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David Hearn
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Default Another heating problem question!

"Phil Addison" wrote in message
On Thu, 11 Dec 2003 18:37:04 -0000, "David Hearn"

I added TRVs this weekend and the week before replaced the boiler
thermostat. Since doing this I've noticed that the downstairs is not as

as it used to be - even with the TRV heads off. I've also noticed a lot

boiler cycling (old Baxi WM 531 RS boiler).

Boiler short-cycling means that the heat the boiler generates is not being
taken away fast enough by the circulating water. Symptoms are that the

fires continuously at first, i.e. when the system is cold, but as the

warms up to temperature the boiler shuts down for a short while every few

The fundamental causes of short-cycling a

1) Inadequate pumping, i.e. pump set to too low a speed, or it is damaged.

2) Restricted flow to the heating circuit caused by (a) partially closed
radiator valves, (b) sludge or other obstruction in pipe work, or (c) pipe
bore too narrow.

3) Excessive by-pass flow.

In the case of 1) and 2), the return pipe temperature at the boiler fails

reach full temperature or even remains quite cool, and some or all of the
radiators will have cool return connections. The return temperature should
reach within 11C of the flow.

In the case of 3), the by-pass circuit is returning too much hot flow

back to the return, and the return at the boiler may be almost as hot as

flow. However, the return water from the radiators, before it joins the
by-pass return, will be too cool.

No 3 might be something - I've covered that below though.

Previously the living room (large room with 2 radiators) used to get

hot with the heating on - now it gets warm, but not warm enough really.
I've tried rebalancing (which previously got the downstairs nice and

but that doesn't appear to make a significant difference. I did

struggle to
get much of a temperature drop on some radiators - some only had as

as a 3 degree drop with just a 1/8's of a turn on with the lockshield.

highest drop appeared to be on the largest (and furthest radiator which
isn't getting quite as hot) which was 20. The majority of the rest were
about 5-8 degree drop.

What procedure did you use to re-balance? Are you starting with all LSVs

open, and turning DOWN the LSVs on the HOT rads according to, rather than just

trying to
open up the cool ones?

Well, my procedure was to open up all the lockshields and then turn the
lockshields on the rads closest the boiler (the hottest ones) to get them as
closed as possible yet still giving an adequate temperature drop and still
hot to the touch. It wasn't a 'to the letter' following of the FAQ.

The pump (UPS 15-50) is running on its slowest speed and always has

Anything more than this and there's a noticeable noise from the
pump/pipework. I did a test last night and increasing the pump speed
altered the boiler cycling times only slightly (5 seconds different) and
failed to make any difference to the temperature of the pipes to the

downstairs radiator.

Specifically, with the boiler thermostat set to 3 (with 5 being highest)

boiler would heat the water for

Boiler thermostat setting Pump setting Time boiler on
Time boiler off Pipe therm 1 Pipe therm 2
3 1
1 min 35 secs 1 min 45 secs 50
5 1
2 min 25 secs 1 min 20 secs 50
3 3
1 min 40 secs 1 min 45 secs 50

[Reformatted for clarity]

Boiler Pump boiler boiler Pipe Pipe
stat on off 1 2
3 1 1m35s 1m45s 50 30
5 1 2m25s 1m20s 50 30
3 3 1m40s 1m45s 50 30

The pipe thermometer readings were from the large (9') radiator which

the one which got cold when it wasn't balanced properly.

50C on the flow is not enough. What is the temperature at the boiler flow

number 5? It should be over 80C. It seems you have hardly any flow to this

so you need to fully open its LSV and close down some of the hotter ones.

I've yet to manage to get the thermometer onto the boiler pipes directly
(very tight squeze above the boiler or floorboards up directly above the
boiler). I'll try to get this done tonight and see what's actually coming
out of it.

As you can see, the boiler is cycling about every 2 minutes and changing

pump speed made little difference.

The radiators closest to the boiler do get hot and the downstairs

don't get as hot, but are hot.

Then the system is not balanced. They should all get equally hot (or

even if the boiler is short cycling. I suggest you study the above

FAQ and try again. Note that all the flow ends should be at virtually the

temperature, and within a few degrees of the boiler flow temp.

I'm pretty sure that most of the radiator flow ends aren't at the same
temperature - I'll work on that part by rebalancing (and following the FAQ
more accurately I guess).

I've got a thermometer (built into a small travel clock) and I've yet to
make the temperature in the living room more than 20.5 even though I've

the wall stat in the hall to anything up to 25.

Is that what you are using to measure the flow and return temperatures? If

the results are probably miles out!!

No, I've got 2 pipe thermometers for the radiator feeds - I'm just using
that thermometer as a basic indication of room temp.

Note, I did these checks
with the TRV heads off - so they're not restricting the flow and I got

same reading with both TRV's set to 5 (the highest on the Pegler Terrier

I'm sure the cycling times dropped when I changed the boiler thermostat,

I'm confused that increasing the pump speed makes little difference.

If the system is unbalanced to the extent that the radiator is not getting
hot, the room will not warm up much above ambient anyway.

What sort of noise would you expect from my pump on speed 3? The pump

very quiet on speed 1 but becomes quite noisy on speed 3 (and its about

away from a bed in a spare room - so I don't want to it be too noisy).

checked that the system has been bled properly including the pump.

I'm going to continue tweaking the balancing of the radiators - but I'm
puzzled that it used to get uncomfortably hot but now it doesn't even

the TRV's are off and that the pump speed makes little difference to the
temperature and boiler cycling.

Any ideas?

It seems to me that (1) in changing the boiler stat you have now got the
boiler set at too low a temperature. It should be set at up to 82C flow

I'll check that tonight hopefully.

and (2) you must have closed the LSVs to install the TRVs.
Therefore you need to re-balance them. Did you record their original


I recorded their original settings, but I replaced the lockshields, and I've
no idea whether the number of turns are transferrable. Initially I set them
to be what they had been on the old ones, but I soon noticed it wasn't
balanced properly - hence doing a (basic) balancing.

In addition, Ed's points about the by-pass and the cylinder coil need

out. If you have no motorized valve or TRV on the cylinder coil, it must

balanced with a series gate valve used as a LSV, just as if it was a

You can get 'locking' gate valves for this purpose.

The system has 2 motorized valves - one for heating and one for hot water.
When I balanced it, I did so with the water off. The water is only heated
for an hour or two morning and evening and wasn't being done when I did the
room temperature check.

I did notice a small 15mm pipe with a gate valve on it which appears to
bypass the motorized valves. After being unhappy with the balancing I did
twiddle it a little and returned it to the same place it was set to before.
Should this be open or closed - or just adjusted? I assume its a safeguard
so that when both valves are shut and the pump/boiler is on, that there is
some flow back to the boiler? If that is the case, I guess it must be open,
but not enough to cause too easy a path back to the boiler.

Since the system was working previously, sludge and inadequate pipe size

probably be ruled out.

Whilst I didn't use any sludge remover before installing TRVs, I did remove
each radiator and flush it through thoroughly, so any sludge etc would only
be that in the pipework (or distubed at the bottom of the header tank when
it refilled).

Thanks for the very helpful hints - I'll report back with the temperature of
the boiler output and I'll give the balancing a better go. I wasn't aware
before that the output pipe of the radiators should all be roughly the same.