Baxi Duo Tec 40 HE A Fluctuating Hot Water Temperature

[Master Mech]

Hello All,

I am currently having an issue with my Baxi Duo Tec 40 HE A not maintaining a constant temperature for the domestic hot water.

With a thermometer under a tap & the domestic hot water thermostat set to 43 °C I get a fluctuating temperature from 51 °C to 38 °C which is clearly not acceptable as it goes from warm to scalding.

I have uploaded a video of the boiler to YouTube below showing what happens when you create a hot water demand by turning a tap on.

As you can see the burner does not stay on but cycles on & off because it seems to me the primary water in the boiler is getting to hot so the PCB is turning the burner off?

What I have found is that if I turn two taps on at the same time the problem is fixed!

The boiler modulates the temperature correctly & I get around 43°C at the tap give or take 2 degrees.

I have boiler cover plan so I contacted them & they sent an engineer out & so far they have changed the following components:

1. Diverted Valve

2. Domestic Hot Water Thermistor

3. Plate Heat Exchanger

4. PCB

All genuine Baxi parts & I still have the same problem so clearly non of these components were faulty.

The warranty company is now saying that because they have spent all this money on these components & it has not fixed the problem the I need a new boiler!

At this point I have lost all trust in them & the engineers capability to diagnose the problem.

I am currently trying to get recommendations for a gas engineer in my area but am struggling to do so & am wanting others opinions on what could be causing this.

This has taken place over a four month period due to COVID.

Here is the video of the boiler with one tap running creating a hot water demand:

As you can see the burner switches on & off causing the temperature to fluctuate.

Here is a video of the boiler with TWO taps running creating a hot water demand:

& here the burner stay lit constantly fixing the problem!

Any ideas what could be causing this & why it is fixed by turning two taps on?

Many thanks

Adam

 

[picasso]

Why are you running the hot water on minimum ? try turning up the hot water dial to max and see what happens. (the numbers on the display are the primary watertemp not the dhw temp)

 

[bernardgreen]

As you can see the burner does not stay on but cycles on & off because it seems to me the primary water in the boiler is getting to hot so the PCB is turning the burner off?

You are correct, either not enough hot water being drawn or the boiler is not modulating down enough ( reducing the rate of burn of gas ) to match the amount of heat required to heat the domestic hot water.

 

[Master Mech]

Why are you running the hot water on minimum ? try turning up the hot water dial to max and see what happens. (the numbers on the display are the primary watertemp not the dhw temp)

I am running the DHW at 43 °C because I find if it is set higher the water is far to hot at the taps & can scald you. Also why heat the water to a higher temperature than necessary? Seems like a waste of gas to me & 60 °C at the tap is way to hot.

I have tried the setting the domestic hot water thermostat to maximum & the same issue exists except the water is ridiculously hot but still fluctuates around 10 °C as the burner is turning off & on due to the primary water in the boiler getting to hot so the only option the boiler has is to turn the burner off to prevent an overheat condition.

The number displayed in the digital display is the primary water temperature as it leaves the primary heat exchanger but when you turn the thermostat knobs for the central heating or domestic hot water the display changes to show you what they are set at which is what I did at the start of each video to show what I have the DHW thermostat set to.

You are correct, either not enough hot water being drawn or the boiler is not modulating down enough ( reducing the rate of burn of gas ) to match the amount of heat required to heat the domestic hot water.

My thinking is that not enough water is flowing through the boiler resulting in insufficient cooling of the primary water in the boiler which causes the boiler to have to switch off the burner to prevent overheating?

This seems to be resolved by switching the second tap on which increases the flow rate through the boiler cooling the primary water enough so the boiler can modulate itself correctly?

I have attempted to check my mains water flow rate but this is difficult as all the taps in the house are high pressure aerated taps that have the very small tap connectors that reduce the flow rate to them.

