I have a honeywell Y plan system.
The way it's been installed means that after leaving the boiler we first
pass the expansion vessel and then travel to the airing cupboard on the
first floor where the "birds nest" lives. Next is the heating pump, directly
after that is a tee piece which allows a 15mm loop to the 22mm boiler
return. After that is the 3 position valve feeding a 50 gallon tank and the
22mm heating loop.
The main problem I have is that occasionally the boiler boils - I can hear
it doing so - it does cut out when this happens, but it's not good.
I have spent a great deal of time researching this problem on the internet
and have read the DIY FAQ in depth - very useful.
As a first attempt to stop the problem - I changed the bypass loop referred
to in the first paragraph for a 22mm loop and added a "differential pressure
bypass valve" to that loop to ensure that there was no possibility that the
system would be pumping against a closed head. Also fitted becuase becuase I
don't think there is enough flow in the system - see below.
Every single RAD in the house has a TRV, (this hasn't affected the
problem) - yes I know this isn't how they say to do it - but that's why I
fitted a "differential pressure bypass valve". Often there are parts of the
house that I don't want heated at all and this was the only sensible way to
acheive this. Regardless of weather this is a good idea or not when the
heating is on the boiler works as it should - and I can make it short cycle
NO PROBLEM - which rules out any defects in the boiler itself. I should say
that the thermostatic system is a CM-ZONE system from honeywell - but 3 rads
have traditional TRV's on them and at least one of these is permenantly open
(towel radiator). The run to this permenantly open towel rad is very long -
this is why I properly balanced the system using the method outlined in
So the conditions under which the boiler boils are - all rads closed except
for the towel rad - and the boiler running at it's 50,000BTU setting as
opposed to it's 30,000BTU setting - in which case it's fine.
Or when were heating hot water only.
I repeat that it has always done this under these conditions before I fitted
TRV's everywhere and the Differntial Pressure bypass valve.
I did adjust the "differntial pressure bypass valve" to ensure that it opens
at 0.1 of a bar - it's weakest setting and all parts of the house are fed
well at this pressure - and also the thing is definately opening correctly
when I was testing against a closed head.
The heating is on a 22mm feed and return, with 15mm feeds to the individual
rads as I expect is the same in thousands of houses and seems to work fine.
The hot water feed is 22mm from the Y valve. But, at the bottom of the tank
someone has used a step down to 15mm to return spent water to the boiler
return - this is only a 2ft run which connects to the main 22mm boiler
return - amalgamating the bypass loop and heating return together with this
spent hot water into one return to the boiler.
1. What was the thinking behind the original gate valve in the bypass loop,
that was set 1/4 of a turn open?
Possible reasons: - Speculation:
1. The "differntial pressure bypass valve" is allowing too much *short*
with not enough dissipation of accumulated energy though the system and the
boiler thermostat isn't sensitive enough - for a quick rise in temp.
2. The 15mm step down in the return from the hot water tank is restricting
flow too much and the boiler is boiling. I guess it was designed to ensure
that the heating gets priority when both ports of the valve are open - is
this a normall and sound idea?
Perhaps - thinking about it after a re-read - the problem is a combination
of all 3 of the contributing elements I describe - low flow to the far
reaches of the house, an insensitive thermostat and a short cycling return.
Or maybe the DPBV is opening a little becuase the 15mm return is restricting
the flow and therefore the extra pump energy must have to go somewhere - I
mean this is hydraulics after all.
If point 2 is correct then the normall practice of having an open radiator
will never work in my system and therefore I fundamental re-design is
Comments and solutions very welcome.
Thanks in advance.
Lastly (if you've got this far - well done) this has to be sorted out
becuase I want to scrap the existing Apollo Fanfare 30/50 si - old noisy and
not powerfull enough - and install a new cast iron boiler in the slightly
damp cellar. Slightly damp means it's workshop with tools which are fine but
you could not keep paper down there. Has anyone any experience of the pros
and cons of doing this. I should say all the pipe work at the moment runs
into the cellar and the from there to the rest of the house and therefore it
will be simplicity itself if only I can sort out the flue. I need according
to diy faq an 88,0000 BTU boiler.