That best practice is about as much use as a chocolate teapot Tony.
System Boiler Size
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That best practice is about as much use as a chocolate teapot Tony.
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24-26kW.
My son's family is the same size there is a 24kW boiler feeding a 125 litre unvented cylinder (Ideal Istor!!) and they have never complained about heating or hot water.
I agree that water heating times will have to be adjusted for specific circumstances, but allowing a 35kW boiler just because the heat loss is 15kW and the cylinder can take 20kW is not sensible.
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Hmmm
So what would you allow if the boiler was fitted with weather comp, so the HW and CH can never be heated at the same time? (Bit difficult heating a cylinder to 60C when the flow temperature is 40C.)
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Google being your best friend DH.
Thanks for all the replies. I have two young kids and a wife. We are likely to be using the baths concurrently. We have three bathrooms in the house and I envisage two of them being used concurrently. Is there an issue with going with a 30KW boiler knowing that it may be over spec'd?
I wasn't aware of this; that may explain how a 120 litre cylinder can supply hot water for six people with an en-suite shower and a bath.
Please explain how the istor differs from a normal unvented cylinder.
My son wants to replace the istor at is beginning to be unreliable. His plumber has suggested a 25kW system boiler and a 120 litre unvented cylinder. I was surprised at this as a cylinder nearer 250 litres seems to be the normal recommendation.
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The cylinder is constantly kept warm by the boiler and has a recovery time of 14-20 minutes depending on which version and will go from 15deg to 70max heat in this time.
It does seem to be a bit quicker than a lot of standard unvented cylinders when it actually works!!!
It does seem to be a bit quicker than a lot of standard unvented cylinders when it actually works!!!
Presumably that's achieved by having a HW thermostat with a very small differential and the HW timed to be on constantly.
What about an appropriate size for a replacement cylinder?
Storing the water at 70 and blending it down in the appliance might have something to do with it too.
I would size the cylinder at 4 people x 50 litres + 50 litres = 250 litres
But I would size the boiler at the house heat loss plus 2 kW and timed to be on for 30 min before the house heating in the morning and either on all day or at least for regular short periods.
I would never put in a 30 kW boiler in a house with a heat loss of less than 24 kW.
There is no point and the minimum power output of the boiler is likely to be an embarrassment and cause short cycling. [ Only exception could be a model of Vokera with a 10:1 turndown ratio or any similar model boiler but most are only about 4:1 max ].
Tony
Excellent advice agile. To many times a heat only boiler is ridiculously oversized. Our 150m2 4 bed house with bathroom and shower room has a 10.5kw heat loss and a 15kw system boiler paired with a 250 litre cylinder. We've never had any problems with it.
OP may be worth looking at a Worcester cylinder to go with your Worcester boiler as you can get a longer guarantee on the boiler if fitted through their network of installers.
OP may be worth looking at a Worcester cylinder to go with your Worcester boiler as you can get a longer guarantee on the boiler if fitted through their network of installers.
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