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

 

[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

 

[John.g]

A ABV is designed to work with a pump running in constant curve (fixed speed) mode so as the flow rate reduces the pump head rises and the ABV will start opening or open more depending on its setting.
The pump above if set to 250 mb constant pressure mode will maintain this pressure irrespective of the flow demand by inc/dec of the pump speed, so a ABV is not really suited to this pump mode but will give some form of control as the differential head across the ABV will vary a little with different flowrates and on where it is installe despite having a fixed pump head.
So if you want to try it, reduce the ABV setting until you feel the piping getting hot, if you are sure that the pump setting is 250mb then reduce the setting to 0.25 and then slowly further reduce it. If one uses the Honeywell DU144 ABV as a guide then with a pump head of 250mb and a ABV setting of 0.2 should give a by pass of ~ 350 kgs/hr or ~ 6 LPM, if the flow demand then increases the deltaP across the ABV should decrease a little and it should start closing in etc.

 

[colin_]

Hi John

So, if the system does not really need an ABV then I can set it to fully high (0.5) without any ill effects?

I am a bit wary of setting it low and allowing it to come in as when it does it is robbing what little heat the boiler is producing (an Evohome bringing it on for one minute and the boiler having sost start so it does not raise the temp by more than 1-2 degrees in this time) away from the radiators on the same level (which are near closed at that point in time)

Thanks
Colin

 

[John.g]

A manual bypass seems to be the recommended way of doing things but you have a bypass albeit a ABV which IMO will give you a measure of control over the bypass if indeed any bypass is required at all, the Evohome probably not but can't answer for your boiler.

Re robbing power a ABV will probably only dissipate 200/300 watts so even if your boiler is on its max turndown (10:1) say 3kw or so?, 250 watts is only "robbing" 6/7% of its output and remember the 1 minute runtime is the minimum run time, I can't see any reason why the boiler can't/won't run for > this time, if required.

Also read post#19 above.

Where is the ABV installed in relation to your boiler?, if adjacent/close then I think it can give reasonable control.

 

[colin_]

Hi John

It is not close to the boiler, it is on the top floor of the house and the boiler is directly below it on the ground floor.

The concern was as there are also radiators on the same level that the ABV would pinch heat from them but, as far as I can tell it is looking like my issue is due to the slow start of the boiler itself as in one minute it barely raises a couple of degrees and once that is pushed around lost in the pipes then not much goes to the radiator.

This morning the system was calling for heat on several radiators and the Evohome doing its one minute on/9 minutes off and the radiators were still cold. Once the hot water requested heat from the boiler it ramped up (after ~ 10 minutes) to 70 degrees and all the radiators that were requesting heat got hot (so valve definitely open enough, simply not enough heat from the boiler in one minute).

It is looking like this boiler will not work with the Evohome (which is a shame as the same installer installed both the boiler and the Evohome).

The heating data from this morning was

Time offset in minutes	Degrees on boiler	Degrees increase from initial	Degrees increase from previous time point
0	27	-	-
3	30	3	3
5	40	13	10
7	50	23	10
9	60	33	10
11	66	39	6
12	69	42	3
12.5	70	43	1

Thanks
Colin

 

[EvilDrPorkChop]

Hi John

It is not close to the boiler, it is on the top floor of the house and the boiler is directly below it on the ground floor.

The concern was as there are also radiators on the same level that the ABV would pinch heat from them but, as far as I can tell it is looking like my issue is due to the slow start of the boiler itself as in one minute it barely raises a couple of degrees and once that is pushed around lost in the pipes then not much goes to the radiator.

This morning the system was calling for heat on several radiators and the Evohome doing its one minute on/9 minutes off and the radiators were still cold. Once the hot water requested heat from the boiler it ramped up (after ~ 10 minutes) to 70 degrees and all the radiators that were requesting heat got hot (so valve definitely open enough, simply not enough heat from the boiler in one minute).

It is looking like this boiler will not work with the Evohome (which is a shame as the same installer installed both the boiler and the Evohome).

The heating data from this morning was

Time offset in minutes	Degrees on boiler	Degrees increase from initial	Degrees increase from previous time point
0	27	-	-
3	30	3	3
5	40	13	10
7	50	23	10
9	60	33	10
11	66	39	6
12	69	42	3
12.5	70	43	1

Thanks
Colin

There’s obviously a problem somewhere, as if the rooms aren’t at the set point the Evohome should be calling for heat longer than 1 minute. I’d suggest doing a full system reset and setting up from the start again.