Strange hike in Boiler Temperature

[killa47]

Yesterday, I was stood next to the boiler in garage and noticed the flow temperature display showing a rising temp from the normal maximum of 72C going up to around 85C. The boiler BDR91 was off and I couldn't hear the pump running but as I was in the middle of fitting replacement downlights, I chose to leave it.

The temp did start to drop back and I think it will have dropped into the boiler gradient limitation routine and fired burner back on when demand was required.

My puzzle and question is - How can the flow temperature show 85C when I have the boiler heat control set at circa 72C.
Surely, if maximum heat out is 72C, there can be no part of the CH system with water hotter than that? [Ignoring slight variation in temps at control]

[By way of added note - around the same time of the day I spotted that the hall zone temp on the Evo controller was displaying 5C which implied either it had reset that zone to off (5C frost protection) or it thought a door/window had opened and temporarily set the xone to 5C. I wondered if the two events were connected.

Confused

As a PS - A week ago I changed pump speed down from high (III) to medium (II) as recommended by DBMandrake in a previous thread.

[DBMandrake]

Yesterday, I was stood next to the boiler in garage and noticed the flow temperature display showing a rising temp from the normal maximum of 72C going up to around 85C. The boiler BDR91 was off and I couldn't hear the pump running but as I was in the middle of fitting replacement downlights, I chose to leave it.

The temp did start to drop back and I think it will have dropped into the boiler gradient limitation routine and fired burner back on when demand was required.

My puzzle and question is - How can the flow temperature show 85C when I have the boiler heat control set at circa 72C.
Surely, if maximum heat out is 72C, there can be no part of the CH system with water hotter than that? [Ignoring slight variation in temps at control]

Do you have zone valves ? If so its perfectly possible for the flow temperature to overshoot the set temperature after the boiler stops firing and the zone valves close - mine does it all the time. I currently have flow temp set to 75 but the reading will sometimes overshoot as high as about 80-82 after the boiler has switched off if it just so happens that the zone valves close just as it is reaching 75.

Think about the temperature of the metal parts of the heat exchanger that are in contact with the water flow - if rapidly flowing water goes in at say 60 degrees and comes out at 75 the heat exchanger mass must be MUCH hotter than 75 where it contacts the water to heat the water by 15 degrees in the couple of seconds that it takes for the water to flow through the boiler. So lets say the heat exchanger body is at about 90 degrees to achieve this.

So long as the water keeps flowing and the water that returns from the radiators stays at 60 degrees 75 degrees will come out. As soon as the zone valve closes all the flow will go through the automatic bypass loop which is a short circuit - now suddenly 75 degree water will go in and the heat exchanger is still, for the moment at 90 degrees, even if the boiler's burner turned off simultaneously with the zone valve you have the thermal mass of the heat exchanger which is now going to heat up that maybe 5 litres of water that is circulating around in a loop. The temperature will keep rising until equilibrium is reached between the heat exchanger and the water.

This is the same reason that hot water overrun (turning off the boiler in advance of closing the hot water zone valve) can be a good idea.

So I'd say that in specific circumstances an overshoot of 5 degrees is to be expected when the zone valve shuts suddenly diverting all flow to the bypass loop, and up to 10 degrees may be possible.

[By way of added note - around the same time of the day I spotted that the hall zone temp on the Evo controller was displaying 5C which implied either it had reset that zone to off (5C frost protection) or it thought a door/window had opened and temporarily set the xone to 5C. I wondered if the two events were connected.

Maybe if the zone went into window mode there was no further heat demand so the zone valve closed at that time, that's the only connection I could think of. I notice a couple of my zones going into window mode rather easily in this cold weather - if I open a door from a cold unused room onto the hallway the hallway zone will go into window open mode even though I've adjusted it to be less sensitive!

As a PS - A week ago I changed pump speed down from high (III) to medium (II) as recommended by DBMandrake in a previous thread.

Did you ever observe this behaviour before changing the speed ? Does it still do it if you change it back ?

[killa47]

Do you have zone valves ? If so its perfectly possible for the flow temperature to overshoot the set temperature after the boiler stops firing and the zone valves close

I have two CH zone valves plus DHW valve but the CH valves are latched open permanently so they would not have influenced the excessive heat. But I do think you further explanation below may be the answer.

Think about the temperature of the metal parts of the heat exchanger that are in contact with the water flow - if rapidly flowing water goes in at say 60 degrees and comes out at 75 the heat exchanger mass must be MUCH hotter than 75 where it contacts the water to heat the water by 15 degrees in the couple of seconds that it takes for the water to flow through the boiler. So lets say the heat exchanger body is at about 90 degrees to achieve this.
So long as the water keeps flowing and the water that returns from the radiators stays at 60 degrees 75 degrees will come out. As soon as the zone valve closes all the flow will go through the automatic bypass loop which is a short circuit - now suddenly 75 degree water will go in and the heat exchanger is still, for the moment at 90 degrees, even if the boiler's burner turned off simultaneously with the zone valve you have the thermal mass of the heat exchanger which is now going to heat up that maybe 5 litres of water that is circulating around in a loop. The temperature will keep rising until equilibrium is reached between the heat exchanger and the water.

This situation could have occurred even though the valves were both open. We have a biggish house with lots of radiators so if heat demand dropped off across the house (with only one upstairs rad on without TRV), maybe the ABV kicked in. Possibly tied in with the hall zone rads dropping to 5C demand. Still looks a bit unusual though but I now appreciate what you say about the much higher temp of the heat exchanger being around 90c maybe.

if I open a door from a cold unused room onto the hallway the hallway zone will go into window open mode even though I've adjusted it to be less sensitive!

This suggestion looks a distinct possibility. I know that no door or window had been open but maybe the much lower temps outside caused some cold draughts in proximity to one of the hall rads near the inner porch door

Did you ever observe this behaviour before changing the speed? Does it still do it if you change it back?

Not noticed this before but I will test at the previous high speed setting if it occurs again.
Appreciate your explanations as always.