Boiler efficiency question

ryanha

I have a 36kw combi Worcester and was wondering about boiler load. The question is, does my boiler use more gas heating 1 zone or 2 zones at the same time or is there no difference?

i.e. is the boiler using more gas when heating 2 zones OR when the boiler is on its just ON and the number of zones is irrelevant?

Jamie Hall

This appears to be a modulating design, so as long as the heating usage is withiin its modulating range, it will use just enough gas to meet the load at any given time. So yes, it uses more gas when two zones are calling that it does with one, and a lot more when hot water is being used. But -- it's matching the load in any case.

hot_rod

Also, the lower the operating temperature the more efficient it will run.

Energy efficiency can also be increased by upgrading the building heatloss areas. Weatherstrip, storm windows, insulation upgrades are often the low hanging fruit. Upgrades in theses areas are a gift that keeps on giving.

ryanha

That's a great help, thanks very much. The boiler is the worcester greenstar 38cdi classic and I'm just trying to work out whether its more efficient to have all 3 zones come on together on a timer (nest) or set the temperatures on each thermostat (nest) and allow the boiler to come on an off individually for wach zone when it needs to.

I was thinking it maybe more efficient to heat all 3 zones at the same time if when the boiler is on it uses the same amount of gas for 1 zone as 3 but what you're saying is it doesn't?

hot_rod

The longer cycles would impose less cycle wear on all the parts inside.
Mod con type boilers do a pre and post purge with the combustion fan, so I suppose a small amount of heat goes out the flue each start and stop cycle.
Also the boiler on low fire, lowest temperature will be the most efficient operating condition, combined with longest on cycles.
Has it been cleaned, burner tuned with a combustion analyzer?

captainco

If a modulating boiler operates between 3% to 5% O2 in all firing rates then modulation is efficient, However if the O2 is much higher in low fire and mid fire then modulating is costing excess energy. Also I have spent 42 years researching heating studies and never have I seen an actual study that verifies cycling equipment on and off uses more energy. In fact, if it operates with high O2 numbers it is definitely costing excess fuel usage. The real reason I found that people wanted to keep their equipment operating continuously was because of flame failures. Remember, until the 1980's we didn't have combustion analyzers that could really watch the total operation of a burner continuously in all firing rates.

Having tested thousands of commercial and industrial boilers, whether for heating or process, modulating was minimized in all cases. Everyone saw less fuel usage and less maintenance.

As to the original question, yes it will take more fuel to heat 2 zones versus one,

hot_rod

Dr Tom at Brookhaven National Labs did some studies on cycle efficiency on fixed firing type boilers, years ago. Maybe for a NYSERDA project?

First calculate run fraction Run Fraction= burner on time ÷ total elapsed time. Use that number on the lower axis of his graph. Below 30% run fraction efficiency drops quickly.
Oversized boilers, micro-zoning, mild should season operation can put you into that condition.

Low mass boilers generally have higher cycle efficiency due to less heat remaining after burner shutdown.
With high mass boilers heat purging, like Energy Kinetics does will help as will flue dampers.

I seem to remember Viessmann talking about mod con boiler efficiency in one of their trainings. Possibly Jody presented that in a Caleffi Coffee with Caleffi on mod con boiler efficiency. I'll look for it.

Notice on the graph the efficiency increase at lower outputs (25%) on a mod con. As the burner output decreases the ratio of heat transfer per unit of heat exchanger area increases.
Running on the red end of this curve results in higher HX surface temperatures reducing the rate of condensation.

captainco

As soon as I see Steady State Efficiency listed, I know they are using fictitious combustion analyzer calculations which are bogus. These are also lab conditions. If we saw numbers like btus in, gpm, Delta T and calculated output it might be believable. No equipment I know of has been tested for actual delivered btus, just assumed btus. ASHRAE did an actual field study that stated oversized equipment uses the same or little difference in fuel usage but that is another story.

I have talked to Tom many years ago and submitted some of my oil diagnostics evaluations. Even when I was working with engineers at AGA and ask about the equipment they were certifying they told me they don't actually measure the output or actual efficiency. They said that was up to the manufacturers. But then the manufacturers say well AGA said it was so.

