Boiler efficiencies

hot_rod

A number of way to look at boiler efficiencies. These are input numbers from a Weil oil fired cast boiler cut sheet.

There are a few different numbers listed, input net, DOE, AFUE and AHRI certified including 86, 75, 87%. Pick your favorite. .70 gph oil input. 140,000 BTU content of #2 oil.

Ideal running condition would be steady state, probably a small fraction of the season even on a properly sized boiler.

Run fraction being burner on/ burner off minutes

Bottom right shows the result of oversizing. I used a 60K design day load, on a 40° day, maintaining 70° inside. 0° being design day. 15K internal gains guesstimate from occupants, appliances, lighting, etc

With a 100K boiler installed on that 60K design day load, running on this partial load condition it is running at about 60%.

Bottom line, with fixed speed appliances the penalty of oversizing can have quite an effect. It is worth the time and energy to do an accurate load calc, don't be sold and upsized boiler just to have a fudge factor, you will pay the penalty for the life of the appliance

PerryHolzman

You are absolutely correct Hot Rod.

Larger industrial installations that are expected to have variable steam loads for many years will usually install 2 if not 3 smaller boilers to meet demand for this and maintenance reasons (being able to do maintenance on a boiler while the facility stays in production).

More expensive to build; but pays for itself in efficiency.

Have a great day,

Perry

hot_rod

The load is the load, only upgrading, a smart investment, the building changes that.

However to Henry's point using the boilers most efficiently has some huge energy saving advantages.

Imagine this example with three 10-1 boilers a whooping 30- 1 turndown 10,000- 300,000. Allows the boilers to run in their optimum steady stat mode as much as possible.

Flue gas temperature tells a lot.

I could heat a space with fin tube 190° SWT flue gas over 400F

OR heat the same space with 120° SWT, flue gas 130F

Common sense would tell which is the best conversion of fuel to usable heat

Solid_Fuel_Man

This is one of the reasons I do not like oil for residential demands...sorry @Jamie Hall and @STEVEusaPA . Even the best oil equipment will be vastly oversized for all but the largest of residential heating loads.


With vapor-pressure atomization burners, ie. nozzles, we cannot get to the low (10,000 btu/hr) that most homes need the majority of the heating season.

This is where gas really shines, the ability to modulate simply and maintain a reliable clean burn at small firing rates.

I believe oil is best suited to larger burners as it avoids the tank freezups LP experiences. Oil can run more efficiently at steady state as hot rod shows!

Many years ago I did an experimental setup by heating my thermal storage with oil when I wasnt burning wood to heat it. Unfortunately oil was expensive and I never ran it long enough to quantify any real data. Basically what if any fuel could be saved by heating bulk storage with oil by running steady state once a day instead of constant short cycles.

We wont even talk about condensing equipment.

The Steam Whisperer

Those efficiency curves are the reason I have always been a fan of step fired, multiple boiler heating plants. Even if using Atmospheric boilers to replace a single large power burner type boiler we've seen fuel savings as high as 60%, with 30 to 40% being the norm. The atmospherics keep the equipment simple ( very little annual maintenance, inexpensive parts and often not susceptible to power issues) and long lived ( 30 year life or more) to keep maintenance and replacement costs down, while the efficiency is exceptional. I believe this is the sweet spot for larger variable load applications. It is unlikely that mod cons used for this application can have as low of overall operating costs as this type of heating plant, especially with the gas prices we have historically had in Northern Illinois ( around $0.30 per therm since the late 1970's. Maybe a couple Modcon lead boilers, but certainly not the whole heating plant.

HVACNUT

I dont know why we haven't seen a small Lo-Hi-Lo oil residential burner yet. It should have been here by now.
I think Riello offers a 2 stage residential burner for limited OEM furnace applications, but that's it as far as I know.
Facts are facts I guess and I must submit to the efficiency numbers, but total cost vs savings on oil to gas conversions has a lot of fuzzy math and each situation should be looked at separately. Not as a whole. IMO anyway.

95% of what I service is high end residential. (South Fork L.I. billionaires) Oil, nat gas, LP, Geo, and electric. Furnaces and boilers. Atmospheric and mod con. Ductless, etc.
I have to say, the amount of service and replacement parts needed fall heavily on the shoulders of mod cons and ductless. Year after year, the modern oil fired equipment and controls hang tough while I (and the client) wait 4 to 6 business days for a gas valve to arrive from South Korea. To me, that weighs heavily.

