Gas boiler 'hunting'.

[Nick]

Visited an elderly friend this afternoon. Heard the boiler striking up more
than a few times, it is located just outside the kitchen. It seems that it
fires up for maybe 60 secs and then goes off. A couple of minutes later it
does the same thing. Is this right, normal or safe?
I asked Lynda about this. She said that the boiler had done so for years.
Had a very quick squint. Potterton Kingfisher 150K btu gas boiler. Fully
pumped vented system.
The house was warm, rads hot. The boiler is serviced annually and continues
to heat the house and dhw well. System is about 20 years old. Boiler gas
valve was replaced about a year ago. Noticed that the room stat was buzzing.
Should the boiler be firing up every few minutes?
I suspect not. If I'm correct, any ideas what the likely causes or solutions
may be please.
Many thanks,
Nick.

[Andrew Gabriel]

In article <aofbde...@mid.individual.net>,

That's how that sort of boiler adjusts it's input to match demand.
It does it by having a thermostat on the water jacket part of the
heat exchanger, and maintains it at a set temperature. The frequency
and duty-cycle of the burner will depend on the demand from the
heating circuit (and hot water cylinder).

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

[Brian Gaff]

Sounds like it needs a spot of hysteresis.

Brian

--
From the Bed of Brian Gaff.
The email is valid as bri...@blueyonder.co.uk
Blind user.
"Chris Hogg" <m...@privacy.net> wrote in message
news:5j45i81eac78k6s4f...@4ax.com...

> Our 12-yr-old Vaillant does the same at times. The burner is
> 'modulated', i.e. flame size is automatically varied depending on
> temperature of the return water from the rads (or that's what I
> believe it to mean). First thing in the morning, the rads are cold,
> and the boiler comes on at full power. As the rads warm up and the
> return water gets hotter, the burner throttles back until it reaches a
> steady setting, when the heat supplied just about balances the heat
> lost from the radiators. As the house gets warmer, the rads lose less
> heat until eventually the return water gets sufficiently hot to cause
> the boiler to cut out. But the house itself isn't up to full
> temperature and the house thermostat is still calling for heat. Water
> in the boiler cools, because the burner is no longer lit, until it
> reaches a point when the boiler cuts in again, at full power. But this
> time the water in the insulated return pipes is quite warm, and heats
> up rapidly. But the boiler modulation can't throttle back fast enough
> to allow for the rapid water temperature rise that results, and the
> water temperature goes above the boiler cut-off temperature, at which
> point, and after only about thirty seconds, the boiler cuts off again.
> This cycle is repeated several times until cooler water from the rads
> themselves gets to the boiler, when it settles down and will burn
> steadily for say twenty minutes. Eventually, the rooms get warm enough
> and the house thermostat cuts the boiler off.
>
> I don't think it does any actual harm to the boiler, but it's my
> understanding that it's inefficient in use of gas when it hunts like
> that. I have asked the service engineer if there's any way he can
> adjust the modulation rate so that it cuts back a bit faster (like
> something as simple as turning a pot on a circuit board), but he
> didn't seem to think there was.
>
> --
>
> Chris

[geoff]

In message <aofbde...@mid.individual.net>, Nick
<nick...@NOSPAM.tiscali.co.uk> writes

Its called "Short cycling" - there are several reasons and plenty of
info about it on't web once you know the right phrase to look for

e.g.

http://www.home-heating-systems-and-solutions.com/boiler-short-cycling.ht
ml

http://www1.eere.energy.gov/manufacturing/tech_deployment/pdfs/steam16_cy
cling_losses.pdf

--
geoff

[Dave Liquorice]

On Mon, 18 Feb 2013 18:49:03 -0000, Nick wrote:

> Noticed that the room stat was buzzing. Should the boiler be firing up
> every few minutes?

Does sound rather too often with the house up to temperature. The room
stat shouldn't buzz, so I'd check that is switching properly and does
inhibit the boiler when satisfied. Is there a tank stat, check that as
well. Any TRV's on te radiators?

--
Cheers
Dave.

[Big Les Wade]

Nick <nick...@NOSPAM.tiscali.co.uk> posted

>Visited an elderly friend this afternoon. Heard the boiler striking up more
>than a few times, it is located just outside the kitchen. It seems that it
>fires up for maybe 60 secs and then goes off. A couple of minutes later it
>does the same thing. Is this right, normal or safe?
>I asked Lynda about this. She said that the boiler had done so for years.
>Had a very quick squint. Potterton Kingfisher 150K btu gas boiler. Fully
>pumped vented system.
>The house was warm, rads hot. The boiler is serviced annually and continues
>to heat the house and dhw well. System is about 20 years old. Boiler gas
>valve was replaced about a year ago. Noticed that the room stat was buzzing.
>Should the boiler be firing up every few minutes?

Much the same problem occurred with my boiler - also a Potterton
Kingfisher - a few years ago. The cutting out and reigniting was
accompanied by loud clicking of relays. I replaced the main PCB, which
fixed it completely.

The reason the PCB failed was probably excessive cycling, caused by
inadequate water flow through the heat exchanger, due to a corroded pump
or flow valve.

Mind you that buzzing room stat sounds suspicious too. Maybe replace it
with a cheap electronic one?

--
Les