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Replacement boiler selection
QUESTION:I've been following the current thread on "Boiler Trouble" which has
meandered into several tracks so thought it sensible to have a more specific
one. I suspect that a lot of people have the same things to figure out so
hopefully it is worthwhile.
Taking all the comments into account regarding condensing boilers I'd like
to apply them to my forthcoming boiler replacement, to make sure I've
understood and also to place the various factors into economic rather than
idealistic perspective.
Having designed and installed a complete CH system in the past, I am quite
aware of the principles of heat requirements for rooms, radiator sizings,
flow and return temperatures etc.
The existing boiler was new with the house, 15 years ago and is a Glow Worm
of that era with 22kW peak rating and according to SEDBUK an efficiency
rating of 65% - a good candidate for replacement since apart from that I
guess it is nearing the end of its design life. I will probably make a
replacement next spring.
The remainder of the system is entirely conventional, being open vented,
diverter valve, DHW via indirect cylinder (also vented), and the radiators
connected via 8mm microbore. The house walls are cavity insulated.
Other controls are a night setback thermostat timer for the CH, normally set
to 14C at night and typically 21C daytime; and a cylinder stat set to 60C.
All radiators apart from the one in the area where the thermostat is located
have thermostatic valves which have worked fine for more than 10 years.
I checked the radiator types and sizings for the rooms together with the
heat losses and it appears that the standard assumptions of 80C flow have
probably been used, with the radiators sized about 10-15% above the
requirement for maintaining room temperatures at -1C outside.
As a whole, the system works well, rooms are at or about design temperature,
and remain warm - i.e. it has to be very cold outside before the CH starts
running at night. Very little apart from one replacement pump and two
diverter valves has gone wrong with the system - touch wood. It was filled
with Fernox MB-1 on day 1 and has been flushed and had that replenished when
works have been done.
From a replacement perspective I am considering some areas and almost
certainly not others.
1) I am not going to completely rip out the system and start again, so
combis and their ilk are out, as far as I can see through needing wide bore
gas and water supplies and fairly significant changes in other areas. I am
also not convinced that they have enough hot water throughput when the
supply water is cold unless one has an enormous one.
2) I am not going to replace radiators (except perhaps one or two should it
be necessary for heat output reasons)
and certainly not the microbore plumbing which would require major upheaval
to change.
Essentially, I certainly want to look at a boiler replacement and possibly a
change to the DHW. As a minimum, considering its age, I might as well
replace the DHW cylinder while I am at it.
As what appears to be a good example, I'll pick a Keston boiler or
equivalent as the starting point.
Considering the above and the other thread, I come so far to the following
conclusions:
a) If I run the system as now, with 80C flow and approx 70C return when
fully hot, according to Keston's performance graphs I should get about 85%
efficiency compared with the 65% of today. I suspect that is the greatest
saving before getting into condensing/non-condensing discussions.
b) In steady-state operation, I am not going to get very far up the steeper
slope of higher efficiency below the magic 52C return temperature for two
reasons. The first is that if I lower the flow temperature too much, I
will not achieve sufficient heat output from the radiators on colder days.
The second is that the temperature would not be adequate to heat the DHW
cylinder unless I drop the temperature requirement from 60C to say 50C which
I am reluctant to do in case there is then an inadequate supply in colder
weather.
c) I am inclined to think that the boiler will spend quite a bit of time in
warm up mode - the current one does, and therefore the condensing mode
savings really happen during those times. I can't see a way during
steady state operation that condensing mode happens to any great extent if
one is heating hot water as well.
The other area to consider is what to do about DHW heating. So far I have
got to the point that probably a rapid recovery cylinder (still open vented)
is a good idea, and probably a larger capacity if I have to reduce the
storage temperature.
There are other practicalities such as a take off for a shower pump
currently via Surrey flange, but I don't regard this as a big deal.
I am not sure whether making either or both of the CH or DHW systems
pressurised has any merit but would be interested in any thoughts. I don't
mind a certain amount of plumbing change, but I am not going to do wholesale
increases of mains water supply etc.
The conclusion so far, therefore, is that I get the greatest benefit out of
replacing the boiler (basic efficiency improvement, non condensing mode) and
that additional effects are fairly secondary.
ANSWER: An outside weather compensator with the curve set so that the "return" temp
modulates according to the outside temp. OR
An outside weather compensator with the curve set so that the "flow" temp
modulates according to the outside temp - this will require that the system
is balanced to the maximum, or as neat to, as the boiler makers recommend.
Always ring their technical dept, e.g, the Ideal Minimiser states 11C
differential on flow and return (same as a non-condenser) yet the say that
15C will do fine. (they aiming the boiler at the replacement market and
covering ignorance on condensers).
Have a priority sytem (I think you have) that diverts all the boilers heat
to a quick recovery cylinder. Initially the return temp from coile will be
very low and promote condsensing.
Can you split the system easily to zone off upstairs and downstairs? this
will give great savings (not heating unoccupied space). When hearting say
the upstairs, before going to bed, the return tewmp will be low (the
efficient warm-up mode)
The extra efficiency a condenser will give may pay dividends when the fuel
prices rise. The Middle East crisis right now may mean that oil will ramp up
soon. These countries are highly volatile. Also a condenser is not that much
more expensive these days that the best non-condensers. It appears foolish
not to take the condenser route.
As I see it:
1. Get a quick recovery cylinder (priority system)
2. Modulating condensing boiler
zone off the upstairs using a timeclock and thermostat (Honeywell CM 67 with
optimisation)
3. Outside weather compensator.
You will then have a very efficient and cheap to run system.
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