Jamie Hall

Trust me, folks -- if there were a variable firing residential oil burner (probably modulating on pressure in the 0 to 4 ounce range) which fit Cedric, I'd probably try it. If @Charlie from wmass would let me! But there doesn't seem to be...

That is... if I could be convinced that the economics/reliability equation made sense. It seems to me -- I could well be wrong, since burners aren't really my thing -- that it would be necessary to have an O2/CO feedback loop in there somewhere, or very precisely set up (and restricted by installation with tight maintenance requirements) fuel/air ratio controllers of some kind. And it seems to me that even such things for automotive and rail diesel applications, where they are universally used now, are neither particularly inexpensive -- nor particularly reliable (on motor vehicles, the second most likely cause of the dreaded "check engine" light is the oxygen sensor)(the first is you didn't tighten the fuel cap...).

And some would ask why stick with oil. Three reasons, highly location specific. First, natural gas is simply not available. The nearest line is seven miles away -- and the reliability is not good. LP is available, but one has the choice of locking in a contract and hoping that the price you locked in is a good one -- or buying the stuff week by week, and around here the price varies all over the map. Some weeks an LP BTU is cheaper than an oil BTU. Most weeks, more or less the same. Some weeks... wow. So oil it is. But as I say, that aspect of decision making is very location specific!

STEVEusaPA

Solid_Fuel_Man said:

This is one of the reasons I do not like oil for residential demands...sorry @Jamie Hall and @STEVEusaPA . Even the best oil equipment will be vastly oversized for all but the largest of residential heating loads.

No reason to be sorry, you're 100% correct. Oil is almost always oversized, and boilers usually need a buffer tank (except for an EK).
I think with the 2 stage oil boilers, low will never be low enough. Europe and Asia use, and don't seem to have a problem with condensing oil-fired equipment, just doesn't seem to work here.

Solid_Fuel_Man

I looked pretty heavily into condensing oil, Firebird if I remember correctly. One of the requirements was that the return temperature not be below 140F for sustained periods! What? That's why I wanted condensing equipment in the first place.

Solid_Fuel_Man

@Jamie Hall there are wood gasification boilers which incorporate an oxygen sensor in the stack and use the voltage output to control primary and secondary air shutters. Viessmann uses their flame rod to adjust the air/fuel ratio as well. They claim to have the only independantly set combustion blower and separately adjustable gas valve.

I've had excellent results with mod/cons, many lasting 10+ years with only cleanings every other year. I dont install the Korean ones....

PerryHolzman

A realistic part of this discussion is the life of modern boiler models, as the labor cost of replacing them is often more than the cost of the boiler itself.

A long life (30+ year) boiler with somewhat lower efficiency will almost always be more economical than many of the short life modern boilers if its properly sized (perhaps with a surge tank).

I was fortunate that I chose to install a Viessmann Vitodens 200 in 2007 (with LL Header, Indirect water heater, move boiler to sidewall & vent out side of house); so I have a potentially long life boiler with replacement part availability in the future to do that.

At year 12 (last year) I had to replace the burner control board, radial fan, and the ignition transformer (which required modifying the boiler). I also replaced the spark rod and ionization sensing electrode as that seemed to make sense (I have retained the old ones as "used spares"). Parts alone were about 1/2 cost of a new boiler (without installation); so well worth it overall (and much better than the folks who have had to replace their boiler at or before 12 years). Now I should get another 10-12 years before that is again required (the fan bearings wore out - and will likely do so again. The burner control card likely died due to capacitor aging - and will likely do so again.

I do have to wonder if a conventional heavy wall cast iron boiler would not have actually been cheaper than the overall cost of switching to the Modcon and given the cost of the needed "rebuild" of the Vitodens 200 every decade or so?

I do know that question has been debated before on this forum. In my opinion; high quality modcons with long term (several decades) part availability seem to me to be the only thing worth really discussing in the modcon world. Given that US Federal law only requires a company to supply parts for 10 years after discontinuing a model - you are looking for a manufacture with a history of doing more than that. That takes research beyond just the price of the boiler and the country of manufacture.

