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]

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.

 

[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.

Hi

It is one room that is acting awkward with respect to the set point. The rest say they are at temperature on the controller but the data says they are calling for heat but not getting warm (as the boiler needs a good 5 minutes to raise its temperature by just 1-3 degrees ( 2 minutes gives only gives around 1-2 degrees more).

I am gathering information for the installers and will ask if they should set it up again (but suspect will still have some issue with the boiler output)

Thanks
Colin

 

[EvilDrPorkChop]

Hi

It is one room that is acting awkward with respect to the set point. The rest say they are at temperature on the controller but the data says they are calling for heat but not getting warm (as the boiler needs a good 5 minutes to raise its temperature by just 1-3 degrees ( 2 minutes gives only gives arounf 1-2 degrees more).

I am gathering information for the installers and will ask if they should set it up again (but suspect willstill have some issue with the boiler output)

Thanks
Colin

Out of interest have you got the weather compensation features and load compensation turned on under the settings? That maybe your issue too. Especially if it’s been set incorrectly.

 

[colin_]

Out of interest have you got the weather compensation features and load compensation turned on under the settings? That maybe your issue too. Especially if it’s been set incorrectly.

Hi

These functions are not set (say disabled)

I have set the min on time to 2 minutes as per previous suggestion to see if that makes a difference but so far not (temperature still not going up by more than a few degrees from the boiler) but will monitor it today (as I am working directly above the boiler I can hear when it comes on)

Thanks
Colin