Oil boiler output power

[Bodgit]

Currently I have 2 boilers: an oil boiler for CH and a small LPG multipoint for DHW. I would like to ditch the multipoint and add a heatbank to the oil boiler system to get mains pressure hot water.

I'm quite happy to do all the pipework & control wiring etc. but I'm a bit confused when it comes to figuring out the output power of the oil boiler.

The boiler is a Perrymatic Jetstreme Mk.II. The burner has been replaced fairly recently with a Riello G3B (rated at 19-35 KW). Looking at the adjustment of the air damper, it seems that it is set to an output power of approx. 22KW.

What factors affect what the output power can be set to? Is it possible to increase the output power or will that damage the boiler?

Cheers, Dave.

[cynic]

Depends on the nozzle size, oil pump pressure, heat transfer surface of the boiler and its combustion efficiency.

See above

Possible but the heat transferred to the water is limited so you may simply burn more oil and lose efficiency instead

[David Hansen]

On 14 Jan 2007 13:08:36 -0800 someone who may be "Bodgit" wrote this:-

What is the result of your heat loss calculation for the house? How does this compare to the boiler power?

How large a heatbank do you wish to have? How quickly do you want to heat it? How will it be controlled? How will the heating be connected to it? What sort of hot water usage do you have?

Having cogitated on that the answer to your questions may be that the current boiler settings will work fine. Provided the boiler is capable of putting heat in faster than the heating and hot water is taking heat out, the bank will remain charged. Even if, for relatively short periods, the boiler is not charging the bank fast enough this may not be a problem as there will then be time for it to "catch up".

None of us will do the donkey work of calculations for you (well I assume nobody is that generous), but if it looked in the right ballpark I would be inclined to give it a try and see. One strategy would be to couple up the heating and measure how it behaves as stage 1. If it works move part or all of the hot water across and see how it behaves as stage 2. Fiddle if necessary. The multipoint can be retained in reserve, with suitable valving to change between the two. Stage 3 is to remove the multipoint heater after everything is working as you want.

[Bodgit]

I've already done the heatloss calculation - The house is a bit complicated and the heatloss programs don't really cater for some of the peculiarities, but I reckon I need about 28-30KW.

I wasn't asking anyone to do the calculations for me. I reckon the boiler is slightly underrated for what I want. My question was that if I was to adjust the burner to increase the output power (I would get someone in with all the gas analysis equipment to do this), is it possible to get a bit more out of the boiler without breaking it. 'cynic' pointed out that I might lose efficiency by doing this - thanks.

So given that my boiler is obsolete - I can't find any documentation - does anyone know how I can find out how high the burner can be turned up?

[Julian]

Once I shared a breakfast table in a B&B with a oil boiler engineer. I had a pressure jet boiler at the time so I picked his brains.

With a pressure jet boiler you can raise the burner pressure to increase the size of the flame, but he said that once the flame starts to touch the metal jacket on the far side of the heat exchanger cavity then you run into smoke and soot problems - so that effectively sets your limit.

The air needs to be set to the minimum that ensures clean combustion - white cloth held over the flue is a good test.

That's all I know WRT your question. HTH

Julian.

[cynic]

There is a very non linear relationship involved. Raising oil pump pressure and air supply will increase throughput but this is not always accompanird by a longer flame. I have seen flames actually shorten but become unstable.

A professional was he or simply worked in the game? I'd not consider that an acceptable test for much at all.

To the OP - there should be some kind of label or badge with a model number indication "somewhere" on the boiler unit, possibly inside the outer casing. Maybe an old nozzle with rating ( us GPH and spray pattern/angle stamped on one of the flats. Also what flue exit temperature do you have at the point of exit from the boiler? (somewhere between 200 and 500 degrees C)

[Julian]

My old Trianco Redfyre behaved as he described.

That's an odd thing to say. I looked after my old boiler for 10 years, kept it serviced and repaired it and running very cleanly based on his knowledge he imparted to me (before the www days) . Should I have purchased pyrometers, pressure gauges gas analysers etc etc to DIY maintain one little domestic boiler? There seems to be a trend nowadays for people to look down their noses at anyone who hasn't got thousands of quids worth of test equipment with bells and whistles. Common sense is 99% as good.

I think (memory fades somewhat) that he was commisioning some large industrial boiler equipment. (Vauxhalls at Luton) So yes, he worked in the game. What's your position and qualifications?

Julian.

[Bodgit]

Looking again at the plate with the serial number etc. on it, the model number is Jet 70/90. Now I've looked on the internet at other makes, I think I can assume that the 70/90 refers to the number of 1000s of BTU/h. So I think it should be possible to get 90,000 BTU/h or 26KW out of it. Is this a reasonable assumption? If this is the case then it's not too far off the lower end of my estimate - I have been a bit generous in places!

[Michael Chare]

Varying the jet size and oil pump pressure will alter the power input.

The boiler manual should tell what range of jets can be used. This has been possible on the boilers that I have owned.

The air supply needs to be adjusted accordingly.

Increasing the water temperature (if not already at maximum) would help the boiler output the heat.

Ask the manufacturer. - Assuming you can contact them Quite often the heat ouput range of a boiler is implicit in the name. Or there may be a plate giving this information on the boiler itself. Knowing the heat output you could make a guess at what jet size you should use perhaps by looking at the specs of other boilers of similar efficiency

If the boiler does not run continuously, increasing the heat input may not do you much good.

[Julian]

I think that it refers to the input and output, ie 90000 BTu's input

70000BTU's output, thus 20000BTU's goes out the flue.

Don't take my word for this it could be BS!

Julian.

[The Natural Philosopher]

That is massive. Insulation will pay off quickly at that sort of heatloss. I have a 6 bed house here, and the calcs gave just 10KW.

The boiler wiill always cycle or modulate back to what is needed. With a heatbank to cope wig peak hot water demands you don't need any extra for showers etc.

You biggest problem is a boiler that has to ruin 24x7 to keep the house warm. Cos its too small. If teh house has a low thermal mass,, then it will heat up quickly..mine doesn't and I DO need to run the boiler for a long time if it gets cold.

Conversely it stays warm when the heating goes off. The house itself is a massive heatbank.

No..just suck it and see.

In your case I would think long and hard about better insulation rather than a new boiler.

[Doctor Drivel]

Using heat bank you can size the boier for "average" use, not peak use as is the norm. So a smaller boiler can be used.

[Doctor Drivel]

The heat bank. Have it vented and the boiler heat it directly (no coiled heat exchanger). The CH. Have a coil in the bottom section of the heat bank (size to the max heat input to the house), to heat the CH. Have the CH pressurised using a pressure vessel, discharge valve etc. They can be bought as kits h

Get your heat loss figurers sorted to size the CH coil. On the CH return to the cylinder have a Magnaclean filter. On the CH have TRVs on all rads and use a Grundfos Alpha pump. No wall stat needed. The CH coil will ensure no sludge will accumulate in the cylinder. Put 1% by volume of cylidner and boioer and pipes of inhibitor into the cylinder. The same with the CH side - put into the rads or fit a filling point.