Automatic Bypass Valve

Hi,

I have a Keston C55 which has been running happily for 3.5 years (unusual, by all accounts) - but when installed it only had a manual bypass fitted - which was too short and caused some problems.

So when I had to have some major work done on the boiler (Heat Exchanger replacement ) I decided to fit a Honeywll DU144 auto bypass valve with a small radiator to provide a big enough bypass circuit.

However, I can't find out what the minimum flow rate through the boiler is to set the ABV. Keston's customer service is pretty poor - they don't bother to answer e-mail queries - and I was wondering whether anyone would know how to calculate the setting? It's a sealed system running at about 1.6 bar - but the graphs and instructions in the Keston manual and the ABV instructions don't seem to match up.

Grateful for any help!!

D...

It certainly is a major job replacing the heat EX !

Keston would probably say it had failed as a result of insufficient water flow rate! That could point to installation issues!

Thats the first time I have heard of anyone fitting a bypass radiator which implies to me that there is no experienced heating engineer involved.

Since I doubt that you have any way of measuring the boiler flow rate then any figure is likely to be irrelevant! I would guess that they might say about 25 litres/minutes.

The important thing is to maintain an adequate flow rate. Following the standard way of adjusting an auto bypass is probably going to be the best thing that youy can do.

As far as a bypass rad...... you are just generating further heating when the house is already hot!

Tony

I put the rad in because the manual suggested that the bypass should be at least 10 metres of 22mm pipework to allow sufficient volume of water to enable the HE to cool properly on over-run (I'm assuming that this is a worst case scenario - boiler running at full power when the demand switches off). I don't have the space to put in a circuit this big, so used a radiator to effectively increase the volume of the bypass circuit.

The instructions on the DU144 refer to graphs and specific boiler information - which isn't in the Keston paperwork - rather than a generic setting procedure. The only other way I can think to do it is trial and error.

Anyway - it's got to be a better bypass system than the 1metre of 15mm pipe with a gate valve partially shut!!

D...

Keston are trying to protect against the considerable power still being transferred into the water from the specific heat of the flame side of the HE.

The best situation is to put the bypass at the opposite end of the heating circuit so a considerable volume of water is available in the pipes.

Your system will not really work properly as that rad will be hot when you least want it to be i.e. when the TRVs start turning off.

The procedure in practice is to set the bypass with about a third to a half the rads turned off and turn the adjustment from max slowly down until water JUST starts passing as evidenced by the outlet getting warm.

Tony

Hi Tony,

Thanks for the info - much appreciated.

The system has about 35 radiators on it, with 7 radiators/towel-rails spread throughout the system without TRVs, so bypassing within the rad circuit when the TRVs start to shut doesn't appear to present any problems.

It's when both the CH and DHW circuits switch off at the same time - and then there isn't a big enough circuit for the boiler to pump water during the 2 minute overrun cycle without overheating. With the old (15mm) manual system the circuit wasn't long/big enough - so when the boiler was running at it's normal operating temperature it would sometimes 'kettle' if it was working hard when the programmer switched off demand for both CH and DHW simultaneously.

The only way we could do it was with the boiler operating temperature at 60 degrees or below. The old bypass also acted as a short-circuit, so unless you set the gate valve carefully, the boiler would be idling along happily at 60 degrees, but the distant radiators in the system wouldn't be getting hot.

Once the ABV is set properly, the ABV shouldn't bypass (much) unless the DHW and CH are both switched off and the boiler in overrun mode - and that's the only time the new little radiator is in the circuit.

I will follow your advice on the ABV setting - and see how that works!!

Thanks for your help.

D...

DGR said:

Hi Tony,

Thanks for the info - much appreciated.

The system has about 35 radiators on it, with 7 radiators/towel-rails spread throughout the system without TRVs, so bypassing within the rad circuit when the TRVs start to shut doesn't appear to present any problems.

D...

Click to expand...

In that case set it to JUST let water pass with ALL the TRVs closed but the seven without still working and then turn it back a tiny bit. Or better still set it as described with ONE of the non TRV rads off!

Couple of silly questions!

I hope the pump is driven by the boiler to provide the over run!

Can you check that the demand for heat from the motor valves ceases almost immediately the stat/clock turns off?

Agile said:

Couple of silly questions!

I hope the pump is driven by the boiler to provide the over run!

Click to expand...

On the C55 the pump is integrated within the boiler.

Agile said:

Can you check that the demand for heat from the motor valves ceases almost immediately the stat/clock turns off?

Click to expand...

Yes - as soon as the programmer switches the circuit off, the motorised valves shut down and stop the demand from the boiler - at which point the boiler goes into overrun (mode '6' on the display).

D...

you dont have to connect the bypass to the return , you can use the bypass to short circuit the central heating 2 port valve,connect the" in "port before the valve and the "out" port just after the valve and you have the whole system to dump the heat , this is only possible as you dont have trv,s on all rads

That was the other alternative - but as I already had the bypass connections I just connected the 2 via a small rad rather than use the whole system.

If the current effort doesn't work properly, that's my back-up plan!!

D...

Or, and possibly rather better in your case, the bypass can be across the cylinder coil.

In that case it does need to be carefully set so that it does not open until both motor valves are closed.

Tony