On 29 Nov 2006 13:03:22 -0800, in uk.d-i-y "Velvet"
wrote:


Well, that's interesting. With the pump set to I (lowest) the water
slowly circulates around, but seems to heat up all the rads barring the
furthest (downstairs in hall, but fed from upstairs circuit) reasonably
quickly (burnyhot within 5 minutes). Boiler temp on the flow reaches
80C and sits there quite happily while it's burning, and with all the
rads wide, it happily sits there for a good long while burning at this
temp, with the pump set to I.

That's a good start, now do the balancing.

This includes the two rads on the ground floor circuit, so I don't
think it's just convection doing the work - I'd expect both downstairs
rads to stay cold if that was the case due to the way the plumbing is
at the point where upstairs and downstairs diverges (sideways T with
22mm continuing up, 15mm continuing down, 22mm sideways horizontally to
boiler).

Where did convection get in on the story? The water is pumped all round,
no convection needed.

However, with all the rads shut except the bathroom (and this wide
open), the boiler kettles. Same if I have the kitchen rad wide

Better check what you mean by kettling!! Its normal for the to make a
bit of a whistling/singing noise when its going flat out, with maybe an
occasional knock sound, but it should not be making serious
banging/clanking sounds. If it is kettling, it might be worth cleaning
it out with flushing fluid, and refilling with a good inhibitor. Google
for posts on flushing and inhibitor.

(closest downstairs one to the boiler, and the only other candidate for
having the TRV disabled on it). So two wide, pump on min, kettles and
short cycles. Set pump to middle and it stops the kettling, though a
little short cycling going on, with periods where it's doing it right
(and modulating up and down in the longer burn cycle).

Again lets check definitions: Short cycling occurs when the flow rate
through the boiler is insufficient to take away the full heat from the
flame, so the temperature rise across the boiler is too great ant the
flow reaches the max stat temperature and cuts it out while the return
water is till not up to temperature. The cool return water cools it and
so it fires up again. With the specified (ie max) flow rate the rise
across the boiler the rise is normally only 11C so the flow temperature
slowly rises as the rads heat up, and is only a steady 11C above the
return.

Shutting down rads obviously reduces the flow rate, and can cause
short-cycling if the bypass is inadequate.

I'm not sure where this has got me. If there were no TRV's I'd set the
pump on minimum, balance the entire lot on the basis there would
probably be enough flow with that pump speed, and if not, knock the
pump speed up to half way, and balance for that.

Nearly! Set the pump on slow, then balance, then speed the pump up is
the overall drop is above the 11C figure. You do not need to achieve
exactly 11, just equal across each rad.

But the TRVs mean that if I balance the rads, when they shut down, it
throws the *entire system* out of whack, not enough flow rate through
the remaining radiator,

No, the system is still balanced. Shutting some rads down simply mans
more pressure available to supply the remainder.

boiler kettles, and I'm back to having the pump
on full tilt (which leads to very noisy valves, and all the other
things too).

Yes, could be. See above. However, the house room-stat is supposed to
turn off the boiler before all TRVs close. And incidentally, the TRVs do
not normally close completely, they shut down linearly to allow just
enough water through to maintain the room at the set temperature. I know
you say yours are going on and off, but I don't know why. See if
balancing sorts that out.

If I leave the kitchen open wide enough to stop the kettling (in
addition to the bathroom rad being wide open), then I can see in order
for it not to turn off on the TRV it's going to be open that wide
constantly, the roomstat will have to be set to turn off before the TRV
starts to shut, which then leads to the roomstat turning off
prematurely if a) the radiator's kicking out heat at a constant rate,
b) I'm cooking, c) for some reason it's warmer in there than another
part of the house.

Indeed, choosing a place for the room stat is a problem. The usual
solution is to make the hall rad the non-TRV one and fit the stat in the
hall - but not too close to the rad. A more recent option is to get a
wireless room-stat and try it in different places.

Fair enough that another part of the house may have TRV's 'open', but
if the flow rate through those other 'on' radiators isn't at least
equal to the flow rate lost from the kitchen one having turned itself
off (or even down), the boiler's going to start kettling again...

I know the theory behind them, but... are TRVs really such a good idea

Are you sure? You don't see to grasp that they are linear devices, and
if they are banging on/off something is wrong.

in practice? I'm *really* beginning to wonder if they're suitable for
this boiler and this house!

Phil
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