I do have an outside tap which is plumbed with 13mm pipe all the way to the tap, its right next to the stop cock where the supply enters the house. Thing is the boiler is in the loft so going up two stories is bound to also effect the flow rate that the boiler is receiving?

The bath & the shower have thermostatic valves on them so I think this could invalidate the results also due to me now knowing if any mixing is occurring?

I read in the Baxi manual that the minimum water flow rate for these boilers is 2 litres per minute which seems crazy to me but I guess this is the bare minimum & I know I have more than that.

With this being a 40kW boiler I am wondering what water flow rate it would need to be able to modulate the DHW to 43 °C.


The other possible cause like you said could be that the low & high settings on the gas valve are set incorrectly?

If this was the case the PCB could not turn the gas flow rate low enough to be able to modulate the temperature of the primary water correctly?

When the gas engineer comes back should he be checking the maximum & minimum rates on the gas valve as if these are incorrect the boiler will not be able to modulate the temperature correctly?

 

[picasso]

I will leave you and Bernard to it then.

 

[bernardgreen]

I will leave you and Bernard to it then.

Your knowledge and experience would be far more useful to Master Mech in solving the problem he has than mine could ever be.

 

[ianmcd]

do you have braided hoses on your taps ? these can be very restrictive, it does seem that you have got insufficient flow rate

 

[Gasguru]

I suspect your boiler is too powerful for your intended use and there is nothing wrong with it.

I'm guessing it can probably only modulate down to say 12kw...and with a low tap flowrate it has no choice but to cycle on/off in order to not overshoot the target temp. With 2 taps on it prevents the cycling.
Measure the flowrate into a measuring jug and so the calcs... Have you checked for blocked strainers/isolating valves etc.

Boilers use cheap thermistors that have a high thermal mass...you're doing very well if the output temperature is plus/minus 5 degrees in my experience. Add in the burner cycling and it will be all over the place.

I'm not sure whether the DHW flow sensor is just a simple shuttle or turbine. With a turbine sensor the boiler software can use the flowrate to help modulate the buner.

The 2 litre figure is just a minimum to get the boiler fired up in DHW mode....with no guarantees to maintain temperatures.

With a thermostatic shower etc it is normal to set the boiler DHW temperature somewhat higher than the desired shower temperature in order that the thermostatic element can operate correctly.
Some showers need at least 55 degrees inlet (allowing for pipe temperature losses).

If you are concerned with the tap temperature plumb in a thermostatic mixer valve under the basin and set the DHW to a high temperature.

The minimum boiler power is normally fixed and not adjustable on modern boilers.

 

[Master Mech]

do you have braided hoses on your taps ? these can be very restrictive, it does seem that you have got insufficient flow rate

Yes all the taps have the braided hoses on the taps so I guess this is where the low flow rate is coming from.

I suspect your boiler is too powerful for your intended use and there is nothing wrong with it.

I'm guessing it can probably only modulate down to say 12kw...and with a low tap flowrate it has no choice but to cycle on/off in order to not overshoot the target temp. With 2 taps on it prevents the cycling.
Measure the flowrate into a measuring jug and so the calcs... Have you checked for blocked strainers/isolating valves etc.

Boilers use cheap thermistors that have a high thermal mass...you're doing very well if the output temperature is plus/minus 5 degrees in my experience. Add in the burner cycling and it will be all over the place.

I'm not sure whether the DHW flow sensor is just a simple shuttle or turbine. With a turbine sensor the boiler software can use the flowrate to help modulate the buner.

The 2 litre figure is just a minimum to get the boiler fired up in DHW mode....with no guarantees to maintain temperatures.

With a thermostatic shower etc it is normal to set the boiler DHW temperature somewhat higher than the desired shower temperature in order that the thermostatic element can operate correctly.
Some showers need at least 55 degrees inlet (allowing for pipe temperature losses).

If you are concerned with the tap temperature plumb in a thermostatic mixer valve under the basin and set the DHW to a high temperature.