Perry

hot_rod

mainly I wanted to show how efficiency should be calculated and the big hit you take by oversizing a fixed speed boiler, since thatqeestion comes upon a lot. Calculate the load and go one size larger boiler

Also cast iron boilers are not what they used to be, wall thickness is thinner, water content smaller, I'm not sure they are 30- 50 year boilers anymore?

EBEBRATT-Ed

We all know how small the hole in an oil nozzle is. Especially now that most oil burners run over the old standard of 100psi which they did for better combustion (and they should have done that back in the early 60s like some commercial burners did)

But imagine trying to fire something down to .2 gph (28,000) btus how small the nozzle opening would have to be and how would we keep that from plugging up.

Not to mention the damper would have to be automatic for lo hi fire. Don't think it would be practical for a high pressure burner.

Maybe we should go back to a Timken rotary burner LOL

mattmia2

you could probably modulate oil with higher pressure, larger orifice and pwm control using a fuel injector like they do in automotive applications. Would need to regulate the air somehow. Probably the biggest factor is the residential market is pretty small.

Quality capacitors properly applied should last several decades. Figuring out which those are is very difficult now. i have identical power supplies that have been in service around 16 years and are still working fine and examples of that same supply that were built around 10 years ago that have all failed. Rebuilding your old control board with new caps that are of better quality and higher ratings is pretty easy if they are through hole mounted.

hot_rod

I think on a cast iron type boiler you would not be able to turn down very much. Too much cast iron mass with a low fire input, it may condense in the sections?
I think the burner size and heat exchange surface relationship are fairly critical? Unless it is designed to condense.

On a job where it is possible the stage multiple boilers.

What is the smallest residential oil boiler available?

Maybe stage a bunch of these 17,000 btu diesel RV boilers

https://www.heatso.com/eberspacher-espar-hydronic-ii-d5s-12v-heater/

Gordy

Any meaningful Modulation of a boiler with considerable mass would be a feat.

Lower water content, and less mass of the HX creates quicker response for modulation. Which can also be a monkey on a mod/cons back at times.

Real time BTU manufacturing, and distribution is the ultimate goal.

mikeg2015

My last electric bill was $0.15.kw-hr for winter rates! My gas steam boiler could be around 25% efficient and I’m still ahead of the curve.

My boss isn’t a big fan of steam and laughed when I put a indirect on my steam boiler to replace my 15 year old electric water heater. HE called it 50% efficient. I laughed... hell,, if it is I’m still spending 1/2 as much. Plus it was a fun project/challenge. I estimate a 3 year payback and even better it’s nearly endless hot water.

mikeg2015

As for condensing flue gases in a cast standard single pass atmospheric cast iron boiler, I think at low fire the excess air is so high, it;s a non issue. But at the same time efficiency drops because of the excess air.

The best concept would probably be like WM did with their 90+ boilers. A auxiliary economizer. Basically a add-on mini fire tube heat exchanger. It makes sense as you are only trying to extract 6-8% of the energy. No need for a huge stainless exchanger pulling 1500F gases down to 120, just need ot pull 350 down to 130 and pull out some of hte condensate. Then you cold vent with PVC even.

I’ve wondered if you cold take a steam boiler, remove one radiator and replace with underfloor radiant (like a kitchen with tile floors.) and a buffer tank on a closed loop, and that would be enough to pull a minimum of 8% to reach condensing and flue temps under 140.

Lets say you have a 200k steam boiler. You now can downsize it to 175k, and 8% is 14,000BTU for the condensing portion to go from 82-90% efficient at the edge of condensing. Practically speaking, so long as the kitchen is 7-13% of the heating load proportionally, then it balances out.

Other advantage is that it’s closed loop. Downside is that if you need more heat for something like DHW, you need another heat exchanger off the boiler itself for more BTU’s.

Would actually be cool if there was a retrofit kit that replaced a flue damper with this heat exchanger and inducer. One kit should be able to cover maybe 100-250k with varying restrictiors on the inducer inlet on 2” vent for 100-150 and 3” for 175-250 I;’d estimate.

Of course the kit would cost probably $1200 including the pump. What’s the payback for gaining 8% efficiency? But it replaces a $300 vent damper and would wire in to the same controls with a relay. SO net cost is maybe $900. For me, that;s around a 8-10 year payback. SO your want it as much for the PVC venting flexibility and ease in adding a low temp zone.

