Boiler help!

FreeBear

ALacoste said:

Would like combi so hot water on demand especially with kids. We don't have a tank.

Get yourself a 5l or 10l bucket and time it to see how quickly it fills from the cold tap on a bath.

Do the same if you have an outside tap.

Do not bother with the kitchen tap, especially if it is a monoblock as the pipes will restrict the flow.

Once you can tell us how long a bucket takes to fill, we can work out the flow rate and advise what boiler power would be appropriate.

ALacoste

So I have filled a 14L bucket and it took 1 min and 14 sec to fill up. What does this tell me?

FreeBear

ALacoste said:

So I have filled a 14L bucket and it took 1 min and 14 sec to fill up. What does this tell me?

You have a cold water flow rate of ~11.4l/min. Most 30KW combi boilers will be able to handle that flow rate with a bit of room to spare.

I have a flow rate of around 10l/min with a Viessmann 050 30KW combi. Could have got away with the 25KW version, but I want a bit more capacity for the day I fit a booster pump. Looking at Viessmann Direct, for some perverse reason, the 35KW model is just £999 compared to £1078.80 for the 25KW and £2385.60 for the 30KW... Crazy.

I would maintain that a 30KW would be more than adequate, but if a 35KW is cheaper (and still modulates down real low), there is nothing to lose by going bigger.

ALacoste

I am leaning towards Vaillant ecoFIT pure 835 35kW Combi Boiler or Ideal Logic Max C35KW combi.. i heard that Ideal Logic uses more gas, does anyone know if this is true?

FreeBear

ALacoste said:

I am leaning towards Vaillant ecoFIT pure 835 35kW Combi Boiler or Ideal Logic Max C35KW combi.. i heard that Ideal Logic uses more gas, does anyone know if this is true?

Both have a minimum output of around 7.5KW, so gas consumption will be very similar - You will never be running at full load with the central heating. I doubt you will need 7.5KW for heating much of the time anyway, so both of these boilers are likely to end up short cycling. Compared to some of the other boilers out there, a modulation range of 7.5-26.7KW (Vaillant at 50/30°C) 7.1-24KW(Ideal) is pretty poor. e.g. The WB Greenstar 8000 35KW will go from 3.5KW up to 35KW.

For heating, the maximum output is not important, what is, is the minimum - Although my boiler will pump out a maximum of ~24KW, most of the time, it is running at 3-6KW (and no short cycling). Short cycling is inefficient, and also increases wear & tear on the boiler.