oil boiler cycling alot

[billybaggins]

hi all, i have just fitted a new oil condenser in place of a 25 year old oil boiler in my house.house size is 5000sq feet double glazed but not insulated well and is draughty, 20 radiators some not convectors and then a coil in the pressurised cylinder.The house is fairly long not wide and the boiler is sited at the very end of the house. the new boiler is a grant with riello RDB 2.2 burner and is set up with a 1.20/80 degrees S nozzle as standard which produces 41.5kw or 142,000 Btu's. All pipes coming from boiler house are 28mm, the old system had four connections, gravity primarys and pumped central heating, now i have 2 pumped with tee off to upstairs, downstairs and dhw cylinder, so three zone valves also. PROBLEM is the boiler cycles about 1 minute 20 seconds on and off for the same, but rads are very hot, and furthest ones are pretty hot too so any ideas why this is cycling? think the boiler is oversized? i can put a 1.00/80 degree EH into the burner to bring it down to 36kw would this be sufficient? the pump is a semi commercial wilo £175 quid jobby it was up full but i now turned it down to middle setting (out of 3) and not sure it made a difference. my biggest question is will this be using more oil starting and stoping so often? ps my thermostat is set at a quarter but even having it on full tilt it'll still cycle like that. CO2 readings are on the button as is the oil pressure.

Thanks in advance if anyone has any ideas, it will be greatly appreciated.
And no smart comments please i deemed competent to fit a boiler as oftec isnt mandatory in northern ireland, just needing advice from somebody who has done a change over to a condenser or who generally knows where my problem lies. many thanks......William

 

[Tibbot]

Have you installed an auto by-pass?

 

[billybaggins]

just a manual bypass. have it closed at the minute while trying to sort this out. any ideas?

 

[crockett]

Your boiler is probably over specced. A smaller gph nozzle will help a little. Who sized your boiler ?

 

[crockett]

Just re-read it. You sized it, yes ? What did you base the size on ? The previous boiler output, or did you do a full heat loss calculation ?

 

[billybaggins]

the old boiler was 110,000 btu's and the heat wasnt enough for the size of house. do you think reducing the 5.5kw to next nozzle size down will make much of a difference? also why is it cycling? should it get up to temp and stay off for a while before kicking in again? thats why i dont understand how a bigger boiler will need to keep heating the water back up again. thanks

 

[billybaggins]

and we have serious heat loss, 250 year old house, very little roof space insualtion and old walls, its always been a cold house. can certainly feel more heat with this new boiler though. is this unefficient having to start and stop?

 

[Tibbot]

By-pass should be open enough to pass heat especially if it is oversized. Also have you positioned pump correctly in the negative flow from when it was altered from semi-gravity 4 pipe?

 

[billybaggins]

By-pass should be open enough to pass heat especially if it is oversized. Also have you positioned pump correctly in the negative flow from when it was altered from semi-gravity 4 pipe?

in what way do you mean the negative flow? all my flows and returns are as they were before and pump on the flow coming from boiler. i have a motorized valve manually open at the minute to upstairs circuit so its acting as a bypass at the minute if the other valves close themselves

 

[crockett]

5.5kW will equate to around 19000 btu's, it may make a bit of difference, but its cycling is possibly due to either poor circulation, balancing, or over specced boiler IMO.

 

[billybaggins]

cheers for your reply ill try toning the nozzle down. circulation seems to be good, and i balanced down the rads at the end of the house the boiler is on. i just dont know how its losing heat so quick to need to reignite. do you think this is very unefficient? or will it just waste a small amount more? i appreciate the help thanks

 

[Tibbot]

Meaning the pump s/be on flow with the feed/vent pipes before it on negative side. eg. boiler, vent, feed then pump. Old gravity usually has feed/vent into cylinder primaries which mean they end up on positive side of pump if not altered during change to fully pumped. This may not be causing your problem but it's good practice.

 

[billybaggins]

the vent and feed go into coil at another cylinder on far end of house. the aga heats that cylinder so the ch coil has a valve closed to stop it circulating that coil, it still circulates the unvented though.

 

[outinthestix]

Have you fitted a room stat ?

 

[snb]

i would say without a doubt that you have oversized your boiler.

You could get a competent boiler engineer to down rate to 0.85/80eh.

This would reduce input to same as you had before but the actual output will still be at least 10% more than you had before but up to 30% more.

From your rads not all getting hot i would guess most is down to balancing and flow resistance at the end rads being too great.

It would be best to fit an auto bypass and also run pump on speed 3. You say have semi commercial pump. I guess it's a 25/80 or greater.

If so these need to connected to 28mm minimum.

hope this is of some help.

It's a shame you didn't get your boiler sized correctly as with oil boilers if you oversize them the cycling is a problem but also causes it to be very thirsty.

You might be able to get an engineer to reduce the output further.

I have a lot of 25kw/30kw boilers reduced to 15kw output with no ill effect but with greatly improved consumption and cycling of 5mins on 2mins off