I can adjust a burner so my analyzer will read 99.9% efficiency but in reality, it will be below 50%.

I don't remember where it is written, a boiler manufacturers manual or just a boiler book that stated operating any equipment more than 15% below its rated will cause a substantial loss of efficiency. Of course at the time modcons had not been developed. But only some of them!

Dave Carpentier

captainco said:

ASHRAE did an actual field study that stated oversized equipment uses the same or little difference in fuel usage but that is another story.

Considering the graphic that hotrod posted, would a 200k boiler running at 50% modulation be more efficient than a 100k boiler running 100% ?

JDHW

Dave,
I think that is true. I have a heating/hot water system with a thermal store running on a 30kW boiler that is limited to 15kW. Quite apart from benefit of condensing, a bigger heat exchanger means to hot side will be cooler for a given power level - more energy extracted from exhaust gasses.

John

hot_rod

These formulas and graphs were not developed by the sales and marketing team. They are based on studies done, actual experience, by people that work in the science of thermodynamics.

The run fraction seems commonsensical to me. Sit in front of your boiler with your phone and time it your self to come up with some actual numbers.
It also makes sense to my non engineer mind the concept of extracting heat from condensing conditions,
it is surface area related like most all heat exchanger is.

I don’t doubt or ignore the fact the burner needs to be dialed in properly and that affects the fuel utilization side of efficiency. My point deals with the heat exchange on the water side of things.

You could contact the authors of the studies and formula, and question them about methodology. And motivation🤓. I’m merely passing on published, widely accepted data. Use it or not as you see fit.

captainco

If a boiler is operating at 50% and its efficiency is 80% or operating at 100% and its efficiency is 85% then it will cost more to operate at 50%.

40+ years ago, I noticed that the calculated efficiencies on analyzers were mathematical formulas rather than measured numbers and are generally mis=representative of the actual efficiency. I remember back then I was asked not to tip over the apple cart. I just happened to find out it was full of lemons!!

If you ever check the cfm blower charts on furnaces or the gpm pump charts, you might notice that flow doesn't affect efficiency? But when you adjust the cfm or gpm incorrectly there is a rise in flue temperature showing less heat transfer. That is not even a hard test but it is ignorred.

The current venting tables in the Codes were written by a computer and mathematics.

fentonc

I instrumented my boiler last season, and the chart @hot_rod posted agrees pretty well with my experimental data. The y-axis is 'fraction of steady-state efficiency' (where I'm taking that as my nominal rated efficiency of 83.5%), while the x-axis is 'fraction of the day the boiler was firing':

The Steam Whisperer

I've used this efficiency curve for years. I've seen it in Burnham's modular boiler information too. It has proven to be highly accurate for predicting the annual fuel savings when going from an oversized gas atmospheric boilers to a properly sized atmospheric boiler. Also, it is very useful for predicting the annual fuel savings by going to staged atmospheric hot water boilers. I combined this curve with weather data in Chicago using bin data analysis (calculating efficiency at a variety of points on the curve) to get a predicted efficiency based on how well sized the boiler is compaared to the actual heat loss of a building (using standard ASHRAE design numbers) The points for properly sized, 2x oversized and 3x oversized are marked in pencil on the curve. This is for atmospheric boiler without stack dampers. It also lines up with the old US Dept. of Energy efficiencies (AFUE) when residential boilers were tested without dampers (usually about70% AFUE). Dampers usually gave an AFUE of around 78% and Spark Pilot around 80%, IIRC. I've recently found similiar curves for oil power burner boilers, but haven't incorporated weather data into them to provide predictions for annual efficiencies. I'd love to find curves for gas power burners, so if anyone has them that would be very helpful to predict the annual efficiency gains from going from typical oversized atmospheric boiler to a properly sized power burner boiler. Feedback from actual buildings where we have made this upgrade has shown about a 20 to 30% reduction in fuel usage from this change. Adding modulation with outdoor reset to a steam system seems to show another 10 to 12% additional reduction in fuel usage.
As an FYI, for typical WWI and later steam systems for typical oversized radiators ( about 60% oversized compared to heat loss), and with the typical oversized atmospheric boiler ( about 60% bigger than the radiation load and 33% pick up factor), the seasonal efficiency is around 54%. If the system radiators are resized to current design heat load using orifice plates and a heat load sized on/off power burner installed, we've seen about a 25% reduction in fuel usage. On systems with the same radiator setup, but using outdoor reset with a modulating 3 to 1 power burner fuel savings have been 40 to 50%.

