In an earlier contribution to this discussion,
ChrisJ chris@youmustbekiddingifyouthinkimusingarealaddre ss.com wrote:

Background:
We live in a 3 bedroom house-on-a-hill. From the front it looks like
a bungalow. Once inside you go downstairs to the bedrooms. The (non
combi) boiler is in the garage on the front. The hot water tank is
downstairs in the airing cupboard (pumping hot water downstairs
against convection, hmm who thought of that one!). There is a header
tank in
the loft. The CH radiators are all on Microbore pipe. In the 6
months we have lived there the CH top up tank has required about 2-4
litres of water to top it up.
We had a combi boiler fitted in our last house (non condensing
£1000). Running a bath took long enough to undress our toddler and
clean his teeth and get him ready for bed, just about acceptable,
certainly not quick. Showers were great with a very good flow and
pressure, enough to make you want to stay in there longer.

Situation:
We want to extend into the loft and so need to lose the header tank.
We intend to have a bedroom, nursery and shower room in the loft as
well as a sizable landing (there's plenty of space due to the layout
of the house).

I've been told that microbore pipes and combi boilers don't mix.
Is this true?
I've also been told that if your system leaks a combi boiler will make
it worse because of the pressure. (Remember its needed toping up
already!) I would imagine the plumber who fits the new radiators in
the loft may want to avoid using microbore and so we would have both
standard and microbore radiators. Is this an issue?
I have no idea how much it would cost to replace all the existing CH
pipes (floor boards not sheets, at least in some rooms, no concrete
floors afaict) and radiators (3 bedrooms, kitchen, dining, lounge,
hall, lower floor hall, bathroom and washroom (10 rad in total) and
whether this would be a better solution than using the existing
microbore.
There will be a plumber coming in to add new rad.s to the loft anyway.
Existing CH and boiler is original fitted in 1992 when house was
built.

Things never seem to be simple though.

ATM my understanding is our DHW comes from the hot water tank in the
airing cupboard downstairs (there's also a pressure vessel in there
too, not sure what for though).

Siting the new boiler in the place of the old bolier in the garage
puts it just the other side of the garage wall to the kitchen sink.
The exiting bathroom is directly below that and the new shower room
will be just above and to the back of the house so the pipe runs to
the majority of the taps from the boiler will be short. (Our main
gripe with the combi boiler in the old house was that it took ages to
get any hot water through and then if you were just rinsing the pots
you had to leave the tap running at a fair lick in order for the
water not to go cold which
it would do if you turned the tap off.)
The current hot water pipes from the boiler go to the airing cupboard
and the DHW pipes come back from there. It seems silly to send the
water half way round the house so we are intending drilling through
the garage wall into the space where the washing machine currently is
in the kitchen and connecting the DHW feed from the new boiler to the
end of those pipes. I'm presuming all the DHW taps are on branches
off one run from the exisitng hot water tank so capping of the other
end at the hot water tank (with a drain plug) will mean we still get
hot water to the all the taps.

For the CH we were going to try and locate the existing loop and break
into it as close to the new boiler as possible and close the loop in
the airing cupboard where the hw tank is.

My questions a

Does the above sound feasible? (IE are there any glaringly obvious
mistakes I've made or things I've overlooked.?)

Are there issues with microbore pipes for CH?

Can you mix micro bore and standard piping in one CH system?

Considering the minimum job for the loft will be addition of 4
radiators, new boiler and the plumbing to (re)move the HW tank from
downstairs would replacing our existing 10 radiators on micro bore
with new radiators on standard pipe add significantly to the price
(+10%?, +50%?, double? triple?)???

Where is a good place to start looking on the net for prices for
boilers/rads etc.

We will then have the possibility of using 2 showers at the same time
(one a floor up from the boiler and one a floor down from the boiler
if than makes any difference). I'm presuming some sort of HW storage
tank (as opposed to a combi) will be required to allow both to be used
together. Is this the case (most "Combi yes/no?" threads so far have
only concerned one shower being used. Could a combi and mini tank be
used or would the best idea be to go for a mains pressure storage tank
solution?
As the back half of our garage has been converted to a store room
there is not a space issue with putting a hot water tank in there
with the boiler but I'd prefer it to be compact as possible.

If I was to redo the exising pipes and radiators myself what's the
best way of getting the pipes laid when their direction is across the
joists? (I want to minimise pulling up the floor. My intention
would be to
use copper pipes from the radiators into the under floor space and
plastic pipes to link these as I've been lead to believe that the
plastic pipes are easier to work with.

Any comments appreciated.

Chris.

PS Apologies for the long post (as the bishop said to the actress)



I'm struggling a bit to understand your current setup! You mention a header
tank in the attic. I assume that this is a large tank (50 gallons or so)
acting as the header tank for the hot water supply?

You also mention a top-up tank for the heating. Is this the *same* tank, or
are there two? If separate, how big is it, and where is it? You also mention
a pressure vessel. I am confused about this, because if the primary circuit
(i.e. the "sealed" circuit taking water from the boiler to the indirect coil
in the hot water cylinder and to the radiators) is non-vented (pressurised),
there won't be a top-up tank - but there will be a filling loop and pressure
gauge instead.

As I see it, you need, in addition to your existing requirements:

* Extra radiators in the attic at a height equal to or greater than your
existing primary circuit header tank (if any!). This means in my view that
the primary circuit will *have* to be pressurised.

* Hot water delivered to the attic for the new shower/washbasin. Whilst your
existing hot water flow is gravity driven from the header tank in the attic,
your new shower/washbasin cannot be gravity fed without raising this header
tank above roof level!

You thus need a mains pressure hot water system for your upstairs
requirements. There are three and a half possible ways of achieving this:
1. Replace the existing conventional boiler with a combi, and use it for all
your hot water requirements. This is a BAD solution - particularly if you
want to use both showers at the same time, or have lots of baths.
2A. Install a mains pressure hot water system, still heated by a (new or
existing) conventional boiler. This is fine as long as you've got adequate
mains pressure/flow - but needs expert installation because you'll have a
*lot* of hot water at high pressure.
2B. Install a heat-bank, heated by a conventional boiler - and obtain
instant mains-pressure hot water via a plate heat exchanger. Almost as good,
and can be DIY'd.
3. Install a combi boiler, but retain your existing stored hot water system.
Let the HW side of the combi provide just *part* of your hot water
requirements - I would suggest the kitchen sink and the new upstairs shower.
"Zone" the CH side of the combi so that it heats both the radiators *and*
the hot water cylinder. Use the hot water from the cylinder for the
downstairs bathroom/shower. You will still need a header tank for the hot
water, of course. You'll have lashings of hot water for baths, and no
problems running 2 showers at once because one will be using the stored hot
water and the other will use instant mains pressure hot water from the
combi.

I think 3 would be my preferred solution - but you'll still have to find
somewhere for the header tank. [Actually, you could go for a combination of
2B and 3 - getting the combi to heat the heat bank. You wouldn't then need a
header tank.]

I haven't attempted to answer the questions about micro-bore and pipe runs -
lets get the basic principles established first!
--
Cheers,
Set Square
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