Boiler now too large?

kamney

You could pipe a buffer tank in between the boiler and the load. The boiler would need protection and sounds like it does now as well - ideally, something like an ESBE 3 way thermatic on the return.

With a temperature rise of 25° for the buffer tank, every 10 gallons will add a minute to your boiler's run time and prolong the off cycle. You'd also need to factor in about one ET-30 expansion tank for every 40 gallons of buffer tank.

Randy_7

Boiler Now Too large?

I have an old 2-story home with a gas boiler and cast iron radiators: Weil-McLain, 150,000 input /120K output, Model PD7. When I purchased the home it had zero insulation in the walls and minimal in the attic, and the boiler heated the home just fine, though not cheaply. I have blown in about 40 bags of cellulose and sealed cracks and crevices. The problem now is that the boiler heats the home quickly and shuts down (short cycles) even in below zero weather.

A new boiler is not in the budget. I am wondering what other options I have to increase boiler efficiency. Now don't laugh but I'm also wondering if it is practicle or reasonable to pull one or two of the six cast iron burners from the boiler(though I expect the "Dead Men" will roll over in their graves at that question). Help is appreciated.

Randy

Gary Segal

Boiler too large?

Randy, I had a few questions that would help clarify your issues:

Since you refer to the dead men, I assume you have steam. Is it a one pipe or two pipe system?

When you say the boiler short cycles, do you mean that from a cold start, the boiler heats all the radiators by running for a while (10 -30 minutes), then cycles off and on for short intervals until the thermostat set point is reached? I assume that the total daily run time has decreased significantly, correct?

If so, I would assume that the boiler is cycling off and on by the pressuretrol or vaporstat. Can you verify?

Also, what type of thermostat are you using? Do you know the temperature differential between cut-out and cut in?

Randy_7

Thank you for the interest. I'm renovating the home and since insulating walls and attic, the hot water boiler requires much less run time to satisfy the thermostat - which is exactly what I was trying to achieve. I may have misused the phrase "short-cycling" (I'm absolutely not a heating pro) but I timed the burner / circulator run time a couple times during the work day and home came up to set temp in 5 to 6 minutes. I am of course concerned with efficient operation of the boiler. Some additional information:

1. System is cozy hot water with Weil-McClain boiler and B&G circulator. It's an old home and 2-1/2" diameter pipes in basement suggest (to my untrained eye) that it may have been a gravity system initially. Boiler is 150K/120K and home is about 1900 sf with storms and now insulated.

2. Thermostat is a standard, circular Honeywell. Anticipator is set at 0.2. The boiler gas valve current draw is 0.4 amps. Aquastat relay is Honeywell L8148E, 0.8 amp pilot duty.

3. Circulator and burner start and stop at the same time. Thermometer on the thermostat changes only one degree during burner run time and then raises about another degree due to heat from the radiaters. Example: If thermostat is set at 60, the burner/circulator will kick in at 61 and off at 62, and temp in home will then rise to about 63.

I'm editing a bit to try and answer some questions:

4. The original piping of the system had 2 pipes leaving the boiler and 2 returning. The piping in the basement is all large diameter (about 2-1/2" OD) and risers to the second floor are all 3/4". There is of course smaller diameter pipe (1-1/4"?) attaching the existing boiler to the system.

5. The longest boiler run time is in the morning (if I'm working on the home that day)when I arrive and crank up the thermostat. The "cold" boiler has a temp reading of 60F and 12 psi (2 story home). The burner and circulator will operate an estimated 45-60 minutes (depending on outdoor temp I guess) to satisfy the thermostat and raise the indoor temp about 12 degrees. At shut down the boiler pressure will be about 26 psi and the temp will be about 138 degrees. The aquastat is set at the lowest setting which is 180 degrees.

6. The thermostat anticipator is set at 0.2 and the gas valve current is labeled at 0.4 amps. Is that the proper anticipator setting?

7. The responses so far are certainly appreciated but appreciate that most technically baffle me. I'm confident a heating pro will understand when the time and $$$ comes.

8. That the burner and circulator operate together bothers me as I envision lots of heat going up the chimney as the boiler is hottest at shut down. This differs from my little boiler at home where the circulator can be running when the burner is off.

I hope this info is helpful and not to wordy. Input is appreciated.

Randy

Mike T., Swampeast MO

Yes, your boiler sounds to be greatly oversized. There is no approved way to downfire or downrate your boiler. Unapproved methods exist, but your combustion efficiency will most likely decrease as the chance of condensation problems increases.

If you see two pairs of 2 1/2" pipes, then your system was almost certainly gravity. If one main pair than it may have been made for forced circulation. Are the branch pipes on the ground floor larger than those upstairs? Are any branch pipes smaller than 3/4"?

Converted gravity systems controlled with burner and circulator operating together on a call for heat have an inherent reset ability. This is why a severely oversized boiler like yours will short-cycle. Unfortunately, there's really no way to stop this other than using a properly sized boiler. The only good thing is that your current boiler likely has decades of life remaining as long as you don't mess with it by downfiring... Reset control (mechanical is fine) might help a bit, but expect little.

Your best option is to install TRVs (thermostatic radiator valves) on all of your radiators and use a condensing/modulating boiler. The TRVs do add to cost but they allow extremely simple and easy boiler installation and likely only a single circulator for the entire system (just as you have now). There are significant labor and parts savings in the boiler room if you use TRVs with a mod-con and you can virtually bank on 50% fuel reduction in your case.





Since you're already getting thermostat overshoot, even increasing the thermostat's differential [likely] wouldn't increase your boiler cycle time significantly.

Mike T., Swampeast MO

Gravity systems aren't easy to buffer. If you merely place a tank in series, it just becomes part of the flywheel and you'll never be able to heat it above the system supply temp when the boiler stops firing.

If you put the buffer on the boiler side of a thermostatic bypass valve type ESBE, you automatically store energy in the buffer but you must gain access to remove that stored energy. Unless I've missed something simple, the easiest way I find to do this is via a tank with heat exchanger. This adds significant cost and extensive piping changes yet does nothing to address the root problem of a severely oversized, needlessly inefficient boiler.

None of these changes would be required if you replaced your boiler with a properly-sized mod-con.

kevin coppinger_15

interesting....

at what temp is the boiler running at? You mentioned that the boiler and burner run at the same time....do you have some pict. that we can see...kpc

Mellow_2

How about some controls?

You could look in to some outdoor reset controls for your boiler. the best way to get more $$$$. from your system is to control it. yes the correct size boiler would be a great investment but if that large boiler only ran when you needed it to and for langer cycles it can save you $$$$. the odr control can tell your boiler when to run and how long to run depending on how cold it is outside. cheak out tekmar controls.

Abro

Woops - wasn't clear enough...

When I said between, I meant for the buffer to be where the primary and secondary flows are separated. The ESBE would keep the boiler from charging the buffer until the boiler's above condensing temps. There's some piping involved but the buffer tank would still make it easy for adding a modcon. Just the ESBE would need to find a new home...

I totally agree that a properly sized sized modcon boiler would be best, but the OP said it's just not in his budget for now.