The Natural Philosopher wrote:
....
So the minimum ROI is
10*.3*2.5/1000*365*24/270 = 2.4%
(assumes you're replacing a perfectly good SG unit and heating with
gas at 3p per unit).


Typo in the above: it should say .03 not .3. The answer is still 2.4%.


The cheaper the fuel the LESS the ROI is..so I am not sure how you got
more..

Because there are three error factors increasing the ROI from the 1% you
originally calculated and only one (the fuel cost) reducing it. (i) temp
diff, (ii) cost of DG, (iii) you said 8% of the heat loss is through the
window when you should have said 14% = 5*2.5/(3.5*25). The net effect
of all these corrections makes it a 2.4% ROI as a minimum case.


To work out the maximum ROI we need to know the cost of SG, which I
don't. Taking it to be £135/m^2 we get
10*.1*2.5/1000*365*24/135 = 16%
(assumes you're building from new or the existing SG unit needs
replacing; assumes heating with electricity).


Yup. I never was actually talking about installing SG from new..you only
do that - as we did - out of an attack of aesthetics. Double glazing
cannot be incorporated into lead lights and applied 'glazing bars' looks
crap. So almost certainly your SG units will be more expensive, cos they
are hand made specials.

In that case the benefit of DG could be even more than 16% of course.



There are also benefits in terms of quality of heat. If you just
replace heat lost from the windows with central heating then you get
temperature differentials - colder nearer the floor than at head level
etc - which is not as snug as uniform heat (in my opinion).

And there are benefits of DG in terms of extra sound insulation too.


Actually, we have high quality sealed SG here, installed from new, and I
have to say the noise reductions is similar. It seems to be the sealing
that is the key, rather than the DG.

Fair enough. If you are comparing with high quality SG which you already
have and are not costing then of course the benefits of DG will be smaller.



Alex

I am not in general trying to say what you seem to think I am..that we
shouldn't be putting DG or even TG in new builds. Of course we should
bne. The *opportunity* cost is low enough, and the gains significant
enough..to make it worth while.

But you said "it's very hard to see why double glazing is so insisted
on". I was just trying to explain why I think it makes a lot of sense in
many cases.


My real thrust was at those people who have bought a nice solid
(literally) walled Victorian property, find it chilly, and costing a lot
to make snug, and instantly go out and fit DG.

The real point is that after whacking in loft insulation and SEALING it
against draughts in the loft,and putting weatherstrip on the windows and
doors, the very next think is to line the walls.

And possibly the floor.

Yes yes, I am completely sold on all that. You are preaching to the
converted. I was just talking about DG and I wanted to fix the calculation.


There are HUGE potential gains to be had here, from not all that much
thickness of insulation. 6" of rockwool or celotex is easy to install
under a suspended floor for example. 2" of celotex on the outside walls
is not going to break the bank. And you could be looking at 70% or more
improvement in energy and bills.

Once that is done, boiler efficiency is not so important, and there is
always a moot point as to how much MORE energy the new boiler takes to
make.

Attention to electrical stuff is fairly trivial in terms of planet
saving. Far better gains can be had by switching off huge amounts of
public lighting between say 2 am and 6 a.m. However CFL bulbs are cheap
enough to pay for themselves in a few months..I use them because they
don't blow up every five minutes.

Likewise teenagers who spend 10-15 minutes in the shower are not saving
water or energy vis a vis a bath, either.


Alex