Baxi Duo Tec 40 HE A Fluctuating Hot Water Temperature

All those bits has no one checked or changed the main sensor
 
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I suspect your boiler is too powerful for your intended use and there is nothing wrong with it.

Totally agree with that

Boilers use cheap thermistors that have a high thermal mass...y

I would say the culprit is the thermal mass of the metal mass in the heat exchanger. The thermistor and associated circuity reports over temperature, the flames are shut down but is then several seconds before the water temperature starts to drop.

From the video in the first post.

The indicated temperature increases up to 56° and then at 53 seconds into the video a click can be heard and then after a few more seconds the temperature starts to fall.

The temperature continues to fall until 1 minute 16 seconds when it reaches 42° and then a double click can be heard and then after a few seconds temperature starts to rise again.

As the OP suggested in the very first post....the boiler is cycling ON and OFF

EDIT also take into account the thermal mass of the water circulating between primary and secondary heat exchangers
 
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We are assuming that the system has actually operated satisfactorily while the low flow taps and flexes have been fitted?

Have there been any other modifications to the plumbing side?

I can say the boiler did once work correctly with the low flow fittings as when talking a shower it is most obvious due to the constantly fluctuating temperature which we never had before.

Only modifications since the boiler was fitted is we have had an extension which added four radiators the the central heating system.

All those bits has no one checked or changed the main sensor

Is the main sensor the safety thermostat?
 
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Totally agree with that


I would say the culprit is the thermal mass of the metal mass in the heat exchanger. The thermistor and associated circuity reports over temperature, the flames are shut down but is then several seconds before the water temperature starts to drop.

From the video in the first post.

The indicated temperature increases up to 56° and then at 53 seconds into the video a click can be heard and then after a few more seconds the temperature starts to fall.

The temperature continues to fall until 1 minute 16 seconds when it reaches 42° and then a double click can be heard and then after a few seconds temperature starts to rise again.

As the OP suggested in the very first post....the boiler is cycling ON and OFF

EDIT also take into account the thermal mass of the water circulating between primary and secondary heat exchangers

So if the boiler is just to powerful how come sometimes it manages to modulate itself correctly & maintain a constant temperature?

I have had a look through the service manual but cannot see anywhere where Baxi state the minimum flow rate through the boiler for it to be able to maintain a constant temperature.
 
The temperature of the mains supply water may now in this hot summer be high enough that the heat required is less than the minimum the boiler can modulate down to.

I do agree with you I am just trying to get my head around why when I turn the mains water supply on & off to the the boiler it is able for a at least 4 hours to modulate itself down to the require temperature not a problem. The burner stays lit constantly when hot water is demanded, minimal temperature fluctuation at the tap.
 
I can say the boiler did once work correctly with the low flow fittings as when talking a shower it is most obvious due to the constantly fluctuating temperature which we never had before.

Why are you specifically saying the shower? Did it work correctly with a single tap on?

Does the shower on it's own now present the problem?
 
This is without rereading previous replies, do you get consistent hot water at other taps?
If yes, then I would be looking at shower mixer being the culprit
 
This is without rereading previous replies, do you get consistent hot water at other taps?
If yes, then I would be looking at shower mixer being the culprit


Perhaps you should quick scan? :sneaky:
 
This is without rereading previous replies, do you get consistent hot water at other taps?
If yes, then I would be looking at shower mixer being the culprit

No all the taps fluctuate in temperature.

Why are you specifically saying the shower? Did it work correctly with a single tap on?

Does the shower on it's own now present the problem?

I say shower as it is most obvious when you are using it, when you turn a tap on to wash your hands it is off again before you can really get a gauge on if the temp is fluctuating or not.

I am confident in the past that with just a single tap on that it did not fluctuate in temp like it is doing now.

With the burner cycling I am getting swings of 15 degrees at the taps.
 
I have previously had to replace the shower mixer for problem you are having. Before rushing out to do that, I would put a clamp thermometer on 15mm pipe right of flow pipe or left of gas pipe under the boiler. If nothing else, run a hot tap full bore and feel above pipe for consistent heat. If I remember correctly, boiler will display primary temperature. It should read around 70 degrees

Now run the shower, do above test.

Then run shower as well as a cold tap in the bath, see if temperature is consistent
Sometimes high water mains can cause problem in the shower mixer
 
I have previously had to replace the shower mixer for problem you are having. Before rushing out to do that, I would put a clamp thermometer on 15mm pipe right of flow pipe or left of gas pipe under the boiler. If nothing else, run a hot tap full bore and feel above pipe for consistent heat. If I remember correctly, boiler will display primary temperature. It should read around 70 degrees

Now run the shower, do above test.

Then run shower as well as a cold tap in the bath, see if temperature is consistent
Sometimes high water mains can cause problem in the shower mixer

Just to clarify you want me to monitor the temperature of the DHW flow pipe under the boiler whilst running a hot water tap a full bore.

Then with the hot tap still running switch on the shower & the cold tap on the bath.

See if the temperature of the flow pipe is constant?

I know if I set the DHW thermostat to 50 degrees the boiler will let the primary water be heated to 60 degrees & then turn the burner off.
 
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I came accross a similar problem at my son's house - previously somone had converted to a combi by running the hot all the way to where the old hot tank was to pick up the hot feeds to the bathroom and kitchen. We re- plumbed directly to the kitchen cutting out about 20m of the pipework which made the hot water more stable.
 
Just to clarify you want me to monitor the temperature of the DHW flow pipe under the boiler whilst running a hot water tap a full bore.

Then with the hot tap still running switch on the shower & the cold tap on the bath.

See if the temperature of the flow pipe is constant?

I know if I set the DHW thermostat to 50 degrees the boiler will let the primary water be heated to 60 degrees & then turn the burner off.

In that case as Gas112 says, control issue. With primary temp at 60 ( are you sure it is primary temperature, some boilers show secondary temperature during hw demand). I think a service and check boiler operation is on the cards.

I would focus initially on a hot tap run full bore. If boiler switches off at 60 for primary temperature, something is telling the controls things are getting too hot and burner goes out. Does the fan keep going full tilt to 60 degrees or does the fan starts to slow down till 60 degrees stop takes place
 
In that case as Gas112 says, control issue. With primary temp at 60 ( are you sure it is primary temperature, some boilers show secondary temperature during hw demand). I think a service and check boiler operation is on the cards.

I would focus initially on a hot tap run full bore. If boiler switches off at 60 for primary temperature, something is telling the controls things are getting too hot and burner goes out. Does the fan keep going full tilt to 60 degrees or does the fan starts to slow down till 60 degrees stop takes place

The videos in the first post will give you an idea of how the boiler is operating.

Fan modulates itself right down the closer it gets to the target temp.

I am confident the temp displayed on the boiler is for the primary water as I have checked with my thermocouple clamped to the pipe next to the primary thermistor.
 

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