On 28 Nov 2006 14:51:19 -0800, in uk.d-i-y "Velvet"
wrote:
I've googled, read the faq's, but I still have some questions.
GCH in house, with a Combi boiler. 7 rads in total, semi-detached
house, all bar one have TRV's. Room stat present in non-rad area.
Boiler located on ground floor, has 'thick' (22mm?) pipes from boiler
which then drop via 'T' pieces to the smaller 'normal' (15?mm) pipes
that run under kitchen floor to the downstairs rads, thick pipes
continue upstairs (at some point dropping to smaller size to the
upstairs rads).
Not 100% sure of order of rads downstairs, despite there being only
two. Upstairs it's bathroom (directly above boiler location) then
through bedrooms, and then drops down to feed downstairs hall rad as
the last one on the 'upstairs' leg.
The physical ordering is of no consequence. Just feel the flow and
returns as the system warms up. If all valves are fully open, the last
to warm up is 'furthest', and its LSV should be left fully open.
Boiler appears very capable of heating entire house and then some.
I've no cold radiators anywhere, though I suspect the kitchen one could
do with being a double rather than a single - this is the coldest room
of the house (not *that* much cooking goes on as I mostly live on my
own, plus it's north facing, and has unheated west-facing hall to side
door too).
I'm finding that the heating has to be on for 3-4 hours in order to get
the whole room up to temp. The TRV's do their thing, cycling on and
off. Lots of cycling on and off in some cases. I suspect this might
be partly down to the radiator being fiercely on due to the boiler
being so energetic (even when set to minimum), so the TRV turns off
prematurely, rad cools, room stops heating up, TRV comes on again...
only to go off again a short while later due to radiator being burnyhot
again.
I'm puzzled by that. TRVs are not on-off devices so they should not
cycle. They are linear and gradually reduce the flow as the room heats
up, until the flow is just enough to maintain the required room
temperature. I suppose what you describe could happen if for reason the
heat from the rad is directly affecting the TRV, say if there is a
draught blowing hot air on it. It is never ideal having them so close to
the rads as they necessarily are fixed, as they are supposed to sense
the room temperature not whether the rad is hot or cold.
So I thought I'd balance the system (it's clearly not, since all
lockshields are fully open everywhere) to see if this would improve
things.
Good plan.
I've borrowed one of those point-and-click temperature sensing things,
which is great. But...
Its called an Infra Red (IR) thermometer. Did you also put some black
PVC tape on the pipes to take readings from? IR thermometers give
strange readings off some surfaces.
What I've found is that with everything wide open, there's about a 6C
drop between flow and return at the boiler. Pump is integral to boiler
btw. On individual rads, there's a negligable drop on the return from
the rads. They'll happily sit there at around 63C both sides of the
rad, with slight fluctuation as the boiler cycles on and off. No short
cycling. Boiler apparently has an integral bypass (a bit of pipe,
unclear if there's any automatic valve on it at all). Manual also
states that a flow valve should be fitted between flow and return in
order to limit flow to acheive design temp drop across system. I
figure that'll be my bathroom rad then, since it's non TRV.
I don't understand what this 'flow valve between flow and return' might
be. It sounds as if they mean bypass, but you say that is integral. It
is common for the bathroom rad with no TRV to be used as a bypass. On
the other hand you say there is a room stat; the rad in the room with
the room stat should have no TRV.
Struggled to get a 11C drop across the radiators unless they're only
just cracked open on the lockshields. Boilers rated for 11-17C drop
across the entire system, depending on rads served etc.
Forget about 11C drops - as I say in my FAQ there is NO requirement for
that - it is a common misconception. The aim is to achieve EQUAL drops
across all rads, whilst one rad (or more) has its LSV FULLY OPEN. You
need to ensure the TRVs don't close at all during balancing. Open them
to max settings, or preferably loosen or remove the TRV head to ensure
this.
What I'm struggling with is the drop measurement though. If the
boiler's firing, it's quite a long cycle, and it brings the flows up to
about 62C at each rad.
62C is rather low for flow temperature if the boiler is set to maximum.
Is that the highest it ever reaches?
I can then get an 11C drop. When the boiler's
not firing and just circulating, the drop reduces quite a lot, though
eventually (now that I've throttled back even the bathroom rad by a
looong way) the boiler's not fired for long enough that I'm seeing a
flow of about 54C and a return of about 48C. Boiler *eventually* kicks
in and fires again. It's not overheating due to lack of flow (all the
TRVs are still wide at this point despite the lockshields beingn
throttled back a long way to almost closed for almost all rads) and
can't hear kettling either.
Did you wind up the room stat? That could be cutting out the boiler.
So, when measuring this drop, at what point in the cycle do you do it?
with the boiler firing, so you get maximum input, and wait for maximum
temp of return from the radiator?
See FAQ. You want the boiler firing, and wait long enough for all flow &
return temperatures to settle to a steady value.
Is it worth me throttling back *every single radiator* in order to get
NO, see above.
anywhere near 10C drop across them, and something between 10-17C drop
across the boiler? Isn't that going to put more stress on the pump?
And will it actually improve the TRV characteristics?
I've balanced a system before in my old place, but that was a) all on
one level, b) no TRV's, c) very clearly defined system with boiler at
one end, radiators in a line away from that, thus making it easy to
balance with the furthest being fully open and still seeing a 11C drop
across it, with progressively closer rads to the boiler being more and
more throttled back.
The 'furhest' is the one that is having the hardest job to get hot. It's
LSV should be fully open.
This place? it's all bizarre and odd and a bit confusing.
I think you need to read the FAQ again!
http://www.diyfaq.org.uk/plumbing/rad-balance.html
Phil
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