I've decided that one of these auto bypass valves would be a good thing on
my all-TRV system, up til now I've had the TRV head left off the hall
radiator as an uncontrolled bypass rad. So I ordered the Pegler valve from
Screwfix. Before I fit it, is there any trick to calibrate these things
without the use of a pressure gauge? I was expecting some kind of indicator
on the valve to show when it was fully open (I seem to remember seeing some
other model that had this?) and thought that this would provide a good way
of calibrating the thing, but alas, the Pegler valve just has an adjuster
and a confusing calibration chart in the instructions. Well, confusing to
an amatuer plumber without a pressure gauge that is.
Should I send this one back and try and source one with an "open" indicator?
Should I buy expensive pressure measuing kit? Should I just set the control
half way and hope for the best? Should I give up and fit a room stat? :-)
To some extent it is a trial and error thing, but a simple way is to
close all the TRVs and then adjust the bypass valve so that it opens.
You may want to adjust slightly more so that it opens just before the
TRVs are completely closed to make sure that it does.
You shoulld have some for of room stat anyway and in a space where
there is not a TRV on the radiator or that it is open fully. On a
simple boiler, this is used to interlock it off when the house is up
to temperature. You need to arrange things so that the rooms with
TRVs are warm and valves are closing before the thermostat operates.
I approached it from the other direction:
Closed the automatic bypass, opened all TRVs fully and balanced the
system as per the faq.
Then eased the setting on the automatic bypass until it just opened
(indicated by its return pipe getting hotter), then backed it off a tad.
Then set TRVs to the desired temps.
Bypass now comes on as TRVs close down.
The "general" idea of the by-pass is to open up when all TRvs are closed
down. It appears that yours is opening up when some, or all, are only
Yes, that's true. As the TRVs close it is when they are reaching design
temperature so I wanted to indicate to the boiler that it was time to back off
a bit (it's a modulator). The action of the automatic bypass allows some
flow water to pass, raising the return temp and causing the boiler to
modulate its output. In the event that this caused any of the rooms to be
not to reach design temp I would back off the bypass another bit.
If I did not have a modulating pump you could consider that I was trying to
create a relatively constant flow of water at all times, irrespective of the rad
load. The other way leaves you with flow through the boiler dropping as rad
demand decreases until the bypass starts to operate which could make for
'untidy' operation on a non modulating boiler. Imagine the case of the guy
with a K130 we discussed recently, I would prefer to have a constant flow
through that beast at all times.
You are living with it and tweaking as you go along until fine.
That boiler modulated, if I recall. Still I would rather have a constant
flow through it, via a primary and secondary loop/pump arrangement. Keston
recommend a 42mm vertical copper tube as part of the primary loop if two
boilers are taken off. Works fine, but 42mm appears overkill. The
intention is to create a neutral spot. I prefer the constant flow back to
the boiler that a good primary and secondary loop system gives.
If the system say, heats a duct heater battery or Meson heaters, these
require a high temperature operation. I would prefer a blending valve to
ensure a constant flow back and that only 80C water reaches the coils.
Yes, but how do I know when it's opened? There's no indicator on the Pegler
I thought the boiler would just switch off when the flow/return temps were
very close, but I guess this would stop it cycling every time the water in
the pipes cooled a bit. I've got the cable in place for a room stat soI
think I will fit one as well.
Set it to around 0.3 bar. Mine had markings for this purpose. It will start
opening even when not all the TRVs are closed, but this is a good thing that
reduces noise through the radiators that are still open. The 0.3 bar left
over should be easily enough to push enough water through those rads.
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