Combi boiler with tanks

[Robert Prewett]

Hi there
I am really struggling to find room for a HWT in our retrofit project of a compact 1960's house. I'd like to go for a combi boiler with integral tanks as this will avoid the classic problem of someone getting a cold blast in the shower when someone opens the kitchen tap!
The glow worm 2 35 has 2 no 21 litre tanks.
However I remain sceptical about how this will effect the energy performance of the system:
1. Heat leakage from the tanks. Has anyone looked into this?
2. Will the relatively high rating of the boiler (8-30kW in case of Glow worm 2 35 boiler) be too high for our heating load (around 2.5kW). We will use normal wet radiator system with a decent volume of water to try to avoid boiler cycling.
Has anyone done the research on this or has any strong views?

Bob

[Anonymous]

Hi Bob,

In my view (though I cannot prove this categorically), if the combi boiler is sized right, the pipework is sized right, and the appliances (showers, sink taps) are suitably low flow, I don't see why this problem can't be avoided.

I expect the problem occurs when the draw of water is more than the combi can heat instantaneously. So if you can keep the draw off low, there shouldn't be an issue.

Particularly since a shower will be mixing down from 50deg to (say) 38deg, so the shower mixer valve should be able to cope with a small reduction in water temp off the boiler by simply allowing more hot water through.

We have a combi in our house and don't have a problem. I might have a play and see if I can get the problem to occur with a combination of outlets being on at the same time as the shower.

Chris

[Derek Monckton]

Combi storage is very much for higher demand households. Vaillant produce the ecoTEC plus 937 with 15 litres in built storage.
Generally storage comes with high output, the Vaillant 937 delivers up to 15 litres/min with a 37kW rating. I have recently specified one of these for a client – a large 4/5 bed house so it fits the bill. For a smaller property the combi should not give cold blasts, it can only reduce the flow and heat exchange to all open outlets if the demand exceeds heat exchange. Typically a 28/30kW DHW rated boiler should work well enough in a smaller property.
I have a combi in my house and that's a 1940s 2 bed semi with rads, I don't get cold draw off of water with bath and kitchen going together.
So it is on demand hot water, where as storage needs to be kept warm and the boiler will be on/off a lot more to satisfy that.
Vaillant are at the top end of efficiency, but they do have this keep warm function which cycles all day just to keep a small amount ready for that tap draw off, apparently very popular in Germany, but I can't work out how to turn that off.
Best thing is do the calculations for x litres of stored water working between a 5 or 10 deg temp variation between the longest interval for drawing off -overnight or when you are out all day for example. Are those extra kWh worth it?

Derek Monckton

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