It seems the MOD of Modcons is by far the most important factor for the fuel savings generated.

captainco

To calculate actual efficiency you need to know the exact btus in the gas, the exact cubic feet being used, the exact gpm and the exact Delta T. At one time I thought IBR measured this but they just use a points system. Also there is no valid test data that shows a flue dampers saves more than 3% and yet they add 8-10 points the AFUE for this. After watching an hour long video from a 40 year engineer from a commercial/industrial boiler company, last year, he stated that the best way to determine the efficiency of their boilers is to look at the calculated efficiency of an analyzer. Hey, I know a guy that would be a good investment for cryptocurrency!!!

Jamie Hall

Don't have a problem with @captainco 's remarks -- not one little bit.

It might be useful to look at the problem slightly differently, however. And that slightly different look starts with the question: what efficiency are you attempting to measure?

The basic definition of efficiency is simple enough: useful power output divided by actual power input. For a mechanical device, the useful power output is defined as the actual work; for a heating system if would be useful power as heat.

But.

A heating system (which is what we are talking about) is a system. Except possibly for advertising -- or regulatory -- purposes, it is quite irrelevant to discuss the efficiency of any one component of that system, such as a boiler or a pump or whatever. And there is a fundamental problem in that: too many variables, most of which aren't controlled never mind measured.

That is not to say that there isn't value in maximizing the individual efficiencies of each component of the system; there is (although one runs into economic considerations pretty quickly). If there are pumps, they should be chosen to operate at or near their maximum efficiency (which, incidentally, varies widely depending on where they are operating on their curves). A boiller size, besides being adequate for the job (which is often debatable) should be also chosen partly for efficiency -- but also adjusted when running in the specific setting for maximum efficiency consistent with reliable operation (not always quite the same). For forced air, choice of blower design and operating speed can make a big difference, as can variations in filters -- never mind duct work.

And so on.

I guess overall what I am saying is that all the talk of efficiency is well and good -- but needs to be treated in the same light as those miles per gallon stickers that are put on car windows. "Your mileage may vary".

fentonc

My boiler is a ~4x-oversized atmospheric CI boiler with three zones - my monitoring system tracks the supply and return temps for each zone, using the average temp to estimate baseboard output based on number of feet of fin-tube. On the input side, my boiler reliably consumes gas at a rate of 120 cubic feet/hour when it's firing (independent of outdoor temperature, from what I can tell).

Looking at a recent 'inefficient' burn - the boiler started out at ~65F, burned for 260 seconds to raise the water temp to 160F when the aquastat cut it off, then after several more minutes it had dropped to 140F and it started firing and burned for 30 seconds before the thermostat was satisfied. The thermostat called for heat for a total of 691 seconds. So the boiler burned about 9700 BTUs worth of gas, and about 3000 BTUs made it into my first floor while it called for heat. It was then several more hours before another heating cycle, so everything had cooled back down to about 65F. Some of the heat in the boiler probably made its way into the rest of the basement and/or the 1st floor, but much of it probably went in to slightly heating up the uninsulated concrete block walls around it, or out the chimney (there is a damper on the boiler, but there is no damper on the connected vent for the water heater right next to it).

captainco

Jamie you are correct as far as system efficiency is as important, if not more, than just combustion efficiency. That is what we teach at NCI, whether it is heating or cooling. All burners burn at over 99.9% efficiency if the CO is below 100 ppm or the smoke is zero. But the efficiency we are lead to believe our analyzers tell us is thermal efficiency and that is a mechanical impossibility and that is what the industry promotes.