On 4 Nov 2003 00:20:16 -0800, (Danny)
wrote:

The temp is 65 degrees on the flow pipe. I've tried closing the
by-pass completely and opening the HW circuit fully, hoping that the
HW circuit will act as a by-pass.

If I put the pump on full speed then water begins to flow into the FE
tank - quite alot too. I have to keep the pump on medium to avoid
this and also ensure that by-pass and HW coil are not both fully open
(if they are water still comes out of the FE tank, but just a trickle)

OK. There are a few possible reasons for pumping over in this way.
However, the root cause is the same, which is that there is a large
enough pressure differential between the feed pipe at the bottom of
the FE tank and the vent pipe over it. The pressure difference
doesn't need to be a lot for this to happen - perhaps as little as
30cm of head. THe problem either manifests itself as pumping over or
sucking down of air, depending on which way round the differential is.
It is bad for this to be happening because it introduces air into the
system and that is an enabler for corrosion.

This can happen because:

- The pipes are connected to the system on opposite sides of the pump.
This one is a real mistake of installation and should never happen.
If the pipework is like that it really needs to be corrected.

- The pipes are on the same side of the pump but there is a pressure
differential between them. This could be the boiler or it could
simply be that they are connected to points too far apart on the
system pipework. It could also be because there is a part
obstruction of some kind between the two places.

Could you trace where they are connected and then post back approx
distances between the major items of the system - e.g. boiler, pump,
motorised valve(s), points where feed pipe and vent pipe are
connected. It's useful to use ASCII art for this to make a simple
diagram, or perhaps you could sketch one and put it on a web server?
It's really to be able to see the layout more than anything else.

As I say, this problem should be fixed but I would deal with it if
necessary after resolving the temperature issue. For diagnosis
purposes of this, it doesn't matter if there is pumping over for a
short while. One solution can be simply to have the pump on the
lowest speed, but that may then not be fast enough to run the system
properly. The issue is that heat transfer rate from the boiler
depends on pump speed. However, if the pump speed is too low for the
system requirement, the effect would be that the boiler flow will get
hot - up to 80 degrees+, as will the flow side of the radiators, but
the return side of the radiators will be relatively cool and the
boiler will tend to cycle on and off - simply because you are not
getting the heat away fast enough.



How do I check the motorised valve? When the room thermostat calls
for heat I can hear the MV opening so I assumed that it works.

OK. You can check its movement by operating the manual lever on the
side. You can also take the head off and turn the valve stem
underneath by hand to see that it is reasonably free. These valves
have either a paddle or a rubber ball inside which is rotated to block
and open the ports as required. I did have one once where some crud
from the original installation (a piece of wood) had found its way
around the system and lodged against the ball, partly blocking the
flow. I would just do a simple check for rotation at this point.
You can also take the cover off of the valve head and watch the
mechanism working. If there is any obstruction or stiffness you will
tend to hear it from the motor and their may be the sound of gears
jumping. However, I don't think that the valve is likely to be No.1
contender at this point, from what you describe.


Is the boiler thermostat something that I (a novice) could do? Do I
have to be corgi registered or something?

The law, in the form of the Gas Safety (Installation and Use)
Regulations, 1998, states (section 3) that you have to be "competent"
to work on gas fittings. It doesn't define "competent", but does
require self employed and employed fitters and businesses (in effect)
to be CORGI registered. There is a definition of "fittings" (gas
carrying parts) and "appliances". If you read through the document

http://www.hmso.gov.uk/si/si1998/19982451.htm

Part E talks about work on gas appliances.

Changing a thermostat should not involve doing anything with gas
carrying parts such as the burner, gas valve, etc. If the boiler
is a simple one with a capillary tube and bulb on the heat exchanger
or flow pipe, it is a relatively simple job to change it. Normally
the control itself, behind the knob, has 4-6 wires with spade tags
that push on. The main point here is to label everything........
There are the obvious points about making sure that any parts removed
are properly refitted, all seals are in place etc. etc.

The acid test is really whether you are "competent". If you don't
feel 100% sure about what you are doing, then it is better to involve
a registered fitter.


..andy

To email, substitute .nospam with .gl