Plus add cost of buffer tank (if needed) and trim.

Gordy

I think a unique spread sheet would be one that can correlate a fuel price cost ,to life cycle of appliance, and appliance cost in determining when that fuel price point creates an exceptional ROI.

@hot_rod has shown how over sizing the appliance creates drastic inefficiencies on fixed output appliances. I think we all knew that, but not in detail. Modulating appliances can be a bit more forgiving.

PMJ

I guess I don't follow the efficiency concern here in residential. Big commercial is different. All of us will have to cope with the 10% run fraction on a fairly regular basis in residential don't we at least in the shoulder seasons? It would make little sense to try to handle those days by running on a continuous basis with fancy burners somehow.

The reality is that most of us even with significant oversizing are easily running in the 25% range on average days which has you at 75% vs a max of 87%. That gas difference even if true won't buy me anything, not even the annual maintenance on a special burner. Even my huge boiler is at 50% in the cold. Nothing on the chart to talk about at all there at all.

I will also say that natural vacuum between cycles changes the game at the low end of this chart. To take advantage of natural vacuum you need burner off time. I'll put it like this - all burner off time is definitely not equal.

george_42

The real efficiency of a boiler is the ability of the units ability of the unit to capture btu`s from the fire that feeds it . Take two boilers using the same burner and firing rate and on has a stack temp of 200 deg and one 500 deg , which is more efficient. Bethlehem dynatherm boilers had along flame pass and stack temperatures that were very low. In my mind one of the best boilers ever made and still made today.

Gordy

PMJ said:

I guess I don't follow the efficiency concern here in residential. Big commercial is different. All of us will have to cope with the 10% run fraction on a fairly regular basis in residential don't we at least in the shoulder seasons? It would make little sense to try to handle those days by running on a continuous basis with fancy burners somehow.

The reality is that most of us even with significant oversizing are easily running in the 25% range on average days which has you at 75% vs a max of 87%. That gas difference even if true won't buy me anything, not even the annual maintenance on a special burner. Even my huge boiler is at 50% in the cold. Nothing on the chart to talk about at all there at all.

I will also say that natural vacuum between cycles changes the game at the low end of this chart. To take advantage of natural vacuum you need burner off time. I'll put it like this - all burner off time is definitely not equal.

Sure it’s all meaningless when fuel is cheap.

When it’s not cheap it becomes an issue.

PMJ

> @Gordy said:
> (Quote)
> Sure it’s all meaningless when fuel is cheap.
>
> When it’s not cheap it becomes an issue.

Agreed.

But rarely do I see the real added ongoing cost of fancy burners and reduced life boilers addressed as part of the cost picture at all. That cost difference is not in the chart presented. When you do that in residential it would take a fuel cost way higher than the average of my 28 years with my boiler to pay for my doing something different.

PerryHolzman

PMJ said:

> @Gordy said:

> (Quote)

> Sure it’s all meaningless when fuel is cheap.

>

> When it’s not cheap it becomes an issue.



Agreed.



But rarely do I see the real added ongoing cost of fancy burners and reduced life boilers addressed as part of the cost picture at all. That cost difference is not in the chart presented. When you do that in residential it would take a fuel cost way higher than the average of my 28 years with my boiler to pay for my doing something different.

I totally agree Gordy: I believe now that had I taken the time to research long life conventional boilers with simple control systems in 2007 that I'd have been solid money ahead than my conversion to the Viesmann Vitodens 200 system. Of course, I do get "green" bragging rights to my decision, and I was able to put a new roof on my house in December without a chimney sticking out.

I add that if I did not have my own abilities to maintain that Vitodens that I likely would have had to replace it by now. No heating contractor one in my area has clue how to maintain it, or understands the necessary "jigsaw puzzle" method to access the control boards, etc. Several have said immediately that their recommendation was to replace it when I inquired with them.

Perry

Gordy

Across the pond in EU there are areas where you don’t have the luxury of hanging on to the standard efficiency stuff.

There is proactive, and reactive. Here in the U.S. we play reactive.

PMJ

The total annualized cost of residential heating systems has gone up not down fuel included. Factories must churn out many more boilers and furnaces than before to get the same job done because the stuff doesn't last nearly as long. Obviously many more installation/repair truck trips for the same total heat output too. I'm told this is really "greener". Count me skeptical.