Hot water too hot

[ma701ss]

Hi, I asked this some time ago but never got to the bottom of it. Is it normal for a combi to give out hot water that is much hotter than the temp set on the boiler and then reduce down? It starts off cold for a few secs, rises to about 60 degrees quickly and then levels out to the correct setting of 45 after about 10 secs.

 

[Last Plumber]

Which Boiler is it ?
Make, Model Size ( output ) ?

 

[ma701ss]

Ideal Logic + C35
But I wondered if it is the general behaviour of combi's?

 

[Masood]

Have you got thermostatic mixer taps? Starting off cold is normal - there will be an amount of water in the pipework that needs to be run off before the hot comes through.

 

[ma701ss]

Yes I realise starting off cold is normal, it's the getting hotter (too hot) and then reducing to a cooler temp that seems odd. TMV's are not fitted but that seems to be the only way to stop it scalding. I'd presumed that if I set the boiler to 45 degrees it would only ever get to that or cooler.

 

[Masood]

Yes I realise starting off cold is normal, it;s the getting hotter (too hot) and then reducing to a cooler temp that seems odd. TMV's are not fitted but that seems to be the only way to stop it scalding. I'd presumed that if I set the boiler to 45 degrees it would only ever get to that or cooler.

TMVs are not the same as thermostatic taps! Do you have thermostatic taps? The boiler will have sensors controlling the hot water temperature but like all sensors they have a reaction time. The heat exchanger will retain heat for a while after the sensor has sent a signal to the boiler to modulate down, so it is feasible that you'll get a slug of hotter water as well. But thermostatic cartridges have a relatively short lifespan in my experience, especially the cheaper ones.

 

[SimonG]

All depends on the flow.

 

[Geyser]

Well it's an Ideal Logic so you get what you pay for. But that aside there is always a delay from heat generated to the thermistors sensing it and modulating the temp and variable flow rate through hx .

 

[ma701ss]

Hey, my Ideal Logic plus boiler is the nuts
No to thermostatic taps but don't they work the same way, by mixing hot cold to achieve whatever temp it's set to?

 

[ma701ss]

All depends on the flow.

What do you mean by depends on the flow? I'm, assuming that if a boiler is set to 45 degrees it should never heat water above that temp, how does the flow affect it?

 

[SimonG]

You reckon. Easy. High flow, low flow equals over 45.

 

[Masood]

What do you mean by depends on the flow? I'm, assuming that if a boiler is set to 45 degrees it should never heat water above that temp, how does the flow affect it?

Your assumption is wrong. See my post above.

 

[chris watkins]

All I would add to the above comments is that it takes a massive amount of energy (gas) to heat the domestic water the boiler can't afford to start off slow & build up it, you would be waiting a long time plus wasting water.

If it gets a call for hot water it switches on or over from heating & goes to full flame before then starting to look at the outlet temperature & ramp down to meet the set temperature.

It may also be in part be due to the fitting which may open slowly before reach full flow & as instantaneous water heating temperatures are also inversely proportionately to the flow lead to the dropping off of the temperature as the flow increases.

 

[ma701ss]

All I would add to the above comments is that it takes a massive amount of energy (gas) to heat the domestic water the boiler can't afford to start off slow & build up it, you would be waiting a long time plus wasting water.

If it gets a call for hot water it switches on or over from heating & goes to full flame before then starting to look at the outlet temperature & ramp down to meet the set temperature.

It may also be in part be due to the fitting which may open slowly before reach full flow & as instantaneous water heating temperatures are also inversely proportionately to the flow lead to the dropping off of the temperature as the flow increases.

Thanks Chris this explains it well, sounds logical
Masood you basically said it (I just re-read your earlier post).

 

[tamz]

It is usually more noticeable when the heating is on as the water is already as hot as 82º when the diverter operates.
Such is life.

 

[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