EDR Between Two Boiler Sizes - Which to Use?

ChrisJ

SWEI said:

ChrisJ said:

Everyone claims the longer the run time, the more efficient the boiler.

I've yet to have anyone explain why this is, or isn't true

When the boiler shuts off, heat goes up the flue. On natural draft appliances this happens mostly due to natural convection (note the increasing popularity of motorized flue dampers over the past couple of decades.) On forced- or induced-draft appliances, the post-purge circuit accomplishes a smaller version of the same effect.

So on atmospheric boilers with a damper, how big is the effect?
Is there any published data on this? Fewer long run cycles vs a bunch of short ones?

If it saves $1 in fuel a year, it's pretty much meaningless.

SWEI

ISTR many (most?) manufacturers adding motorized flue dampers to their natural draft boilers about 15 years ago, almost certainly in response to one of the DOE rule changes. I think that got them from 79-80% AFUE to 82-83% AFUE.

Actual savings would depend on the building in which they are installed. A tall chimney paired with a leaky basement in a cold climate can suck a LOT of hot air up the stack.

ChrisJ

SWEI said:

ISTR many (most?) manufacturers adding motorized flue dampers to their natural draft boilers about 15 years ago, almost certainly in response to one of the DOE rule changes. I think that got them from 79-80% AFUE to 82-83% AFUE.

Actual savings would depend on the building in which they are installed. A tall chimney paired with a leaky basement in a cold climate can suck a LOT of hot air up the stack.

Yeah...

My 30' 6" B-vent + drafty basement is pretty impressive when it's below zero out.

It pulls a decent draft when it's 50 out actually.

ChrisJ

Hatterasguy said:

What I'd like to know is why oil fired boilers never adopted the motorized flue dampers to any significant degree. I would have thought it would have also helped them immensely.

Because the flue sucking a ton of air through the barometric damper doesn't count as losses from the appliance so it doesn't effect AFUE numbers.

Pretty simple really.

Fred

@ChrisJ said: "I've yet to have anyone explain why this is, or isn't true but until that happens I'm going to follow that rule."

Chris, I guess we each get to pick and choose which "proven" rule we decide to stand with. I have no problem with that, just understand that everyone is entitled to do the same, without criticism. Everyone has there own experiences. As I've told you, on numerous occasions, both in these posts and in our exchange of messages, I too have an over-sized boiler and I run at 1 CPH and my typical boiler run is @ 1 ounce of pressure, longer runs may get to 2 ounces and one time this past season, when the weather was -10, about 11 degrees below Design day, my boiler cycled on pressure, 12 Ounces, twice during a single heating cycle. I know I can eliminate that just by going to 2 or 3 cycles per hour but I'm happy with the way my system runs @ 1 CPH. I'm not saying I'd intentionally over-size my boiler when it comes time to replace it but I guess, by the standards of a few, on this site, I would be because I intend to replace it with the traditional Piping and pick-up factors. That's OK, I respect everyone's opinion. I just wish they'd realize that their view is an opinion, just like everyone else's and that there is more than one way to get excellent results. I know your system works exactly as you want it and is efficient. I just happen to feel others are in that boat as well, using different techniques. I'm hoping the OP uses all of this input in making his decision and doesn't view the input as distracting from that goal. I've said a couple times before, people come here for help and we should try to stay focused on their issue, but I digress.

ChrisJ

Fred said:

@ChrisJ said: "I've yet to have anyone explain why this is, or isn't true but until that happens I'm going to follow that rule."

Chris, I guess we each get to pick and choose which "proven" rule we decide to stand with. I have no problem with that, just understand that everyone is entitled to do the same, without criticism. Everyone has there own experiences. As I've told you, on numerous occasions, both in these posts and in our exchange of messages, I too have an over-sized boiler and I run at 1 CPH and my typical boiler run is @ 1 ounce of pressure, longer runs may get to 2 ounces and one time this past season, when the weather was -10, about 11 degrees below Design day, my boiler cycled on pressure, 12 Ounces, twice during a single heating cycle. I know I can eliminate that just by going to 2 or 3 cycles per hour but I'm happy with the way my system runs @ 1 CPH. I'm not saying I'd intentionally over-size my boiler when it comes time to replace it but I guess, by the standards of a few, on this site, I would be because I intend to replace it with the traditional Piping and pick-up factors. That's OK, I respect everyone's opinion. I just wish they'd realize that their view is an opinion, just like everyone else's and that there is more than one way to get excellent results. I know your system works exactly as you want it and is efficient. I just happen to feel others are in that boat as well, using different techniques. I'm hoping the OP uses all of this input in making his decision and doesn't view the input as distracting from that goal. I've said a couple times before, people come here for help and we should try to stay focused on their issue, but I digress.

