Firing rate vs water capacity

Fizz

Does water capacity of boiler effect rate to fire?  Ex:  Two years ago we converted WM SGO 6 to gas.  The boiler was oversized to connected load of 518, so we fired at lower rate than mfg suggest(Wayne 250A) for gun to GPH of 1.75 for oil.  Suggested fire rate was 240K BTU.  Fire to load was 165K.  We initally fired to load, but burner short cycled, so we increased to 195K, with good results.  The water capacity for boiler I believe is 14.2 gal.  Assuming a firing rate of 165, did water capacity account for longer heating to bring to steam resulting in increased pressure and short cycle?

Jamie Hall

If you increased

the firing rate and the short cycling stopped, it was something else for sure.  Increasing the firing rate will shorten the time from burner on to steam (which is related to the mass of the boiler and the amount of water) and increase the pounds of steam produced per minute, either of which should bring on short cycling.



Somethings odd here.

Tim_Hodgson

It is math

I would calculate that 165K input would get a cold boiler to steaming in 10 minutes. It would then evaporate 2 pounds of water each minute until the system was completely filled with steam and the end of the main and radiator traps closed. At that point the pressure should start to build. All providing the traps and main air vents are working well.

Raising the firing rate to 195K will bring the boiler from a cold start to steaming in 8 minutes. It would then evaporate 2 1/3rd pounds of water each minute.

What were the CO2 and CO readings?



Best regards,



Timothy Hodgson

Fizz

At the time

my HVAC guy(I'm a homeowner) wanted to raise pressure and water level above nwl. Having a Richardson system I refused, but he agreed to raise fire rating.  Don't know if he checked CO2 or CO levels.  Since the SGO 6 rating is  654 sq ft, and suggested Wayne firing rate of 240BTU's, the current rate is well below their figure.  Perhaps there was another issue, I have since added a vaporstat and system runs between 11/2-3 oz.  Wayne reccommended rate was to rating of boiler, so current rate of fire was compromise between size of boiler and next lower model, which has 21/2 gal capacity less than my boiler.    Sometimes a little knowlege is dangerous. 

VA_Bear

You said...

... you have since added a vaporstat. What controlled the cut out of the boiler before? What was the problem with the original control and what was it set for (cut in and cut out)?



Increasing the firing rate in a given boiler should not extend the firing cycle; I agree with Jamie that something is weird...



VABear

Fizz

A pressuretol

controlled cut-out before v-stat.  If i remember correctly, it was set at11/2lbs and 1/2lb.  Since change, system runs much better, as Richardson's love v-stats as Dan says.  I agree increasing fire rate should lessen time of cycle.  If you  change the amount of BTU's to a lesser rate, does the fact that the water capacity which had been heated at a significantly higer BTU input(240K BTU's) prior to conversion, effect efficiency? 

Dave in QCA

One more Chiming in....

I'm in complete agreement with all that have commented. Increasing the firing rate would NOT reduce short cycling.



There must have been some other factor going on. Do you know for sure what was causing the short cycling? Was it in fact caused by pressure? Low water? Loose wire? or something else. One thing about the pressuretrols is at very low setting they are very inaccurate and unreliable.



Now that you have a vaporstat, does it ever shut the boiler down, or does the pressure just run along at a very nice low level as you have reported. The vaporstat only functions by shutting the burner off when the pressure rises above the setting. When the burner is firing, the vaporstat isn't doing anything.



Most boilers equipped with power burners can be derated without a loss in efficiency as long as it is not too extreme. Generally 50 - 100 % firing rate will be fine. The boiler will also last longer. Extra water content when compared to other boilers that are designed for the firing rate that you are now running is not really an issue. While I have heard many comments on this forum that low water content is necessary for modern boiler efficiency, I suspect it has more to do with reducing the cost of the boiler and making them easier to install.



I recently installed a WM 680 and it has a HUGE water content when compared to other boilers and it has a relatively high efficiency. The WM 88 series also has a high water content, but has an even higher efficiency. It seems to be made possible by baffle plates that are inserted between the sections.



Remember, when you're in the heating season, your boiler stays hot between cycles and the amount of time that it take to cause the water to boiler is very little.

Fizz

Vaporstat never

shuts-down boiler. Whatever caused boiler shut down prior I don't know. Possible pressure issues.  So, did I make mistake by increasing fire rate?  If so, is it worth the effort to get it re-fired to attached load?

Dave in QCA

You're not overfiring

If your system pressure does not go above 3 oz, you are not over firing when compared to your radiation. Your numbers further indicate that, if anything you are still a little underfired. Remember ---- Your firing rate is NOT the output of the boiler. about 20% goes up the chimney.



Let's assume your burner is tuned reasonably well and you are getting 82% combustion efficiency. You might be getting better.... maybe worse... but lets use that as an assumed number. Let's also assume that you have insulated mains in the basement and so a 34% piping and pickup factor is appropriate.



You have 518 sq ft of radiation, so.... 518 sq ft x 240 BTU / sq ft x 1 BTU input/0.82 BTU output x 1.34 (piping and pickup) = 203,157 BTU Input.



If your system is running well with even distribution, then you probably hit the actual perfect firing rate by accident. Your actual needed piping and pickup may be less the 34%. These are all just rules of thumb anyway. If you crease your firing rate you will get to the place that your boiler cycles off an on after prolonged firing. That does not help anything. Now, if on the other hand you have uneven distribution in mild weather when steaming cycles only partially heat the radiators, then you might need to increase your firing rate a bit to improve the distribution. That is the purpose of the "pickup factor".

Fizz

Greatly appreciated guys!

Even a blind squirel gets an acorn??? Feel much better after reading posts.  Thanks to all!

Fizz