oil boiler cycling - normal?

[warwickwater]

Having moved to this (large old) house in the summer we have finally had our (old inefficient, oil fired) boiler serviced and the heating is back in commission. I have replaced the wall mounted room stat in the draughty hallway with one in the lounge, which seemed sensible. All is now fine (house is warm) but one thing seems a little strange.

The room stat turns on and off roughly as expected, and the circulation pump turns on and off with it. The boiler will cut in as expected as the stat calls for heat, but will then turn off a few minutes later, although the pump continues to run and the stat is stall calling for heat.

The engineer who recommissioned the system explained that this was the boiler switching off as the water circulation temp reached that set by the boilers internal stat (it has a control comprehensively marked 'off.. medium.. high'.

Is this behaviour to be expected - is it simply my boiler heating the water faster than the radiators can radiate it and thus allow the return to be cool enough to warrant heating? I currently have the control set midway between medium and high and the rads are 'painful hot' to touch.

My understanding was cycling an oil boiler was inefficient - it certainly sounds like its taking a lot of oil with it every time it fires up with a little shudder!

Any info much appreciated, and yes i do realise the most efficient resolution would be a new boiler, just not this winter!

Thanks in advance.

 

[GPlumb]

If the rads are painfully hot then turn the boiler control down a little. Measure the temp of the flow pipe if you can and report back.

Might be a good idea to balance the radiators if its been out of commission for a while.

 

[warwickwater]

Whilst appreciating the reply, I dont understand any link between boiler cycling and balancing radiators?

 

[simond]

You have to lose the heat from the boiler to prevent it quickly reaching jacket temperature and turning off.

Your radiators are designed to discharge heat into a space. If they are not all doing this effectively your boiler will short cycle.

It could be that your room thermostat is calling the boiler on when most of the heating task is complete, you may wish to buy a radio linked unit and site it where it is more representative.

Finally, in a large old house, some rooms will lose heat very quickly. Zoning is a way of using the oil energy much more effectively.

A Kidd boiler works more on an insulated thermal store principle and cycles less often, but you don't want to replace it apparently

 

[wilhelm]

might be overfired.

 

[warwickwater]

whats 'overfiring' - calling for heat too often?

We have a wireless stat (in the lounge) and the temp seeems very stable. All rads seem hot so I assume heat can be lost quickly if needed?

 

[wilhelm]

overfiring..too bigger nozzle or too much pressure (on oil pump) for nozzle fitted. therefore heating water jacket up too quickly, before water pump dragging it away.

details of your boiler, and maybe the size nozzle that has been fitted. may help.

I have to say, if it is overfired, it may also start with a boom....

 

[oilman]

wilhelm is probably right. However, if a boiler is expected to keep the house warm at the back end of February, then it will be overfired for the rest of the time. That's why they have an internal stat.

 

[warwickwater]

theres a definite house shaking boom when the boiler starts up, so that along with the fast cycling means the wrong nozzle is fitted?

Would/ should this nozzle have been changed at the recent service I just had? On a side note should the braided hose from the incoming oil pipe to the 'pump assembly' also have been changed (i think this was included on the quote from one engineer we didnt use).

Thanks once again for the knowledgable replies - pity you guys (oilman / willhelm) arent close and can come and do the work for me!

 

[Kes]

I thought that this was normal. Our oil boiler will fire up from cold, and - after about 40 mins - turn off on the boiler thermostat with the pump still running. The rads may be up to temp but the rooms aren't, so the roomstat is still calling for heat. The boiler then cycles five mins on, three or four off, until the room stat tells it to shut up. There's no law that says the boiler must be on continuously whilst the room stat asks for it.

 

[warwickwater]

Kes - mine is cycling off after maybe 1 minute, not 40. I cant see how the water can have even made a circuit by then!

 

[Kes]

That's most peculiar, it almost sounds as if the pump is not running, and even then I would think it would take more than one minute to bring the boiler up to temp (I'm assuming free-standing boiler, not a flimsy wall-mounted combi). The pump is working, I assume, as the rads get red hot (unless the water is circulating by thermo-syphon, which is unusual).

Do you have an unpumped flow and return to a hot water cylinder? Do the pipes get hot straight away the boiler fires? Does the boiler seem to be up to temp after that first one minute from cold?

 

[warwickwater]

sorry kes, not explaining myself - pump seems to be working and is new. theres 'water moving' noises around the house, the pump makes a gentle hum etc.

from old (ie off all day turns on at 4pm) the boiler wont cycle quickly, but will still turn off before room hits temp, but (with rads very hot so maybe thats to be expected).

once the room is at temp or nearly temp the cycling from the boiler can drop to sub one minute - and it is the internal stat turning it off (green power light stays on) and not the clever stat doing some duty cycle thing (when the room stat goes off it turns off boiler power and the circ pump with it).

This is a big house with 18 rads over 4 floors, but with 3 or 4 awating a new TRV and the rad stuck off. Is it as simple as a good (but old) boiler being too good at heating water quickly (with the rads less than efficient at losing heat) or are we (bearing in mind our 'boom' start ups) overfiring and wasting our precious oil ?

As 'oilman' pointed out earlier would the cost of reducing the nozzle (is this possible) be a less efficient system when we really need it in cold Feb?

Thanks once more for you help everyone!

 

[oilman]

There's no law that says the boiler must be on continuously whilst the room stat asks for it.

There isn't, but to have the boiler overfired is inefficient. The ideal is to have the heat input equal to the heat output, with the burner running, not switching on and off every minute.

Oftec say the nozzle is changed every service, but then a nozzle manufacturer is a member of oftec, and judging by some of the posts here, some who service oilers don't seem capable of assessing if a nozzle NEEDS changing. It also means more £ for the servicer.

You probably don't need new TRVs, I expect the pins need a light tap.

 

[Agile]

In a rather haphazard way, you have been given all the information that you need !

Your boiler is rapidly heating your rads to perhaps over the 80°C thats expected.

What we dont know, because its more difficult to measure on oil than gas, it could be that your boiler could be turned down to give less power and cycle less.

But, as you have been told, it needs little power today. Outside here in London its 9.2 ° now and thats a degree HIGHER that at lunchtime.

Your boiler would need to be on for twice as long when its -1° outside.

Whats more important than how long its on for, is how long its off for ! I would suggest you need the boiler to be on a 50:50 on/off cycling at the moment. If its a much lower duty cycle then the power could be reduced but you have already said that you have many rads off at the moment.

That leads me to wonder if you are wasting the planets resources ( and your bank balance ) by heating unoccupied rooms? Whilst you can probably afford it, its still wasting the world's resources which is totally unreasonable in my green book.

18 rads implies perhaps 14 rooms! Thats a lot of rooms to be heating! Do you really have so many people living there or do you just like to feel good wandering around a large house which is fully heated regardless of the real need?

Tony Green