Sizing a boiler - short cycling?
 

Just wondering if anybody can help me out here. I have an oil fired boiler for the central heating and hot water. We've been in the house for a few months now and we seem to be using a lot of oil and I'm trying to see if I can get the boiler running more efficiently

Now it's a big house (2600 sq feet) and I understand that a house that size will be hard to heat. The heating would be on inthe morning for an hour and in the eveniong for 2-3 hours. There is 15 radiators and the hot water cylinder. The boiler is 120000 Btu (35Kw) with a 0.65mm nozzle in the burner.

A few days ago I timed the cycling on the boiler when it was up to temperature and found that the burner was coming on for 2.5 mins and then of for approx. 2.5 mins which I thought a bit excessive.

I've tried searching for info but find it hard to find anything on the boiler cycling question. I had a look at the sedbuk website but don't have all the measurments to size the boiler - need to find the drawings again.

Any inputs much appreciated.................

Sizing a boiler - short cycling?
 

"Fatboise" wrote in message
...

Just wondering if anybody can help me out here. I have an oil fired
boiler for the central heating and hot water. We've been in the house
for a few months now and we seem to be using a lot of oil and I'm
trying to see if I can get the boiler running more efficiently

Now it's a big house (2600 sq feet) and I understand that a house that
size will be hard to heat. The heating would be on inthe morning for an
hour and in the eveniong for 2-3 hours. There is 15 radiators and the
hot water cylinder. The boiler is 120000 Btu (35Kw) with a 0.65mm
nozzle in the burner.

A few days ago I timed the cycling on the boiler when it was up to
temperature and found that the burner was coming on for 2.5 mins and
then of for approx. 2.5 mins which I thought a bit excessive.

I've tried searching for info but find it hard to find anything on the
boiler cycling question. I had a look at the sedbuk website but don't
have all the measurments to size the boiler - need to find the drawings
again.

Any inputs much appreciated.................

You have three alternatives:

1. A new modulating boiler (expensive)

2. A heat buffer. A cylinder, can a cheap normal direct domestic cylinder,
that the boiler heats up directly, not via a coil with two cylinder stats
that prevent boiler cycling. The CH can be taken off the cylinder and TRVs
on all rads using a Grundfos Alpha auto modulating pump. The DHW can also
be taken off the buffer cylinder, or have a quick recovery coil cylinder,
again with two cyl' stats and taken off the boiler directly.

3. If you have good mains pressure install an 'integrated' heat
bank/thermals store, again with two cyl' stats, CH off the store directly
with Alpha pump. The cold mains water is heated via the heat of the stored
water. Explanation: http://www.heatweb.com

With 2 and 3 above the boiler comes on for one long efficient burn to heat
the stored water. Only when the water cools off substantially does the
boiler come in to fully reheat. With 2 and 3 above, install a magnaclean
filter on the CH return to the buffer or thermal store.

Sizing a boiler - short cycling?
 

Fatboise wrote:
Just wondering if anybody can help me out here. I have an oil fired
boiler for the central heating and hot water. We've been in the house
for a few months now and we seem to be using a lot of oil and I'm
trying to see if I can get the boiler running more efficiently

Now it's a big house (2600 sq feet) and I understand that a house that
size will be hard to heat. The heating would be on inthe morning for an
hour and in the eveniong for 2-3 hours. There is 15 radiators and the
hot water cylinder. The boiler is 120000 Btu (35Kw) with a 0.65mm
nozzle in the burner.

A few days ago I timed the cycling on the boiler when it was up to
temperature and found that the burner was coming on for 2.5 mins and
then of for approx. 2.5 mins which I thought a bit excessive.

I've tried searching for info but find it hard to find anything on the
boiler cycling question. I had a look at the sedbuk website but don't
have all the measurments to size the boiler - need to find the drawings
again.

Any inputs much appreciated.................


If you have a 2500 sq ft house, with only average insulation, you could
end up burning up to 600 quids worth of fuel a month.

The fact that you have a 35KW boiler shows that someone thought you
needed one...

A 50:50 duty cycle implies you are needing 17.5KW of heat ..whilst the
shortish cycling is not hugely good, the fact that a 50% duty cycle is
needed shows you are indeed needing a LOT of power.

To get a linger on and off period, change the thermostat to one with
more hysteresis.

Sizing a boiler - short cycling?
 

Fatboise wrote:
Just wondering if anybody can help me out here. I have an oil fired
boiler for the central heating and hot water.


The boiler is 120000 Btu (35Kw) with a 0.65mm
nozzle in the burner.

A few days ago I timed the cycling on the boiler when it was up to
temperature and found that the burner was coming on for 2.5 mins and
then of for approx. 2.5 mins which I thought a bit excessive.


These boilers (at least for mine & several others I know) use an
optical flame/ignition sensor. The sensor on mine lies several inches
behind the ignition electrodes.

Your short cycling is a symptom off the sensor glass being sooted up or
dirty. On mine I have to unbolt the Riello unit (amazingly only 1
nut!) from the front of the boiler and clean the glass. I use a long
child's poster paint brush. At the same time the whole of the
combustion chamber should be scraped, brushed & vacuumed clean of soot
at the same time On my boiler, access is from the top via several lift
off plates & a few more screws/bolts.

