Down firing an oil burner

johntrhodes81

I have a Thermopride OL16-125 furnace currently with a 1.25 gph nozzle. I could get by with half that size based on heat loss.  If I switch the head and nozzle to 0.65 gph do I risk condensing in the chimney or some other issue? The instruction manual only gives max nozzle size not minimum.  Cutting in half seems drastic.

The oil burner is the reheat for the heat pump defrost as well as stage three heat.

John

jesmed1

You have just opened a large can of worms. I'm a homeowner who asked a similar question here a few months ago and got an earful from pros telling me why that was not a good idea.

I'll let them speak for themselves and just share what I found as a homeowner who is a mechanical engineer and did a lot of my own research. I did find a few pros who said they have downfired oil boilers by as much as 50% with no bad effects. I even found one pro who said he had downfired a boiler rated at 1.0 gph input down to 0.3 gph, again with no ill effects.

On the other hand, you will get many pros telling you that you cannot downfire that drastically without losing efficiency, risking condensing, etc etc.

I was willing to take the risk and try downfiring from 1.2 gph to 0.7 gph for one year as an experiment, and see if all the bad consequences I was warned of came to pass or not. But in the end, my boiler company refused to downfire it that far. They consulted the boiler mfr (Weil-Mclain) who would only OK about a 20% downfiring, so that's what we did, and I gave up trying to go lower.

So we have a boiler (actually 2) rated at 1.45 gph input, but running at 1.18 gph input, even though our heat loss is only about 1/3 of that.

My "solution" is to have solid heat loss calculation numbers ready for when a boiler conks out and the oil company wants to install the same size boiler we now have. I'll be ready with proof that we only need a boiler 1/3 to 1/2 the size.

johntrhodes81

Thanks. I have no service company so I can do what I like. It's a furnace not a boiler so that may make for less issues.
John

Jamie Hall

The boiler manufacturer will have, somewhere -- may be on the nameplate -- a minimum firing rate which a particular boiler has been checked to operate at. This is not to say that with very careful choice of spray pattern and pressures from the nozzle that you may be able to go lower. You and your technician, however, will own the job completely -- actually, you will if your tech. does it at your direction. Further, he or she (or you) must be able to do very thorough combustion checks and analysis of the flame pattern to ensure that the flame properly fills the firebox and the heat exchanger, whatever type it may be.

Unless the flame pattern is very carefully adjusted to fit the firebox and heat exchanger, there is a very real, though not inevitable, risk of increased corrosion on the fire side.

You will see a drop in real efficiency (not the usual measured efficiency, which is calculated partly on the basis of flue gas temperatures) as the fixed losses from the boiler will not change (the absurd extreme is to consider placing a candle in the firebox and seeing how things work).

The flue gas temperature may drop significantly. This will be a problem in terms of corrosion of the flue, though how much of a concern this is depends on how the flue is built.

In short, yes it may be possible to downfire beyond the manufacturer's testing. You will not gain efficiency. It is not simply a matter of unscrewing one nozzle and screwing a smaller one in. You will no longer have any warranty on the boiler at all. And it takes real skill and the time and equipment to try various combinations to come up with one which will work -- if one can be found.

HVACNUT

You might need to lower the fan speed to be within heat rise specs.

STEVEusaPA

Good luck. You won't save any money and will probably cause more harm than good. Unstable, less efficient, if it works at all.
You probably boil water on the stove at 'low' to 'save money'.

johntrhodes81

It's a furnace and not a boiler. Most usage is for heat pump defrost reheat not supplemental heat.

JMWHVAC

I am always trying to fire boilers/furnaces lower, but to cut in half may be extreme. If done right you WILL save fuel but other issues may arise. When I went from .75 to .55 on my boiler the water literally started running down the chimney. Downsize the fire, replace the burner head with the proper one for the fire and suddenly you may not get enough light to the cad cell. So think small steps not drastic, and other components & settings may also need changing. There is really nothing wrong with going as small as can be made to burn & operate properly.

HVACNUT

Is the fan control a fan/limit control or time delay? If fan/limit it might come out if defrost before the HX is hot enough to even turn the fan on.
I don't think you'll see any "folding money" savings.

johntrhodes81

Speed 1 is fan wire from stat.
Speed 2 is 1st stage compressor
Speed 3 is 2nd stage compressor
Speed 4 is the fan and limit switch.  Doesn't trigger much.

EBEBRATT-Ed

@johntrhodes81

I would say cutting in half would be too drastic. Don't know what oil pressure you're running but I would go with a .85 nozzle and then bump the pump pressure, so your firing rate is somewhere between .85 and 1.00gph.

But you really need a combustion test kit and know how to use it to make sure combustion is ok. You need the stack temp @330 or higher. And check the temperature rise while it is running to make sure that is within range.

You could also call Thermopride and ask them.

If you look on line many of there furnaces show 3 different firing rates and they usually ship their furnaces with the nozzle with the middle rating. Again, the oil pressure also changes the firing rate.

They do not appear to have your furnace listed it's probably an older model.

In short if you have the original furnace manual that may help. See if you can get some info from Thermopride. Thet would cut down on the experimentation.

johntrhodes81

Thanks