Pull your pigtail off and hook up a quality gauge and tell me what your pressure is. As has been proven, and Dwyer themselves actually told me, pigtails skew pressure readings at lower pressures. The pressure my Magnehlic model 2002 shows connected directly to my boiler with nothing more than an air gap isolating it is very accurate even at fractions of an ounce. If you tried to use this gauge with a pigtail it'd likely end up showing a vacuum the entire thing with the needle pegged in the negative.

I don't pick and choose anything, I go by data and nothing else. I have no way to measure the efficiency of my boiler and I've been trying to come up with a way.

jumper

Holohan wrote that undersized boiler equals long run time equals more fuel consumption. OTH when old big boiler was replaced with multiple boilers owners were pleased with reduced fuel consumption. Just about all winter only one is used. My gut feeling is to undersize replacement boiler. How many times have we heard about problems stemming from undersized boiler on steam system?

As for draft heat losses,I don't see that it's a big deal with steam. OTH with HHW and always on circulator those rads and baseboards can get COLD. Same if HHW heater uses low limit.

ChrisJ

jumper said:

Holohan wrote that undersized boiler equals long run time equals more fuel consumption. OTH when old big boiler was replaced with multiple boilers owners were pleased with reduced fuel consumption. Just about all winter only one is used. My gut feeling is to undersize replacement boiler. How many times have we heard about problems stemming from undersized boiler on steam system?

As for draft heat losses,I don't see that it's a big deal with steam. OTH with HHW and always on circulator those rads and baseboards can get COLD. Same if HHW heater uses low limit.

I agree.
The only way an undersized boiler can possibly use more fuel is if there are balancing issues and somehow the radiator starved is in the thermostat room.

With a properly balanced system an undersized boiler cannot possibly use more fuel. It goes against physics.

Fred

@ChrisJ , I clean my pigtails regularly and have a quality 0 - 3 PSI gauge and a 0 - 16 ounce Vaporstat on my system. Even if my gauge is off by 100%, I'd still be operating well within acceptable ranges. I certainly don't mean to argue any of the points you made. I'm just saying the others who have commented about how their boilers are running have some credibility as well, given how they have set their systems up and my own personal experience.

PMJ

Fizz said:

Agree with PMJ. Have 478 SF of radiation, with a WM SGO 6,which has a steam heat rating of 654 SF. I switched to gas and had contractor fire the Wayne conversion at 186K. My pressure runs 1-2 oz. Two mos ago put system in vacuum and the numbers run 4-9Hg. Also changed CPH from 1(steam setting) to 3CPH(hot water) and ther results are as PMJ states. Heat is much more comfortable and without significant swings. No issues related to oversized boiler.

Great @Fizz. Glad to know someone else tried this and and has experienced what I am saying. Really nice way to run isn't it.

All I am trying to do is share my experiences and certainly not to diminish others' success with their systems. My work led me to a very different place than I expected it to when I started. A lot of the balancing act is removed running this way. The size of the boiler is not nearly so critical, there is much reduced investment in vents, and traps really aren't needed either. All in, it is pretty simple. Yes, I am programming a PLC, but there are much simpler ways to spread the burns out and make more cycles.

Anyway, good to hear that you are enjoying this approach.

Charlie from wmass

I often the undersized boilers use more fuel than an oversized boiler because they simply don't ever shut off and they never reach the proper temperature for the building. An oversized boiler will at least keep the building and then shut off and stay off. Most homeowners do not have this issue this comes in when we're dealing with larger buildings with multiple units also multi-story buildings end up with this issue being more of a problem because all the steam goes to the top radiators and very little goes to the second and third floor radiators.

Charlie from wmass

350mbtu for 18 hours a day versus 750kbtu for 5 hours a day.

SWEI

Hatterasguy said:

What I'd like to know is why oil fired boilers never adopted the motorized flue dampers to any significant degree.

http://www.fieldcontrols.com/oil-vent-damper?page_id=93

ChrisJ

Hatterasguy said:

Charlie from wmass said:

350mbtu for 18 hours a day versus 750kbtu for 5 hours a day.

6300MBTU for the former versus 3750MBTU for the latter. Total consumption for the day.

Kindly explain where the additional 2550MBTU disappeared in the former system. The energy went somewhere.............where did it go? Without a good answer to that question, the claim cannot stand on its own.

I might believe it if there was a huge set of uninsulated pipes in unconditioned space. Otherwise, the physics don't add up.

I believe he's saying the unwanted heat was wasted in someone's apartment or apartments and they likely had the windows open.


It goes with my "the radiators in the thermostat room were starved" theory.

ChrisJ

Hatterasguy said:

ChrisJ said:

I believe he's saying the unwanted heat was wasted in someone's apartment or apartments and they likely had the windows open.


