Down Firing Oversized Boiler

BrooklynMike · 5:56PM

I have a WcM WGTO4 4 section boiler with a 1.0 gpm nozzle. I have taken some data on oil usage during this recent very cold winter. I have an elapsed timer on the oil nozzle solenoid valve coil so I get accurate run data without the pre and post purge time effecting the run time data. I have gotten run times between 4 and 5 hours per day . This works out to heat loss of 20,000 to 25,000 BTU/hr for the whole house. In reality, it is lower since I am not accounting for BTU's used for DHW. For a 6 hour run time in single digit temperatures, this equates to a duty cycle of about 25%. You think maybe the boiler is oversized? The nozzle installed is .85 @ 140psi. I have an electric hot water tank in series with the tankless coil , so boiler temp is not critical for my DHW needs. Your thoughts and recommendations on installing a smaller nozzle with the correct pressure as recommended. BTW, when we got this house, the boiler had an equivalent 1.2 gpm nozzle installed and it sounded like a blast furnace and rumbled a lot . I had to replace the combustion chamber material ; it was cracked totally. Thank you.

HVACNUT · 6:14PM

Interestingly the dimensions on the 3 and 4 section GO boilers are the same. The middle section on the 3 is double wide. The 3 section actually holds more water (14.9 vs 13.4 gallons) but the 4 has more fire side surface area.

Firing rate on the 3 section is .95 GPH, so as long as the stack temperature is above 350° net at steady state, the 4 section can fire at the same rate.

jesmed1 · 7:55PM

Is the boiler oversized? Based on your report of a 25% duty cycle in single digit temps, yes it is, massively.

We're in a similar situation with two WGO-5's heating our 4-unit condo building. Each has an input rate of 1.2 gph, and coincidentally they also run 25% duty cycles in single digit weather.

The WGO-5's are rated for 1.45 gph input, and I asked our heating oil company to downfire them as much as possible. They consulted Weil Mclain, who would only OK downfiring to 80%, or 1.2 gph.

Even with this admittedly modest downfiring, our gross stack temps are routinely 400+, with net temps 350+.

Our ultimate solution will be installing smaller boiler(s) when these crap out.

EdTheHeaterMan · 8:14PM

So you are saying in any given 24 hour period, the oil burner flame is only on for 5 hours. even near design temperature. You are also saying that this boiler maintains a minimum temperature for a tankless DHW coil. That can happen when you are using the same boiler for heat and hot water. often a boiler is sized to the hot water demand when the heating demand is much lower than the boiler size indicates.

A boiler with a 0.25 GPH firing rate (if that were available) would not produce enough heat to maintain a sufficient boiler temperature for taking a 6 minute shower unless you had it restricted to about 0.75 GPM shower head. The standard water saver shower head in the USA is about 2 to 2.5 GPM flow rate. So that small boiler will not heat your water.

This is one of the reasons that I do not like the “combination heat and hot water" boiler systems. Even those wall hung gas Combi boilers are problematic for me. Your best system is a high efficiency boiler that matches the heat loss of the building with an indirect water heater storage tank that has some storage in order to allow the smaller input boiler to do the heating of the tank and the home more efficiently.

There I go rambling on about the virtues of an indirect water heater again. This is left over from my last comment on another post

Jamie Hall · 8:21PM

That's a hot water boiler. However, one can't say whether or not it is truly oversized. How does it's output at that firing rate compare with the radiation installed? It is not a condensing boiler, so there is little to be gained by reducing the circulating water temperature, so…

If the boiler is bouncing off its high limit, indicating that the radiation can't absorb its output, then yes you might say it is oversize. Otherwise while the system as a whole is oversize for the building, the boiler alone isn't.

Now the next question is if it is oversize, can you downfire it? You have it firing at very close to its design BTUh input rate, and some — but not all — boiler and burner combinations can be downfired. Somewhere in your boiler manual or even on the nameplate it may say.

If it appears you need to, want to, and can, then if you have a very good technician you can experiment with a smaller nozzle size — but this takes a certain amount of experience and knowledge and often experimentation to get right.

BrooklynMike · 10:01PM

Thanks to all for the interesting comments.

EdTheHearterMan,

I think you are on the right track. I suspect the prior owner put a priority on DHW at the expense of cost of oil and over fired the boiler.

Jamie Hall,

Yes the boiler is bouncing off the high limit. This is not a warm home. The baseboard radiation is a very old type with steel fins not the modern aluminum slant fin we are used to. I already had to put the cold kitchen/ family room on a separate zone to make it comfortable for us. The rest of the house is not over radiated. Boiler output exceeds installed radiation by a big amount.

We are a couple of senior citizens who are trying to make the best of what we have here without making a big investment. In our previous house, we installed a Buderus 115W3 and an indirect hot water tank and loved the efficiency but could not take it with us when we moved. Trying to make small incremental changes to get the heating costs reduced without huge investments. Luckily, I do have a good plumber and oil burner technician who uses a good combustion analyzer to work with me as I make improvements. Will try a smaller nozzle and adjust pressure as needed in the fall.

Jon_blaney · 10:06PM

Help me understand the the issue. Does it make a difference if the boiler delivers the required heat in ten minutes or fifteen minutes. Does that much heat go up the chimney?

hot_rod · 10:50PM

Does the home ever maintain a comfortable 72° in all the room?

If not it could be under-radiated. A room by room het looss calc would determine that.

If some rooms are warm but not others it could be a balance issue.

Most agree a boiler should fire for at least 10 minutes once it lights off. On a design day if everything was sized correctly, it would run non stop.

The run fraction determines how efficiently the boiler operates

If you could watch the operation, or record it.

Run fraction= lets say it runs for 2 minutes, then 10 minutes off

The elapsed time is 12 minutes 2 ÷12 = .16 or 16%

The chart below, enter the bottom axis at 16% run up to the red line. The boiler is running 65% or so. A tuned oil boiler should run closer to 83%

So run your numbers to get a good idea of how efficient the boiler runs.

The worse number may be on a mild day, when the boiler is most over-sized.

Combi boilers rock for efficiency. An oversized mod con, (sized for DHW) adjusted to the heat load running 180 swt would be mid 80% efficient, and would not short cycle. The lower graph shows a mod con at 100% and 25% firing. so with a 160° return to a mod con you jump a few points above 82%.4. A gas fired mod con running outdoor reset is hard to beat for efficiency.

At the end of the day the building itself determines how many BTU/hr you need to put into it. Before you buy a new boiler tighten the home up as much as possible.

The heat emitters size to the heat load number and the boiler sizes to the heat emitters.

Screenshot 2023-07-28 at 11.38.08 AM.png

Jamie Hall · 11:06PM

Do see if your technician can downfire that boiler. Most WMs can be, possibly quite substantially. It just takes a bit of fiddling.

On the radiation. Have your plumber or heating person take a look at how the heating is plumbed. There may be a possibility for a small investment of breaking the radiation up into zones and then trying to balance the flow between them to get more to colder areas. That said, you may be stuck with the simple lack of enough radiation in some rooms.

Yes you could consider a new boiler, or even installing more radiation, but see what you can do with what you have before thinking about that.

Down Firing Oversized Boiler

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