Automatic bypass query

[colin_]

Dear John

The boiler is newly installed and the only user control is to set the desired water temperature (set to 70) and view the bar pressure (for topping up) and the temperature of the heat it is sending.

I have set 0.4 for the ABV for now but will do a proper test this weekend.

Many thanks for you help.

Regards
Colin

 

[John.g]

I think you will have to keep the ABV out of the picture as the boiler circ pump is constant Head (pressure) which means it will stay the same whatever the flowrate so very difficult if not impossible to set up the ABV, also the pump factory setting of 250mbar (2.5M) seems a bit low to me especially if your system is old, have a look and confirm that it is in fact set to this. page 56 of manual.

 

[colin_]

Dear John

I was hoping that it would be something simple with the ABV not cutting in that made the boiler not ramp up but unfortunately not loking the case from what you are saying.

When the ABV was set at 0.15 it got extremely hot and noisy but the boiler was outputing 60 degrees. Now it is at 0.4 bar the boiler does not get over 36 degrees as it cycles one minute on, 9 minutes off. During this time the radiators are not stone cold, so they are getting some heat, just not the full whack of +60 degrees to give them more heat.

The installers say the Evohome has to "learn" and this can take a few weeks but if it is still doing the cycling after that then I shall have to get them back to see if they can find out more (at the moment they are very busy elsewhere due to the weather so I will wait the couple of weeks).

Thanks
Colin

 

[John.g]

All is not lost just yet, one of the reasons for no rad output is that with the pump set to 250mbar and the ABV set to 150mb then you were bypassing 600 kgs/hr say 10LPM and with that low pump head probably next to nothing through the rads so confirm if possible the pump head setting, I would then increase it to 350 mbar and set the ABV to just barely pass with everything open, if unable to increase the pump head then do the same with the ABV with pump head at 250mbar OR close the ABV fully and open up fully (TRV etc) one small rad/towel rail which will act as a permanent by pass.

 

[colin_]

Dear John

As the Evohome only brings it on for a minute (every 10) I would be a bit worried that the small bypass radiator would take all the heat out (as the boiler doesn't get much chance in a minute to warm up) before it got any to the (further away) ones, but it is one that I can try in the future.

I will give it 2 weeks for the Evohome to "learn" and see if it changes, if not I will get the installer back and now that I know more (thank you very much for the information) I can at least ask them to check the settings you have recommended.

Regards
Colin

 

[John.g]

Assuming you have roomstat(s)( then if you were to set them all to max, say 30C then under no circumstances should the boiler just fire for 1 min on 9 mins off (6 cycles/hr) this should only happen if you have a control like the Honeywell DT90E which does a form of this but if the stat/room difference is > 1.5C the boiler will fire continuously and when the difference is 1.5C or less will cycle the boiler and does do a learning process. If your boiler is not firing continuously with a high differential then something seriously amiss and IMO nothing much will be learned in 2 weeks, in the meantime I would just get the ABV to pass a little with everything opened up.

For interest you might just enter Honeywell DT90E in the Central Heating forum search box and you will get a feel for how these fancy controllers are supposed to work.

 

[EvilDrPorkChop]

Personally for me, on a system that has a modulating pump and modulating TRVs such as EvoHome I prefer to fit a Manual Bypass.

 

[John.g]

On your system are the evohome modulating TRVs closed even after the system has been shut for say the night and does the controller then open them gradually until the desired room temp is reached and does the boiler reach its setpoint temp rapidly (from a cold startup).?

 

[EvilDrPorkChop]

On your system are the evohome modulating TRVs closed even after the system has been shut for say the night and does the controller then open them gradually until the desired room temp is reached and does the boiler reach its setpoint temp rapidly (from a cold startup).?

On system shut down the TRVs close. However this can take 1/2 minutes. The heat demand if using the Boiler Relay will switch off the demand first then close the TRV valves.

The system will open and close the TRVs dependant on the set point of the room. Anything greater than a 1.5oc difference will get a 100% demand. After that the system learns the characteristics of the room and will adjust accordingly over the coming weeks.

 

[colin_]

I think you will have to keep the ABV out of the picture as the boiler circ pump is constant Head (pressure) which means it will stay the same whatever the flowrate so very difficult if not impossible to set up the ABV, also the pump factory setting of 250mbar (2.5M) seems a bit low to me especially if your system is old, have a look and confirm that it is in fact set to this. page 56 of manual.

View attachment 64111

I think you will have to keep the ABV out of the picture as the boiler circ pump is constant Head (pressure) which means it will stay the same whatever the flowrate so very difficult if not impossible to set up the ABV, also the pump factory setting of 250mbar (2.5M) seems a bit low to me especially if your system is old, have a look and confirm that it is in fact set to this. page 56 of manual.

View attachment 64111

Hi

I am awaiting confirmation of the setting but if it was at 250mbar how would I use the above to work out the setting for the ABV?

Thanks
Colin