Minimum DHW output of combi boiler

[Tulip]

Hi,

I have recently decided to ditch my electric storage heater and immersion heater and switch to a gas central heating system, as I had in Holland. Not sure what boilers are available, I have searched the WEB for various makes of (condensing?) combi boilers. My problem with some of the boilers is that the minimum output might be too high, quite the opposite from the problems most people seem to have and I couldn't find a topic discussing my issue.

The point is that I am single and live in a small 2 bedroom mid-terrace house and don't need a lot of power from the boiler . Being single it is very unlikely that two hot water taps will be running at the same time. I am always having a shower instead of a bath and this uses only 5 liters/minute water at ~38ºC. So the low power consumption suites a combi boiler very well.

However I have a problem with the minimum power output. The principle is the same for all the boilers, but take for example the Vaillant ecoMAX 824. This boiler can supply DHW at 9.4 liter/minute for a dT=35ºC, using a 23kW. I need, 5 l/min with let's say a dT=25ºC. My shower therefore requires only 38% of the full power of the Vaillant, or 8.7 kW. The specs of the ecoMAX 824 specify that the minimum output to CH is 11.6 kW, so I assume that the minimum output for DHW is also 11.6 kW. This is much higher than the power that I need for my shower.

So I am confused now and have 2 questions :
1) Am I right that in general the minimum output for DHW is the same as CH?
2) Please, could some one explain to me what would happen when I use less power during my shower than the minimum output power of the boiler? I wouldn't like to buy a boiler that keeps switching the burner on and off during my shower.

Any help would be greatly appreciated .

 

[ChrisR]

1) Am I right that in general the minimum output for DHW is the same as CH?

Yes and No! Though the CH output should be adjusted at installation to suit the property, you can't turn down the HW output manually. The boiler will turn down the wick if the tap water gets too hot though.

2) Please, could some one explain to me what would happen when I use less power during my shower than the minimum output power of the boiler? I wouldn't like to buy a boiler that keeps switching the burner on and off during my shower.

You CAN reduce the HW temperature withthe knob on the front, but if your flow rate is such that the boiler is required to give less than about the lower limit of 11.6kW the boiler has to switch off and on. It will do it quite quickly, such that with a thermostatic shower valve you shouldn't notice it. If sludge in the system gets significant though, the sensor & heat exchangers become somewhat insulated and you do start to notice the retarded performance. This normally happens to some extent even in treated systems, it seems to me.

The question is begged - why do you want a "slower" shower?

Delta T 25Celcius won't be much good - in the winter the mains can be 5C!

 

[PEDANTICVINDICTIVEMAN]

Most modern (decent) combi boilers modulate the gas rate at the burner to suit before tottaly switching on and off.
I get the feeling this a joke posting by someone.

 

[ChrisR]

PVM - I 've never measured it but I imagine they modulate down to about the same level they can go to on CH - which is what I meant above. You ever checked it?

Even if it IS a joke it has raised an interesting point.

 

[Tulip]

Dear ChrisR,

Thank you very much for your very quick reply, it has helped a lot. I must say that I didn't have a problem with low DHW in Holland (I don't know anything about that boiler) and was just alarmed ready the specs of the boilers. I didn't know that the boiler could switch the burner on and off so quickly . So the minimum output is not as much a problem as I first thought. Saying that, I understand that it is still worth considering the minimum output of the boiler.

The point is that all manufacturers do boilers with about the same maximum output, which you can choose from quite a range. However, for a given manufacturer, the minimum output is always the same fraction of the maximum power. So if I choose a boiler with a 1.5 times higher maximum output, the minimum output will also be 1.5 times higher. The ratio between minimum and maximum output power varies a lot (a factor of 2 or more) from manufacturer to manufacturer for boilers with the same maximum output. The ratio between the maximum and minimum output could therefore be another selection criterium.

Reading you comments, I might consider a thermostatic shower valve.

The question is begged - why do you want a "slower" shower?

I measured the maximum flow rate on my shower (when I have a comfortable water temperature) and it turned out to be 5 l/min. This is enough for me, although a little bit higher would not harm. The low flow rate could be due to a low hot water pressure on my vented system, but at least it keeps the bills low.

Delta T 25Celcius won't be much good - in the winter the mains can be 5C!

I do agree that a delta T of 25ºC in the winter is not enough, but all 24 kW boilers can deliver a 9.8 l/min water at delta T=35ºC giving water of 40ºC. This is still about twice the flow I need, so the maximum output is therefore never a problem. At a nice and sunny spell (like it is now here in the South) mains water might rise and the minimum output might be too how. You see, I want to buy a system that is comfortable at any time of the year .

Thanks a lot again ChrisR.

--------------------------------------------------------------------------------------

Dear PEDANTICVINDICTIVEMAN,

Most modern (decent) combi boilers modulate the gas rate at the burner to suit before tottaly switching on and off.
I get the feeling this a joke posting by someone.

How can you believe that ??
Have you read the boiler specs and seen the minimum output spec?? I cannot call a minimum output of 11kW (or most boilers go to 7 or 9 kW) a modulation that adjusts the gas flow at the burner to suit, if you require only a few kW. Maybe that you think a 11kW - 24 kW modulation is a "full modulation". A boiler doesn't modulate the gas as much as your gas cooker. The only reason why I think they can call it fully modulating is because they fully modulate the on/off times of the burner and not the gas flow .

Greetings,

Tulip

 

[PEDANTICVINDICTIVEMAN]

Dont think it will matter soon as from next april everyone will be either packing up or working illegally, I should get an electric shower and electric storage heaters as the only legal gas fitters will be too expensive for anyone to employ, there will however be plenty of cowboys as always.

 

[bripl]

most combi boilers that modulate drop down to min fire approx 1.1mb it will be stated in mi. worc for example approx 1.5mb,ferroli 2.5 .these are the min fire rates so on hot water they drop to these values as req .if you have max hot water=max gas pressure so min hot water draw say 3 litres=min gas hope this helps