Combi water getting cold

[TD]

We've just moved into a new flat. It's a new-build, fitted with a Geminox THI-5-25-M75 boiler. I understand that this model is a combi with a 75 litre tank fitted as well - I'm not up on boilers but I assume that it tries to offer the "best of both worlds"? It's also got the Geminox QAA73/REG73 control panel and outside air temperature sensor.

Anyway, the problem is that when I run a bath, it's hot enough for the first third to half, then it gets noticeably cooler and I have to switch the water off, and leave it to heat up again before running the rest of the bath. Needless to say, this is rather annoying.

I thought that combis were supposed to limit the flow in order to keep the temperature up, but according to the status display on the REG73, the "DHW actual" temperature is 50 at the start (as set), but then drops to below 30 after I've run a lot of the bath. Which would explain the situation. At that point, the "Boiler actual" temperature goes up to around 70.

Geminox tell me different things depending on who I speak to, but I think something's not right, so I'd like to ask here before I get their engineer to come down, so that I can't be fobbed off. Is something broken here, or is this normal behaviour?

The bath has what looks like a thermostatic valve, FWIW, and it's set so that when it starts running, the temperature is what I want.

[John Rumm]

Alas no, most combis don't have any automatic flow rate control. The output temperature setting is a maximum limit and not a set temperature. Once it has run out of preheated stored water, it can only deliver hot water at a rate governed by the power of the boiler. (which for a 25kW boiler is only around 10 lpm if the incoming water is cold)

It does not sound unusual...

An experiment worth trying is to run the bath with less water flow. It will take longer, but you give the boiler more chance to keep pace with the demand.

[newshound]

A problem I have had in the past is that if you throttle the water flow down too much, the gas burner cuts down/out on high primary water temperature. Well worth listening to the burner when doing this experiment. Don't confuse the sound of the exhaust fan (which will stay on longer) with the sound of the burner. You may be able to see the flame by opening the front cover, or feel the warmth through the casing.

In my case the solution to this problem was to put a pressure regulator on the cold water feed to the combi, then set the pressure here so that with taps open to full flow you get sufficient temperature.

[TD]

Thanks for the reply, and sorry for the delay in my reply. I think that running the bath with less flow did help, although I need to try it with a thermometer to be 100% sure (currently 75%).

But this boiler has a 75 litre tank built into it! Surely that is designed as some sort of buffer, so that when you start running the bath, it delivers 75 litres of hot water, and heats up more all the time? Let's assume a bath is

110 litres (I'm not sure on that, but I was advised it's thereabouts). This should mean that it only needs to heat up another 35 litres, which according to your "10 lpm" statistic, should take 3.5 minutes, much less than the time to run a bath.

According to

"The THl M75 is available in 5-25 Kw. The combined boiler and hot water unit is the ideal solution for two bathroom properties. The inbuilt 75 litre un-vented hot water, fast recovery tank lifts from 10°C to 60°C in only 10 minutes on the 5-25 model and provides an amazing 160 litres of hot water in the first 10 minutes of draw off and 712 litres of hot water in one hour."

So, do I not understand something? 160 litres of hot water should be more than enough for a bath, no? Surely something is faulty?

[John Rumm]

That's a bit on the conservative side for a bath unless it is very small or you only fill it very little. 140l might be closer. A little depends on how hot the stored water is. If its very hot then it will blend down to more bath temperature water than if its only 60 say.

Yup 160L ought to be enough for a bath - assuming the boiler is kicking in as soon as you start drawing water from the store. Depending on store temp (and cold water temp) you might get 100L of that out of the store, and 60 from the boiler directly (although note it won't be able to recharge the store if you are using all its output for HW).

How long does it take to fill the bath?

[Doctor Drivel]

Some do. The Alpha CD 50 does.

[Doctor Drivel]

Something is wrong. It can dump 75 litres immediately from the cylinder. The burner gives about 9-10 litres/min. So in 10 mins the burner can deliver 90 litres plus the 75 = 165. About right.

This combi is not a common product and Geminox are a quality French make. The docs are all on the web site. I haven't looked at them, but it may have a flow restrictor to reduce flow when the cylinder temp drops.

[Doctor Drivel]

Pressure regulator? Find out the litres/min of the combi and fit the correct sized flow regulator in the cold inlet. Costs about £5.

[John Rumm]

The relevance of that being what exactly?

[Doctor Drivel]

The relevance is that the Alpha CD 50 does.

[John Rumm]

So completely irrelevant then.

[Doctor Drivel]

I the eyes of a know-it-all DIY imbecile I suppose, yes.

[John Rumm]

So if its irrelevant in your eyes as well, why mention it?

[Doctor Drivel]

You really don't know.