The minimum boiler power is normally fixed and not adjustable on modern boilers.

I have not checked the strainer in the main water supply tap under the boiler or the one that is part of the restrictor in the hydraulic inlet assembly but I would have thought if these were clogged the flow rate at the taps would be severely reduced?

Both hot & cold at the taps is around 9 litres a minute which suggests the filters are OK?

The only way I think the boiler could know the flow rate is from the flow turbine that triggers the hall effect sensor to activate the hot water demand. But I am unsure.

Here is an image of the hydraulic inlet assembly.

https://ibb.co/Npvtc7G

After speaking again with the insurance company they have basically said they will do no more work on this boiler & I need a new one as I have exceeded the amount they are willing to spend to fix it.

After explaining to them that clearly all the parts they have fitted were not needed as they have not fixed my problem they offer me some compensation.

I then got recommended a Gas Safe engineer from a good friend & he came out & after doing some tests though along similar lines that the flow rate is to low from the boiler.

At the kitchen & bathroom taps I am getting around 9 litres a minute. Outside garden tap which is right next to the stop cock is delivering about 25 litres a minute so I am guessing these braided hoses that feed the taps are restricting the flow rate at the taps significantly.

I hear these are standard now to reduce water consumption?

He changed the primary thermistor for a slightly lower rated one so it reports the primary water is actually a little cooler that it actually is & this seemed to stop the burner cycling on & off due to the primary water getting to hot.

That lasted a day & then it was back to its usual self with the burner cycling on & off so I don't think that actually did anything due to what I found out below.

I have noticed is that if I open a hot water tap then turn the mains water supply tap off under the boiler & then switch it back on again with the hot tap open the boiler manages to modulate itself correctly & the burner stays on constantly when hot water is demanded!

The eventually stops happening & it goes back to cycling the burner after a few hours but I thought it was strange that this seems to temporarily fix the issue & have not clue why.

 

[picasso]

Just had exactly the same symptoms on a Vaillant, bath/shower mixer was passing cold water into the hot water system.

 

[Master Mech]

Just had exactly the same symptoms on a Vaillant, bath/shower mixer was passing cold water into the hot water system.

Funny you mention that in the past I have had the thermostatic valve for the bath tap passing cold mains water to the hot side.

Found this by switching the cold water mains supply tap off under the boiler & turned a hot tap on.

Water was still coming out the tap at a rapid rate which was obviously mains cold water.

When I turned the thermostatic valve on the bath tap to cold it slowed to a dribble.

I have since changed the thermostatic cartridge & now when I turn the cold water mains supply tap off under the boiler & switch a hot tap on nothing comes out so does this prove I have no mains cold water passing over?

 

[picasso]

The thermostatic valve does not effect the hot/cold cross over, there are non return valves built in to the tap but you can cut a separate one in on the hot supply to the tap.

 

[Master Mech]

The thermostatic valve does not effect the hot/cold cross over, there are non return valves built in to the tap but you can cut a separate one in on the hot supply to the tap.

I don't follow you, I understand that these thermostatic valves have non return valves in them but the one I changed had most certainly failed.

If I understand correctly any thermostatic valve has to have a cold & hot water supply to regulate the water temp which intern gives the possibility of the two streams to pass into one another if the valve (including it's non return valves) fail.

The thermostatic valve cartridge I fitted was for an in wall mixer unit which supplies a bath & hand shower, link for it below.

https://www.amazon.co.uk/gp/product/B005DKC53W/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Any way once it was replaced & the main cold water supply was isolated to the boiler no water comes out of the hot tap which I think is how it should be as the only source of water to the hot tap should be from the boiler.

So I am pretty sure I do not have cold water passing into the hot water, if you can tell me a way to test I would be more than happy to give it a go.

 

[dilalio]

Jeez.
Too much theory, not enough practical!

 

[FiremanT]

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?