On 2007-12-22 10:43:32 +0000, "neverwas" said:

No open vent boilers were best buys but the top of list was the
Vaillant and at the bottom was the Potterton promax

From the online version they appear to have made no comment on or
allowance for (i) the effect of the return temperature on efficiency or
(ii) the cost of upgrading radiators in order to make it practicable to
reduce the return temperature. Is it naughty of them to show potential
savings on heating bills based on 90% efficiency and costs for boilers
alone?

For me (with 20+ year old albeit unvented system and separate multipoint
water heater) in a small Victorian terrace with suspended floors the
need to upgrade radiators in order to get anywhere near 90% has been a
big factor in doing nowt. Or (as is increasingly likely) have I
misunderstood what wd be involved?

The issue is a comparative one.

Increasing radiator sizes does allow the return temperature to be
reduced for a given heat output. However in terms of what is needed
and achieved, there are a number of factors:

- Assuming that the radiators were only just adequately sized for 82/70
conventional operation in coldest weather, then the boiler will still
run at relatively low temperatures during the 6-8 months of the year
when only small amounts of heat are needed - i.e. max operation is only
usually for 1-2 months of the year.

- Some or all of the radiators may well be oversized anyway.

I found that I was able to redesign for 70/50 operation by leaving most
radiators alone, moving three to new positions and replacing those.

In any case, even if the 90% figures are not met if the existing boiler
is one of the older wall mounted natural ventilation types like I had,
the starting point was 65%, and there would easily be an improvement in
that.