Hot water too hot

[flatcrank]

Same thing happens with my new Baxi Luna Duo-Tec 40GA, so I am presuming this is the nature of the unit. Seemed odd to me as well that the temperature set is not the temperature you get, plus or minus a few degrees. But my initial water is too hot by more than a few degrees, until it moderates down to the set temp. Not good for seniors and others with scald risk, none of our taps have thermostatic valves. Surely they could design the PCB board to accommodate the delay a sensor needs to register the correct temp and modulate the boiler accordingly? Interesting this issue is across different brands. What bugs me is the manual, or even internet info, does not explain what "normal" behaviour of these units is, and the installer I have does not know either, so you can't get a straight answer on what to expect with these combi units.

 

[tamz]

As there is nothing worth watching on the telly............

All boilers work on pretty much the same principles for hw.
Tap opened activates a diverter valve and lights the burner. Water is pumped around the main heat exchanger (the bit with the flame acting on it) and through a plate heat exchanger transferring heat to the water for the taps. The flame size is controlled basically by a resistor of one kind or the other that reacts to temperature.

In the summer time when the heating is off the water running through the boiler is cold and heats gradually to the set temperature giving the electronics a chance to react as the water temp rises and settles to your desired temperature. It will run all day fairly stable + or - a degree or 2 from the desired setting.

Many boilers have what is called a pre heat which basically means even if the boiler is off, it is sitting with a slug of water at around 60º ready to roll.
This means when a tap is run there is no (very little) delay in hw going down the pipe but the hot is followed by some slightly cooler as the burner reacts then it stabilises.
You can turn the preheat off on most boilers. Personally i think it is a waste of gas having it on but everyone to their own.

When the heating is on the hw reacts in the same way as when preheat is on, only this time the "slug" of water may be as high as 82º depending on how high you have the heating thermostat (the one on the boiler) set.
The electronics on the boiler has no control over this. It can't cool it so that 82º water is diverted straight to heating the HW. It drops a few degrees as it transfers the heat but you could still get some 65 - 70º water through before it stabilises.

I hope that kinda explained things without getting too technical.

 

[flatcrank]

As there is nothing worth watching on the telly............

All boilers work on pretty much the same principles for hw.
Tap opened activates a diverter valve and lights the burner. Water is pumped around the main heat exchanger (the bit with the flame acting on it) and through a plate heat exchanger transferring heat to the water for the taps. The flame size is controlled basically by a resistor of one kind or the other that reacts to temperature.

In the summer time when the heating is off the water running through the boiler is cold and heats gradually to the set temperature giving the electronics a chance to react as the water temp rises and settles to your desired temperature. It will run all day fairly stable + or - a degree or 2 from the desired setting.

Many boilers have what is called a pre heat which basically means even if the boiler is off, it is sitting with a slug of water at around 60º ready to roll.
This means when a tap is run there is no (very little) delay in hw going down the pipe but the hot is followed by some slightly cooler as the burner reacts then it stabilises.
You can turn the preheat off on most boilers. Personally i think it is a waste of gas having it on but everyone to their own.

When the heating is on the hw reacts in the same way as when preheat is on, only this time the "slug" of water may be as high as 82º depending on how high you have the heating thermostat (the one on the boiler) set.
The electronics on the boiler has no control over this. It can't cool it so that 82º water is diverted straight to heating the HW. It drops a few degrees as it transfers the heat but you could still get some 65 - 70º water through before it stabilises.

I hope that kinda explained things without getting too technical.

That is a great explanation and makes sense, and confirms what is "normal" for a combi.

Thank you.

 

[tamz]

Your welcome

 

[chris watkins]

As there is nothing worth watching on the telly............

All boilers work on pretty much the same principles for hw.
Tap opened activates a diverter valve and lights the burner. Water is pumped around the main heat exchanger (the bit with the flame acting on it) and through a plate heat exchanger transferring heat to the water for the taps. The flame size is controlled basically by a resistor of one kind or the other that reacts to temperature.

In the summer time when the heating is off the water running through the boiler is cold and heats gradually to the set temperature giving the electronics a chance to react as the water temp rises and settles to your desired temperature. It will run all day fairly stable + or - a degree or 2 from the desired setting.

Many boilers have what is called a pre heat which basically means even if the boiler is off, it is sitting with a slug of water at around 60º ready to roll.
This means when a tap is run there is no (very little) delay in hw going down the pipe but the hot is followed by some slightly cooler as the burner reacts then it stabilises.
You can turn the preheat off on most boilers. Personally i think it is a waste of gas having it on but everyone to their own.

When the heating is on the hw reacts in the same way as when preheat is on, only this time the "slug" of water may be as high as 82º depending on how high you have the heating thermostat (the one on the boiler) set.
The electronics on the boiler has no control over this. It can't cool it so that 82º water is diverted straight to heating the HW. It drops a few degrees as it transfers the heat but you could still get some 65 - 70º water through before it stabilises.

I hope that kinda explained things without getting too technical.

There is no pleasing some people, I remember when combi's first came out one of the main complaints is just how long it took for the water to come through hot. If you were lucky the gas valve was a two stage HI /Low which was all the temperature adjustment you got without adjusting the water flow at the fitting.
You could always have a TMV blending valve installed on the outlet of the combi to limit the temperatures or ensurer your bath (& other fitting) have one installed to limit the outlet temperature as required under Building Regulations Part G.

 

[tamz]

1st combi's i ever fitted were the old flowmatics. It eventually dawned on me if i bled the hw heat exchanger you got better hw lol