A full service would include an instrument check on combustion
efficiency and oil supply pressure. Going rate here is 50-100gbp for a
full service.

Only do 1 or 2 DIY cleans before getting a full service, because the
condition of the electrodes and jets can deteriorate significantly over
a longer time span. Replacing these is not difficult but you do need
the correct gear. Cost of the right instruments is 200gbp up plus
regular calibration fees.

HTH

Sizing a boiler - short cycling?
 

"Fatboise" wrote in message
...

Just wondering if anybody can help me out here. I have an oil fired
boiler for the central heating and hot water. We've been in the house
for a few months now and we seem to be using a lot of oil and I'm
trying to see if I can get the boiler running more efficiently

Now it's a big house (2600 sq feet) and I understand that a house that
size will be hard to heat. The heating would be on inthe morning for an
hour and in the eveniong for 2-3 hours. There is 15 radiators and the
hot water cylinder. The boiler is 120000 Btu (35Kw) with a 0.65mm
nozzle in the burner.

A few days ago I timed the cycling on the boiler when it was up to
temperature and found that the burner was coming on for 2.5 mins and
then of for approx. 2.5 mins which I thought a bit excessive.

I've tried searching for info but find it hard to find anything on the
boiler cycling question. I had a look at the sedbuk website but don't
have all the measurments to size the boiler - need to find the drawings
again.

Any inputs much appreciated.................

Do you know if it's cycling on the room thermostat or on the boiler's own
thermostat?

Have you measured the temperature on the flow and return near the boiler?

Is the pump providing sufficient flow of water thro the boiler (when firing)
so as to provide 11 deg C across the boiler?

Donwill

Sizing a boiler - short cycling?
 

"jim_in_sussex" wrote in message
oups.com...

Fatboise wrote:
Just wondering if anybody can help me out here. I have an oil fired
boiler for the central heating and hot water.


The boiler is 120000 Btu (35Kw) with a 0.65mm
nozzle in the burner.

The oil pump pressure is included in the heat production equation. The 0.65
usgph is the oil throughput at 100 psi but your burner may be set anywhere
from 90 to 200 psi. what does your manual specify? I'd guess a 120000BTU
boiler would have started life with a 1.00 usgph nozzle and someone has
"turned it down" although this is only speculation on my part.


A few days ago I timed the cycling on the boiler when it was up to
temperature and found that the burner was coming on for 2.5 mins and
then of for approx. 2.5 mins which I thought a bit excessive.


These boilers (at least for mine & several others I know) use an
optical flame/ignition sensor. The sensor on mine lies several inches
behind the ignition electrodes.

Your short cycling is a symptom off the sensor glass being sooted up or
dirty.

This assertion staggers me - on every other pressure jet burner the symptom
of a dirty photocell is locking out of the burner. There is no way I can
conceive of a dirty photocell causing shutdown without the burner going to
lockout and requiring manual intervention to reset. The signal is read by
the burner sequence control, this decides whether flame is present or not
and either allows the burneer to run or locks it out, no ifs no buts.

On mine I have to unbolt the Riello unit (amazingly only 1
nut!) from the front of the boiler and clean the glass. I use a long
child's poster paint brush. At the same time the whole of the
combustion chamber should be scraped, brushed & vacuumed clean of soot
at the same time On my boiler, access is from the top via several lift
off plates & a few more screws/bolts.

A full service would include an instrument check on combustion
efficiency and oil supply pressure. Going rate here is 50-100gbp for a
full service.

Only do 1 or 2 DIY cleans before getting a full service, because the
condition of the electrodes and jets can deteriorate significantly over
a longer time span. Replacing these is not difficult but you do need
the correct gear. Cost of the right instruments is 200gbp up plus
regular calibration fees.


Jets are advised replacement at annual service although 2 years is not
unrealistic. The boiler should NOT be making soot.

To the OP Short cycling as described is likely to be related to the rate of
water flow through the boiler with a possibility of a poorly adjusted
bypass. The number of radiators is meaningless, it is the sizing (load)
which matters

Sizing a boiler - short cycling?
 

On Sat, 11 Feb 2006 10:17:55 +0000, Fatboise
wrote:


Just wondering if anybody can help me out here. I have an oil fired
boiler for the central heating and hot water. We've been in the house
for a few months now and we seem to be using a lot of oil and I'm
trying to see if I can get the boiler running more efficiently

Now it's a big house (2600 sq feet) and I understand that a house that
size will be hard to heat. The heating would be on inthe morning for an
hour and in the eveniong for 2-3 hours. There is 15 radiators and the
hot water cylinder. The boiler is 120000 Btu (35Kw) with a 0.65mm
nozzle in the burner.

A few days ago I timed the cycling on the boiler when it was up to
temperature and found that the burner was coming on for 2.5 mins and
then of for approx. 2.5 mins which I thought a bit excessive.

I've tried searching for info but find it hard to find anything on the
boiler cycling question. I had a look at the sedbuk website but don't
have all the measurments to size the boiler - need to find the drawings
again.

Any inputs much appreciated.................

Hi,

Are you able to measure the return temp to the boiler, eg with an IR
thermometer?

cheers,
Pete.

Thanks for the replies - very helpful!
Pete, Haven't any way to measure the temp but will look into it this week.
I'll get back with any info that I find - once again thanks I have a better understanding of the problem.