FrostStat connected to High-Limit Pipe Stat?

[MadDragon]

I've just moved into a house and have an issue where the conventional boiler system heating and pump seems to be running constantly, generally the pump seems to run 12 minutes on, 2 minutes off, 24 hours a day. Radiators won't heat up (unless programmer/room stats on), but the boiler and pipes from the boiler are always very hot regardless of what the programmer and roomstats are demanding. I've previously noted the issue here without a solution. HOWEVER, it got me the thinking that a FrostStat to a high-limit pipe thermostat seems odd and could be the cause? Here are the key details of the system:

- Boiler: Vaillant VU ecoFIT sustain 415 (VU 156/6-3 OV) - GC No: 41-694-33
- FrostStat - Honeywell Home FrostStat (positioned next to boiler) - Set to 5 degrees
- High-Limit Pipe Stat: L641BI012 (positioned on left hand hot water pipe above the boiler) - Set to 65 degrees
- Hot Water Cylinder: EvoCyl 210L EVO STD 2Z (on 1st floor airing cupboard)
- CH Pump: Wilo Para (positioned next to Hot Water Cylinder)
- Programmer & Controller: ThermaQ TQX0027 (keypad in utility, control unit on hot water cylinder) - For the purpose of this case, assume I've set everything to OFF.

What I've noticed is that the frost black wire seems to be wired up to run through the high-limit stat (which in turn is wired to break on temp rise). Doesn't that seem weird? I've thought of the following logic depending on the theoretical ambient/water temp (ignore the fact ambient/water temp won't be the same):

0 degree C - Frost Stat ON - High-Limit Stat ON > therefore heating ON
30 degree C - Frost Stat OFF - High-Limit Stat ON > therefore heating ON
70 degree C - Frost Stat OFF - High-Limit Stat OFF > therefore heating OFF

Therefore, the boiler/pump would be working away any time the CH water pipe drops below 65 degrees?

Is my logic correct based on the way the original plumber (or previous home owner) has wired it? In any case, the Honeywell Home FrostStat instructions seems to suggest linking it to a low-limit frost pipe stat (L641B1004), NOT a high-limit stat. What's the point of a high-limit stat if the boiler has this sensor and function built-in?

In a nutshell, could this be what's causing the heating to stay on 24/7? Any quick fixes or suggestions of things to try? Thanks

 

[Harry Bloomfield]

Therefore, the boiler/pump would be working away any time the CH water pipe drops below 65 degrees?

Have you tried turning that stat down, to see what difference it makes?

The boiler itself, includes it's own frost protection.

 

[CountryFan]

Remove the 24v link on the RT side at the boiler pcb. It should stop your issue.

 

[MadDragon]

@CountryFan , you are amazing, really well spotted. Removing the 24v-RT link has actually fixed my problem and now everything seems to be functioning as expected, it's now nice and silent at night! What I don't understand is how the original installer left that link in place, and how the previous owners didn't question it for 5 years.

Such as simple fix in the end, THANK YOU so much!

FYI, I've attached a better photo of the link I removed in case anyone has a similar issue in the future.

 

[CountryFan]

@CountryFan , you are amazing, really well spotted. Removing the 24v-RT link has actually fixed my problem and now everything seems to be functioning as expected, it's now nice and silent at night! What I don't understand is how the original installer left that link in place, and how the previous owners didn't question it for 5 years.

Such as simple fix in the end, THANK YOU so much!

FYI, I've attached a better photo of the link I removed in case anyone has a similar issue in the future.

No idea, but it happens more than you think

 

[AdrianUK]

I've just moved into a house and have an issue where the conventional boiler system heating and pump seems to be running constantly, generally the pump seems to run 12 minutes on, 2 minutes off, 24 hours a day. Radiators won't heat up (unless programmer/room stats on), but the boiler and pipes from the boiler are always very hot regardless of what the programmer and roomstats are demanding. I've previously noted the issue here without a solution. HOWEVER, it got me the thinking that a FrostStat to a high-limit pipe thermostat seems odd and could be the cause? Here are the key details of the system:

- Boiler: Vaillant VU ecoFIT sustain 415 (VU 156/6-3 OV) - GC No: 41-694-33
- FrostStat - Honeywell Home FrostStat (positioned next to boiler) - Set to 5 degrees
- High-Limit Pipe Stat: L641BI012 (positioned on left hand hot water pipe above the boiler) - Set to 65 degrees
- Hot Water Cylinder: EvoCyl 210L EVO STD 2Z (on 1st floor airing cupboard)
- CH Pump: Wilo Para (positioned next to Hot Water Cylinder)
- Programmer & Controller: ThermaQ TQX0027 (keypad in utility, control unit on hot water cylinder) - For the purpose of this case, assume I've set everything to OFF.

What I've noticed is that the frost black wire seems to be wired up to run through the high-limit stat (which in turn is wired to break on temp rise). Doesn't that seem weird? I've thought of the following logic depending on the theoretical ambient/water temp (ignore the fact ambient/water temp won't be the same):

0 degree C - Frost Stat ON - High-Limit Stat ON > therefore heating ON
30 degree C - Frost Stat OFF - High-Limit Stat ON > therefore heating ON
70 degree C - Frost Stat OFF - High-Limit Stat OFF > therefore heating OFF

Therefore, the boiler/pump would be working away any time the CH water pipe drops below 65 degrees?

Is my logic correct based on the way the original plumber (or previous home owner) has wired it? In any case, the Honeywell Home FrostStat instructions seems to suggest linking it to a low-limit frost pipe stat (L641B1004), NOT a high-limit stat. What's the point of a high-limit stat if the boiler has this sensor and function built-in?

In a nutshell, could this be what's causing the heating to stay on 24/7? Any quick fixes or suggestions of things to try? Thanks

The frost stat in series with the high limit stat was intended to solve the problem of what happens when the frost stat & boiler are in an unheated space eg a garage. In this scenario, when the frost stat triggers the boiler will fire & prevent the system from freezing BUT, since the frost stat is in an unheated space, the frost stat will not be satisfied until the weather warms up & the heating will run 24/7 until the spring! The addition of the high limit stat prevents this - when the boiler return pipe gets hot, the high limit stat is satisfied & shuts the system down again.