Automatic bypass query

[colin_]

Hi

I have an automatic bypass valve (ABV) fitted on my system but I am having some issues understanding how it does its job when the TRVs are restricted by the Evohome control system

Lets say it is set at 0.4 and the TRVs are nearly closed (say they are 0.35), when the boiler fires I presume that the ABV will cut in.

Does it come in fully and shunt all the heat through it (simply on/off) and so none gets to the radiators or does it simply take 0.05 of it (variable) and the 0.35 flows through the TRVs as they need, so they can heat up.

What is a normal setting for it (initially the heating company set it at 0.15 but when the Evohome came on with restricted valve positions all that happened is that the ABV got hot and the radiators remained cold), they have now just set it at 0.5.

Thanks
Colin

 

[John.g]

That's way too low, try it at 0.35/0.4

0.5 (5M) setting should certainly force water through the rads.

 

[colin_]

High John

Many thanks for replying, they came back and set it at 0.5 (the max as they seem to think the boiler does not need it) but ideally I want to know what happens when it starts the bypass, does it fully open and bypass everything (so the radiators get none) or does it just bypass "a little" so that the radiators still get some through the almost closed zone valves whilst it takes a bit through it?

Thanks
Colin

 

[John.g]

No, it depends on the setting and the circ pump characteristics, with a setting of (example) 0.4 then if the pump head is 3M (0.4bar) the ABV is fully shut, when TRVs start closing in the pump head will then start rising and as it approaches or reaches 4M the ABV will just start to lift and by pass a small amount and so on. If/when the boiler shuts down, the ABV will then fully open to cool down the boiler HX on pump overrun.

Is the circ pump external to the boiler or internal? (system boiler, if gas fired). If external post make/model/mode and setting.

 

[colin_]

Dear John

Many thanks for the explanation and your time to reply.

The boiler is a Worcester Greenstar 8000 Life (GR8300iW 30 S) with its temperature set to 70 degrees.

I have an Evohome that controls the TRVs and these can be nearly closed when at/near temperature. What was happening at the 0.15 setting was the ABV was making a noise and getting hot but the radiators not warming, hence why I am trying to get the correct setting (the installer simply bumped it up to 0.5 and said I would have to notch it down if radiators making a noise?)

As an aside, I have also noticed that this boiler (and I presume a lot of others?) can take over 10 minutes to get from say 30 degrees to 70 degrees, even when full open demand (presumably soft starting up) which means that when the Evohome (when at/near set temperature to try and maintain temp) brings it on for 1 minute every 10 minutes, then the 1 minute is not enough to raise the temp much past 33 degrees (certainly the raiador just stay not cold) before it turns it off for 9 minute. Seems Evohome unstallation was a mistake I have made (I have posted in the other forum to see what others have found with Evohome just in case).

Thanks
Colin

 

[John.g]

Yes, maybe set the ABV to max then with everything calling for heat gradually decrease the setting until the ABV downstream pipe just starts to warm, you can get final setting from this starting point.

From a boiler output point only, most boilers do not take 10 minutes to increase SP from 30 to 70C, they may limit the boiler output to 75% of max for 10 minutes or so after fire up. You say the Evohome controls the TRVs so presumably the boiler flow temp SP is set manually?, I have read about proportional control roomstats that switch the boiler on/off depending on the deltaT between the room temp and the stat setpoint, others like opentherm? I think actually control the boiler SP temperature to achieve constant room temperature, others here like SJB have good knowledge of these systems so you should find what exactly the Evohome is controlling.

Of course your boiler may be just firing up for 1 minute and the flowtemp is exceeding the SP temp by 5C, the boiler will then shut down for the anticycling time, when it next fires up observe the setpoint/flowtemp difference not exceeding 5C.

 

[colin_]

Dear John

The Evohome only brings the boiler on, it is the boiler that is taking its time to warm up.

When downstairs heating fully on and control system keeping boiler on, if I watch the boilers display then it takes 5-10 minutes for it to show it get from its 30 degrees to 70 degrees that has been set for max temp on the boilers control. It seems to slowly modulate upwards as time passes.

If the valves were close to off and the ABV was not cutting in, would the boiler itself deem that it did not have to put any extra heat in and so would not get to a higher temeprature (in which case the ABV setting would be crucial)?

Thanks
Colin

 

[John.g]

If the boiler is range rated, ie, if the maximum boiler output has been restricted to say 10kw and the actual heat demand is 15 kw then it will take a long time to reach its setpoint temperature but someone would have had to do this range rating, can be done from the menu on some boilers.

If the TRVs closed to such a extent that the boiler minimum modulated output was more than the heat demand then the flow temperature will rise until it is SP+5C and the burner/boiler will cut out.

The boiler may also cut out if the flow/return temp deltaT exceeds 30C so the ABV setting will play a part in this situation.

 

[colin_]

Dear John

Sorry to be a pain but hopefully this is the last query as I believe that the ABV setting may be important with how the EVohome is running the boiler.

You say

"maybe set the ABV to max then with everything calling for heat gradually decrease the setting until the ABV downstream pipe just starts to warm, you can get final setting from this starting point"

So I should do

1. Set all radiators to call for max heat so their TRV valves are fully opened by Evohome (out of interest is there any reason why all and not a few)
2. Let it have a few minutes to send around the water
3. Decrease until pipe between in and out flow at ABV starts to warm a bit

When I have found the point in 3, I presume I need to back it off a bit (so say it was at 0.36, what would be a good number to take it to 0.37, 0.4?, one or two turns?)

Regards
Colin

 

[John.g]

No pain Colin, all interesting.

With all rads etc calling for heat the pump head will be at its lowest so you don't want the ABV to start opening then but you have to set up its initial setting somewhere so would think that this is a good starting point, so yes. if say the starting point is 0.36 then increase it a little, maybe as you suggest 0.37 to 0.4 max, you can then monitor it and make adjustments if required, around this initial setting.

Have you watched the flow/return temps on boiler fire up?.

 

[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