standing heat loss across cylinders/thermal stores

[Jeremy]

I'm trying to spec out a central heating solution, and looking at either:

 

(a) 250l unvented direct cylinder

(b) 450l thermal store OR

(c) combi-boiler with integrated store (like the Viessmann Vitodens 111/222)

 

These vary quite widely in price,  so a key deciding factor for me is how efficient the upgrade to a thermal store or the combi will actually be compared to the unvented cylinder. I've found it surprisingly difficult to chase down explicit ratings for standing heat loss on thermal stores. For the unvented cylinders, I'm looking at something like a Telford Tempest Indirect Plus at 250l which they suggest has a standing heat loss of 1.34kWh/24hrs. I'm planning to steal an idea from @Jeremy Harris to have the manufacturer add additional external insulation and see if I can get that down to 1kWh or so.

 

It stands to reason that someone else has already worked this out for different models of thermal store, but I can't seem to find it "out there" - can anyone advise?

[SteamyTea]

The problem is the standards used.

If I remember correctly, a standard test just looks at the standing losses over a short period of time.

You can estimate the losses over any time period by calculating the losses in the same way you would a walk i.e. the conductivity, times the surface area, times the mean temperature differences.

[pdf27]

Another issue is applying the correct boiler efficiency - because a thermal store heats the water indirectly (you don't store the water you're using, but rather it's heated by a heat exchanger within the tank) then you typically need to store the water at a higher temperature. Since boiler efficiency is a function of flue gas temperature (itself a function of return temperature), running a cylinder hot will reduce the boiler efficiency even before you account for the increased losses of it being hotter.

[joe90]

Losses are also directly related to water temperature. My Telford DHW cylinder has little losses (no figures but SWMBO says the airing cupboard is not warm enough ?) as I heat my DHW to 48’ only, but the cylinder is larger as I don’t blend with cold as much.

[JFDIY]

It needs to be considered in context surely, for example my sister has an unvented cylinder in an outbuilding, which the losses are heat that has gone forever.

 

If the cylinder or boiler etc are within the heated envelope of the building then you can argue that they aren't losses at all, (unless the windows are open)  because the heat is retained inside the property.

 

As mentioned above it needs to be in context, the more efficient the house, the more you may consider matters I guess.

[joe90]

5 minutes ago, JFDIY said:

If the cylinder or boiler etc are within the heated envelope of the building then you can argue that they aren't losses at all, (unless the windows are open)  because the heat is retained inside the property.

I have said this many times, it’s only wasted in summer when house heating is not required and the windows may be open .

[SteamyTea]

The only thing about a cylinder adding energy to a house in winter, is that it is an uncontrolled source and may be adding it into an area you do not want.

 

What you can do is fit it into a well insulated airing cupboard, then put one of the MVHR extractions in there.

[joe90]

19 minutes ago, SteamyTea said:

The only thing about a cylinder adding energy to a house in winter, is that it is an uncontrolled source and may be adding it into an area you do not want.

my airing cupboard is central to the first floor which has no heating apart from towel rads in the bathrooms so any heat lost from the cylinder helps the bedrooms a little (good planning ??..).