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Phil Addison
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Default Another heating problem question!

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 hot
as it used to be - even with the TRV heads off. I've also noticed a lot of
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 boiler
fires continuously at first, i.e. when the system is cold, but as the system
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 to
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 water
back to the return, and the return at the boiler may be almost as hot as the
flow. However, the return water from the radiators, before it joins the
by-pass return, will be too cool.

Previously the living room (large room with 2 radiators) used to get quite
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 toasty)
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 little
as a 3 degree drop with just a 1/8's of a turn on with the lockshield. The
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 wide
open, and turning DOWN the LSVs on the HOT rads according to, rather than just trying to
open up the cool ones?

The pump (UPS 15-50) is running on its slowest speed and always has done.
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 largest
downstairs radiator.

Specifically, with the boiler thermostat set to 3 (with 5 being highest) the
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 was
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 on
number 5? It should be over 80C. It seems you have hardly any flow to this rad
so you need to fully open its LSV and close down some of the hotter ones.

As you can see, the boiler is cycling about every 2 minutes and changing the
pump speed made little difference.

The radiators closest to the boiler do get hot and the downstairs radiators
don't get as hot, but are hot.

Then the system is not balanced. They should all get equally hot (or cool!!)
even if the boiler is short cycling. I suggest you study the above balancing
FAQ and try again. Note that all the flow ends should be at virtually the same
temperature, and within a few degrees of the boiler flow temp.

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 set
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 so,
the results are probably miles out!!

Note, I did these checks
with the TRV heads off - so they're not restricting the flow and I got the
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, but
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 is
very quiet on speed 1 but becomes quite noisy on speed 3 (and its about 3"
away from a bed in a spare room - so I don't want to it be too noisy). I've
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 when
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
temperature, and (2) you must have closed the LSVs to install the TRVs.
Therefore you need to re-balance them. Did you record their original settings?

In addition, Ed's points about the by-pass and the cylinder coil need checking
out. If you have no motorized valve or TRV on the cylinder coil, it must be
balanced with a series gate valve used as a LSV, just as if it was a radiator.
You can get 'locking' gate valves for this purpose.

Since the system was working previously, sludge and inadequate pipe size can
probably be ruled out.

Phil Addison
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Phil Addison
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