Baxi 105 HE hot water

Horizon81

I have a Baxi 105HE combi boiler. Handbook states that when cold, pressure should be between 0.5 and 1.0 bar. Mine is at 0.9 when cold. When operating (hot taps open) it is supposed to be between 1.0 and 2.0 bars. Mine is at 1.1 bars.

Problem is, that when the hot tap is fully open (e.g. when running a bath) the water gets hot but not really hot. When the tap is only opened halfway, the hot water does get hotter. I'd like it to be this hot when the tap is fully open!

Is it the pressure which is causing this or something else? I know the pressure is within tolerance but I'd expect to see it at more like 1.5 bars when the hot taps are on. Would increasing the pressure make any difference?

ormus

no. you would need a bigger boiler.
the burner/heater can only heat so much water per second/minute. up to its max designed power rating.
if you force more water through the pipe (LPM or GPM) itll end up colder.

Horizon81

Thanks. There is a temperature scale on the boiler which goes up to 60 degrees when the hot tap is fully open, but increases to 70 degrees when the tap is only half open. I've watched it and as soon as I open the hot tap fully it drops from 70 to 60 degrees. If I then close the tap halfway again it shoots up to 70 degrees. Why can't it stay at 70 with the tap fully open? Does the rate of mains cold water going into the boiler influence what heat the burner achieves?

ormus

yes. slower water flow = hotter water.

ie
the burner has more time to heat the water, inside the heat exchanger.

penrhyn

Also, the temperature of the water entering the boiler also affects its performance, during the winter the cold water supply will be a lower temperature.

Electric showers suffer in this way too, faster the flow lower the temperature.

BTW have you tried turning up the hot water temperature control on the boiler a bit?

fred7777

One other thing to note is that usually the water pressure gauge on a combi is for the central heating water not the hot water or mains pressure.

So the pressure gauge isn't necessarily telling you anything about the hot water pressure.

When you turn on a tap for hot water the boiler starts heating the central heating water but rather than send it around the radiators it pushes it through a small heat exchanger which heats the incoming mains water before it reaches the taps.

As more tap water goes through the boiler the boiler burns more gas but a some point the burner will reach maximum and any increase in water flow will mean more water but at a lower temp.

The problem isn't due to pressure, how old is the boiler, how fast is the bath filling when the water is hot enough & when was the boiler & heating last serviced?

Horizon81

penrhyn - yes I've turned the gauge for the hot water up to max but it still behaves the same. I'd read about the external temperature having a effect on the mains water but thought it was pretty consistent throughout the year (even in summer my cold water is really really cold).

When the central heating is on, the temp guage reaches 80 degrees (which is the max) and then when I turn the hot tap on full the water comes out boiling, which is nice.

Boiler is 4 years old, last serviced about 15 months ago. By the sounds of it though, it's working it should. I'll see how long it takes to fill a 'hot' bath later.

ormus

my vaillant condensing combi, takes ages to fill a hot bath.
(think its a pro 24).

24kw.

gas4you

Yours is a 30kw combi. It should give you 12 l/m at a 35c temp rise of the incoming cold water.

Let the heating go off and the rads/pipes under the boiler go cold, then run a hot tap, if the 22mm pipe under the boiler on the left (heating flow) gets hot, then the diverter valve is stuck mid position and you won't get the full power from the boiler heating the hw.