On Tue, 14 Sep 2004 16:42:09 +0100, 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?

Old wive's tale. With respect to the heating system, a combi
boiler is not really any different to a system boiler.
Microbore pipe is a form of pipe - there is nothing magic about it
either positively or negatively. What you do have to do, and should
do with any system design is to make sure that the bore of pipe used
is adequate to support the water flow rate required for each radiator.
If you look at the web site of the Copper Development Association,
there is a downloadable paper on how to calculate this.
Where 15mm pipe is used, it is possible to get away with some degree
of murder in that a reasonable length will support flow for about 6kW
of radiators. Similar lengths of microbore will support 1-2.5kW
which is normally one radiator. It is why microbore systems are
often done with manifolds but there is nothing to mandate that.
If an installing firm is giving you stories about microbore, it either
means that they don't know how to calculate for it or can't be
bothered. Either is a flag to consider shopping elsewhere.


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!)

Not exactly. it's more a function of the primary radiator circuit
being sealed and pressurised. Mainly combis are used on sealed
systems, but this is a separate issue.

If the system has old radiator valves, it is worth changing them for
good new ones like Pegler, anyway. Then with competent plumbing, you
won't have leaks.

In any case if you do, the situation is better than with open vented
systems because the volume of water is limited.


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?

No it isn't, but there is no reason not to use microbore. If the
space needs a lot of heat (i.e. 2.5kW or so) then it mivht imply two
radiators, but that's probably a good idea for heat distribution
anyway.

If you add radiators using 15mm and the rest of the system is
microbore, then in order to balance it, you will need to turn down the
lockshield valve on the new radiator(s) so that the flow becomes
comparable to microbore anyway - somewhat defeats the object.


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.

If the radiators are in good condition, there is absolutely no need to
replace them or the pipework. If the plumber is leading you on this
path, you are being taken for a ride.

It certainly does make sense to give the system a good clean by either
emptying and taking the radiators outside for a good flush, or by
using a powerflush machine. Don't get suckered into paying multiple
hundreds for this service - you can rent the machine and DIY it quite
cheaply.



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).

Is there a large and a small tank in the loft? If just a small one,
then it means that your HW system is pressurised.
Unless you go for a very large combi with production rate of at least
15litres per minute, you are going to get inferior results to it.
In that scenario, with young kids, I would try to keep the cylinder.
You could go for a combi to supply the kitchen requirements perhaps
and keep the cylinder to rapidly fill the bath and give good showers.


Otherwise, if you have a large tank, then the implication is that the
primary circuit is sealed already.


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.

This is reasonable as long as you make sure you insulate the pipes in
the garage.



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.?)

Yes, although I would check into what you have as a cylinder. If it's
pressurised and you have the space, I would keep it.


Are there issues with microbore pipes for CH?

None apart from those mentioned.


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


Yes, although bear in mind the balancing issue.


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?)???

I would check the radiators. Don't replace them for the sake of it.
If the system has had inhibitor in it, then they should be fine.

If pipework is changed, it would add a lot to the cost and is not
really necessary. There is no reason to change it to 15mm, even if
the radiators have had it. I would just take one off and have a
look. and see if there is much sludge. Careful because it stains.



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

Discounted Heating.


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?

If you have a high end combi at 22 litres per minute, it would imply
11 litres per shower at shower temperature in the winter.
I think that that is marginal.

Some form of storage of the hot water, or a heatbank would be a much
better choice if you think that you are going to use two showers
simultaneously and decently.


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.

You could look at a CPSU. This is a floor standing unit with some
water storage inside. Whether that is enough to last for the
duration of a shower where women are involved is another thing.



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.

If you are using microbore it is soft and you can thread it through
holes in the joists. With 15mm, you would have to line up the holes
perfectly and joint together short sections - not a good idea.

You can thread plastic through joist holes as well.

15mm copper is normally put in by notching the joists. If you can
avoid that then it's a Good Thing.





Any comments appreciated.

Chris.

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


..andy

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