DIYbanter - View Single Post - Please help

On 18 Aug 2003 00:19:23 -0700, (Rebecca) wrote:

OK, I am so fed up with different plumbers and all their different
opinions. Can any of you help?

I have a very simple question and I hope its a straight forward
answer. I will miss out on the problem and suggested solutions and
hope this can be answered without the details.

Q Would you be happy keeping 8mm microbore thoughout your house (with
flow problems) or would you upgrade all to 15mm?

I realise that you may want more details but I'm not gonna give them.
This is the most basic difference 3 plumbers have told me.

No. 1 wants to change all pipes to 15mm
No. 2 wants to change only the 6mm pipe to 15mm
No. 3 wants to keep the 8mm

Please help. Its doing my head in!

Many thanks
Rebecca

I have 8mm pipe throughout my house, Rebecca, and there would be no
need to change it.

The question can't be comprehensively answered without some level of
background, and some detail of explanation is needed to make the right
decision but I'll give you the following::

- Systems using microbore are not any different in principle to any
other. The typical implementation difference is that systems using
22mm and then 15mm tube to radiators are connected like a tree with
the 22mm forming the trunk and the 15mm the branches. In microbore
installations it is typical to have one or more manifolds which are
distribution points attached to the 22mm pipes and then 8mm (or
sometimes 6mm or 10mm) are run from those to the radiators. In effect
you might connect our or more radiators to a single point. There is
no reason that manifolds have to be used, however - a microbore system
could be designed like a tree.

- In any system, the amount of heat that can be delivered from a
radiator is determined by its size and the room temperature (plus some
other factors), but assumes that a certain flow rate of water is going
through. The larger and higher output a radiator is, the more water
flow is required. Tube of any kind has a restricting effect on the
flow of water. The longer the length, and the smaller the diameter,
the greater the restriction. To some extent, the pump makes up for
this, but there is a limit and a properly designed system should have
the flow speeds through all pipes under 1.5m/sec. Think of it like a
stream which flows quickly and a river, with greater dimensions
flowing more slowly.

- You can relate the pipe diameters as a result, to the amount of
heat. In a proper design, the heating engineer will have worked out
the pipe sizes based on the amount of heat required to be delivered
through them - there are tables to do this. However, as a rule of
thumb, 15mm tube in an average house can carry up to 6kW of heat and
8mm up to about 2.5kW. 6mm is about 1.5kW. These numbers are
reduced if the lengths are overly great. In the case of microbore
and radiators connected to a manifold, each can be considered
separately and you can work out immediately whether pipework is
adequate. In the case of 15mm connected as a tree with more than one
radiator on a run of it, you add the requirements of the radiators.

- You can work out radiator heat outputs by measuring them and looking
up the manufacturer's data sheets. The nominal output in the tables,
which comes from a European measuring standard needs to be scaled
downwards by multiplying by 0.9 for most UK heating systems.

- In a practical case where you already have a system, it may be
working perfectly well. If it isn't, the usual problem is that one
or more radiators are not getting hot enough or at all.
There can be three causes of this:-

a) The system wasn't designed properly and there are one or more
radiators that are too large for the size of pipe. The only fix for
this is to put in larger pipes or to reduce the radiator size, perhaps
adding a second radiator connected back to a manifold.

b) The system isn't properly balanced. This procedure involves
adjusting the lockshield valves on the radiators (these are the ones
at the opposite end to the ones you turn) so that the flow of water
through each radiator is adjusted to what is required by it. Doing
this properly is time consuming and requires a thermometer to measure
the temperature at each end of a radiator. You then go round the
house adjusting a little at a time until the correct temperature drop
of 12 degrees C is measured at each radiator. The problem is that
adjusting each affects the others so this can take a very long time.
Hence CH installers often take a short cut and just adjust the hottest
radiators down until the coolest ones warm up.

c) The system has become silted. This happens from the corrosion of
steel radiators and produces a brown-black sludge. In a microbore
system, this can tend to reduce the flow through the pipework more
quickly than 15mm because some is carried from the radiators into the
pipes. The smaller size of microbore will obviously silt up more
quickly. However, the problem is entirely curable by flushing the
system with clean water. In a badly silted system, it's helpful to
take the radiators off and flush them outside. This does require care
to avoid silted water, which is a great brown dye from dripping on the
carpets. Another favourite game of plumbers is power flushing to do
this task. They connect up a powerful pump and some cleaning
chemical to do it and charge several hundred pounds. This is a rip
off considering what is involved.
The whole situation of silting is very easily preventable by the
addition of corrosion inhibitor (about £20 every three years) to the
system. I've done this in my system (18 years old) since new and it
has remained virtually pristine.

I can see no reason at all to change everything to 15mm unless the
system was so badly designed that all the radiators are inadequately
connected. I think that plumber (1) is just looking to expand the
work and the the price. It wouldn't be British Gas would it?

There may be some merit in changing 6mm pipes if the radiators using
them are not warming up, provided that balancing and silting have been
eliminated.

If everything is properly designed, clean and balanced there should be
no reason to change from 8mm.

..andy

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