed across flow and return, just one note if you have an unvented cylinder, you cannot bypass the 2 port valve.

 

[Agile]

If you try to retrieve that last bit of energy by fitting the ABV across say one of the valves then you have a possibility of heat leaking that way when the stat/time control wants it to be off.

It will over run longer but is less critical and problematic to set up if you just connect to flow and return.

Tony

 

[groov]

Thanks.

Installing across flow and return would be really too short (don't you need at least around 10 metres of pipes for this?) - I can only see the pipes below the boiler .. the rest would mean redoing some ceilings or some floor boards or maybe I just don't understand what you mean, sorry..

I have a DPS heatbank cylinder which is basically a vented thermal store .. the more heat the better so I don't think it would actually matter if the extra heat was dumped in it as 15 seconds of flow circulation wouldn't probably even reach it ?

The overrun kicks in at 92c and goes off at 75.. I tried it by keeping the valve open manually after overrun started and this was when the cylinder stat had just turned off the boiler and the temps dropped to below 75 within seconds.

 

[matt1e]

But.... why can't I simply connect the zone valve to the overrun signal (I have no clue how but I'm theorising) so that instead of installing this bypass valve I can simply have the normal zone valve open when the pump goes into overrun mode... this basically would be ideal as it will save on both having to buy the £40 bypass valve and the piping required to accommodate for it.

Please tell me if I can do this and your thoughts about my theory.

yes you can do this but not directly, you would need to install a couple of relays and rewire the control system, in all honesty it will be cheaper and easier to install a bypass and it wont cost you £40 if you are doing it yourself as picasso has said bite the bullet and install a bypass

Matt

 

[groov]

Ok - I have a plumber coming in this afternoon.

I will be asking him to fit a bypass (autmatic valve) from the flow after the boiler system pump to just after the hot water zone valve.

I presume we will have to drain the whole system down ?

Anything else I should ask him ? .. I am just hoping that he won't say it has to go from flow to return.

 

[kevindgas]

there is no reason why you can't just use the bypass to go round the C/H zone valve, all that will happen is the heat will go into the radiators for a short while until the heat in the boiler reduces to the limit set by the manufacturer for the overrun.
in the summer it will help to circulate the water in the rads for a very short while probably not enough to put any heat in them and in the winter it probably won't matter anyway.
similarly you can use the H/W valve again with no real adverse effect.

but the best way is flow to return

 

[groov]

Just was quoted £333 (live in Surrey)... I personally think it is a ridiculous quote. What do you guys think ?

All the piping is below the boiler - extremely easy access with no obstruction whatsoever in our clean utility room.

 

[heatingman]

Tell that chancer to take a hike, that price is over the top. Even without seeing your system £333 is just plain stupid.