It goes with my "the radiators in the thermostat room were starved" theory.

That would do it if no attempt was made to balance the system.

But to make the blanket statement that a tiny boiler will consume more fuel must be factually incorrect if the two systems are similar and properly balanced.

If one system is balanced (because it is relatively easy to do) and the second system is not at all balanced, how can you condemn the second system for using more fuel when the windows are open in the overheated rooms?

Because that's what people do.

Charlie from wmass

The extra energy is lost due to the top floors being 75 plus degrees and the heat simply dumping out into the world. Windows, walls ceilings, everywhere heat can be lost.. I was not charged to find the lost energy, just there to get the boiler running right so the thermostats could shut off the boiler. Also you are assuming all the energy went into the building. Due to combustion efficiency 20% goes up the flue the longer the boiler runs. A boiler that is shutoff is 100%efficient.

ChrisJ

Charlie from wmass said:

The extra energy is lost due to the top floors being 75 plus degrees and the heat simply dumping out into the world. Windows, walls ceilings, everywhere heat can be lost.. I was not charged to find the lost energy, just there to get the boiler running right so the thermostats could shut off the boiler. Also you are assuming all the energy went into the building. Due to combustion efficiency 20% goes up the flue the longer the boiler runs. A boiler that is shutoff is 100%efficient.

A larger boiler still loses 20% up the flue.
That 20%, get's larger with the larger boiler.

a 50,000 btu boiler running 100% loses 10,000 btu up the flue.
A 100,000 btu boiler running 50% loses 10,000 btu up the flue.
A 200,000 btu boiler running 25% loses 10,000 btu up the flue.

And they all in theory deliver the same amount of heat into the structure per hour.

Also, a boiler that is shut off still loses energy into the surrounding air, and up the flue, damper or not. So it's far from 100% efficient.

Charlie from wmass

20 percent is not lost when a boiler is shutoff.

ChrisJ

Charlie from wmass said:

20 percent is not lost when a boiler is shutoff.

This can easily be calculated.
Assume the ambient air is 50F, boiler water is 212F, what is the loss through 1/2" fiberglass, since that's what our manufacturers splurge on.

And doesn't matter, like I said, all of the boilers in theory, lose the same amount when running. Energy can neither be created or destroyed.

Charlie from wmass

I am referring to the loss due to combustion transfer to the water through the cast-iron. The jacket loss is there if it is running or off. Until.the boiler is at room temperature which means less hours run a day means less heat pumped up the chimney.

Charlie from wmass

My experience has also been when the small boiler also did not match the heatvloss of the building.

ChrisJ

If a smaller boiler puts out less than needed for the heatloss of the building, is connected to a balanced system the fuel bills will have to be smaller.

People are forced to keep a lower indoor temperature and therefore heatloss is less.

John Mills_5

Back before the 78% requirement, a furnace maker might sell a standing pilot model and call the AFUE 65-67%. Putting a spark ignition system on it, it would raise to around 70%. Put an electric damper on it and AFUE would be around 80%. Not sure how that applies to boilers but since furnaces blower air to cool their heat exchanger in a couple minutes, the damper is more keeping heated basement air in the house rather than heat in the furnace. Since boiler sections can stay hot for hours, more critical to have a damper closed during the off cycle.

As for smaller boiler doing less cycles being more efficient... when is the burner the most efficient? During warmup? No, steady state. Like a car, slow & steady uses less gas than floor it and stop at the next red light.

ChrisJ

John Mills said:

Back before the 78% requirement, a furnace maker might sell a standing pilot model and call the AFUE 65-67%. Putting a spark ignition system on it, it would raise to around 70%. Put an electric damper on it and AFUE would be around 80%. Not sure how that applies to boilers but since furnaces blower air to cool their heat exchanger in a couple minutes, the damper is more keeping heated basement air in the house rather than heat in the furnace. Since boiler sections can stay hot for hours, more critical to have a damper closed during the off cycle.

As for smaller boiler doing less cycles being more efficient... when is the burner the most efficient? During warmup? No, steady state. Like a car, slow & steady uses less gas than floor it and stop at the next red light.

I'm a bit confused.
The house I grew up in had a 1958 forced air furnace in it that said it was 80% efficient. I still have a picture of the tag somewhere. That had a standing pilot that didn't even have a thermalcouple, only the main gas valve did. No damper.

SWEI

ChrisJ said:

The house I grew up in had a 1958 forced air furnace in it that said it was 80% efficient. I still have a picture of the tag somewhere. That had a standing pilot that didn't even have a thermalcouple, only the main gas valve did. No damper.

That was the (nominal) thermal efficiency when it was actually firing. Pre-AFUE, standby losses were not even considered. Annual efficiency likely somewhere around 55-65% depending on how tall the flue was and what else was out of whack.