2 over-sized boiler questions

Brian_74

According to my calculations, my present boiler is more than 50% oversized. If that's right, does that mean that I'd cut my fuel usage in half if I had a properly-sized boiler at the same efficiency? (I'm thinking it's not that simple, but I can hope).



Right now, the boiler is never shut off by pressure, only by the thermostat. It never gets above 5 oz/in2. When I raise the thermostat 1° after I've added water, then it does shut off on pressure. Right now I have the thermostat set at a constant temperature 24/7. Would it be more efficient to program in two 1° set backs instead?

JStar

How was the boiler sized?

Did you measure all of your radiators? How did you come to the conclusion that your boiler is oversized?



I would personally keep steam temperatures constant throughout the day. I like to keep all of those pipes and radiators from getting too cold when the boiler first cycles on.

Brian_74

Data

Thanks for the reply. I thought it might be simpler to keep the particulars out of it, but since you asked…. I used the info in TLAOSH plus a replacement boiler sizing chart that's been posted on the wall numerous times. According to my calculations, my system uses 81.6K BTus but the boiler is rated for 168K.



I agree about keeping the temp constant, but even on the coldest days, the boiler fires no more than once every two hours and only for 20 minutes at a time.

JStar

BTUH

Your boiler rating of 168K is for input or output BTUH? You need a boiler whose output rating matches your connected load. So maybe you're not 50% oversized...maybe you're only 50% efficient.

Brian_74

Thanks for checking

I appreciate your double checking, but I did cite the output BUTH. The input is 210K BTUh.



I think I was asking a more theoretical question than where we seem to be going. I'll have to think about how to reword my question.

Charlie from wmass

The answer is no to cutting it in half

If your boiler was over sized it would short cycle on pressure. It maybe the pressure trol is not shutting off the boiler now due to a clogged pigtail. If the boiler was 50% over sized then it would heat up shut off on pressure then after several starts and stops it would satisfy your thermostat. Do you have the square feet of steam for your boiler? also what fuel do you use? If it is oil the nozzle may be downsized and the boiler is not producing the steam it is rated to.

Brian_74

525 Sq ft of steam

Thanks, Charlie. What you wrote about short cycling makes sense. It just seems as if the boiler "simmers" more than boils, although obviously I haven't been inside to see what's actually going on.



The pigtail is clear, and the pressure never reaches 6 oz/sq in., whereas the pressure control is set at 1lb. The pressure control does cut off at the right pressure when I bump the thermostat up a degree.



The boiler runs on natural gas. When I had it serviced at the beginning of the season, the technician upped the pressure, which seemed to make bigger flames overall, but I could be imagining that.

Charlie from wmass

Why did he mess with the pressure?

Did he analyze it or just go by eye? Also how are your return line vents?

Brian_74

Maintenance, I thought

This was part of an pre-season check up. He used a manometer but didn't do any exhaust analysis. It was not changed to correct any problem.



I have 5 Hoffman 75s on my two return line vents. Four of them are new this season. The other passed the "blow test" and seems to be working fine.

JStar

Forgot to ask.

Did you add a pick-up factor to the total radiation load? Usually add 33%. Some guys add as much as 50% if you have a lot of piping in the system, especially uninsulated. If your main concern is efficiency, you can install a boiler with less input BTUH, that runs at a higher efficiency.



Are you having any problems with the system? Are you just figuring out if anything could be done to be more efficient?

Brian_74

I forgot too

I need to dig out my worksheet to see what I did. But even if I didn't include an extra 33%, that gets me to 108,528 which is still more than 30% too big.



The system runs ok. Like everyone else, I wouldn't mind if it were more efficient, but I also don't have a target. What happened was that someone on the Wall was answering someone else's question about keeping his old boiler in service. He pointed out that in one sense running an old boiler is costing you extra fuel dollars every month compared with a new one, so hanging on to an old boiler even though it's running might not be the best choice. I looked into how old my boiler is, and discovered that it's at least 44 years old (it's a W-M H series 4, which I found out was manufactured August 1961 − November 1966). My goal was to get a ballpark idea how much a new boiler would save me each month so I could think about replacement even though it's running fine. And I thought that I needed to know not only the efficiency of both the old and the new boiler, but also what a properly sized boiler would save just by being smaller. That's where my original question came from.

JStar

No question.

You would be saving with a new boiler. How much? Who knows? But you would be cutting down on your gas usage. Have you had a combustion test to check efficiency?

Brian_74

But I want to move

Thanks for reading through my long post, JStar. The additional wrinkle is that I would like to move. I don't have to, which is fortunate given the market, but it is something to think about. So I was hoping that I could crunch some numbers to give me a better idea of how long it would take for a new boiler to pay for itself.



I didn't have a combustion test done. It wasn't offered, either. This was my first (and last) time using this particular company. I'll have to remember to have one done in the fall.

JStar

Well...

...if you're just going to replace the boiler for better resale, I wouldn't go crazy with a high efficiency boiler.



1 Therm = 100,000 BTU



Maybe you can use that to figure how much you would be saving.

Brian_74

I'm probably making this overly complicated

But, here's what I've been thinking. First, with the real estate market the way it is, it might be a few years before I can sell my house. A HE boiler might pay for itself during that time, or at least pay for the difference between it and a less efficient model. Second, between the green movement and everyone's desire to save money, a HE boiler might be a selling point in a way that it never would've been in the past. I just don't know. So I was trying to get some data about what I could know.

Charlie from wmass

Not just combustion efficiency

The old boiler may have piping issues and will have lots of sludge or scale or both from distilling those thousands of gallons of water or the past 4 decades. This creates an insulation effect that makes it harder to boiler the water.

Brian_74

Another good point

But I happened to have cleaned out the wet returns this past weekend. While there was some crud (enough to cause a flow problem when the system was at full pressure), it didn't seem too bad. Point taken, though, that a spanking new system will be more efficient for a variety of reasons.

djthx

1 sq. ft EDR = 240 BTUH

in steam heating systems.  525 sq ft = 126,000 BTUH.  When you add the pickup factor, the amount of BTUHs needed is 167,580 BTUHs.  Accordingly, your boiler is right on.

Brian_74

I think I'm missing something

These numbers all came from the plaque on the boiler, so I'm not surprised that they're compatible. Or am I missing something?

Steamhead

What boiler

do you have now? If it's oil, what burner is on it? 

Brian_74

Weil-McLain

It's a W-M H series 4 running on natural gas.