Boiler Clocking, Any Advice?

Rodney Summers

Volume of boiler and boiler-loop piping is 28 gallons.

A 280 MBH (output) boiler at steady state (80% efficiency) should take 1 1/2 minutes to raise 28 gallons by 30 degrees?

Current run times are 4 1/2 minutes (extra Btus reheating the boiler each cycle?).

Current efficency is 1 1/2 over 4 1/2 = 33 1/3% of 80% or just 27% overall?

So, an 80 to 100 gallon buffer tank in the primary loop should increase run times by 3 to 4 minutes, substantially improving overall efficiency of the existing system?

gasfolk

Any advice on recent design day clocking?

Our 99% design temp is 10*F. Any advice on a range of heat loss from the boiler clocking based on the graph below?

Thanks,

gf

Mike T., Swampeast MO

Let me make sure I have this properly:

At 8.6° outdoor temp your boiler was running 22½% of the time thus output averaged 39 mbh during the period? Which "output" rating did you use for the bottom scale? Net? I=B=R?

If so, would suggest that you're getting reasonable boiler efficiency in that weather. This is only a guess, but would suggest that your actual heat loss is somewhere around 85-99% of the net rated output or 65-79% of the gas input.

gasfolk

The bottom scale is A.G.A. gross output.

The boiler's Net I=B=R rating is 243.5 (for water), or 87% of the gross rating.

Is it possible that my design heat loss could be 39.7 MBH * 87% = 34.5 MBH? Lower if adjusted for some inefficiency of oversizing? Manual J gives MUCH higher numbers (three to four times higher).

The outside temp was steady between 6.8 and 8.6 F for 8 hours before the clocking, which started before sunrise, and which was fairly steady for two hours. These numbers don't seem to make sense. Am I missing something obvious here?

Ron Schroeder

The most obvious thing is that at only 22.5% runtime at a temperature below the design temperature, the boiler is at least 4 times over sized and your efficiency is less than 40% at 8.6 degrees and worse durring the rest of the season.

You would benifit from wider boiler temperature differential, outdoor reset, vent damper or a buffer tank if you don't want to change the boiler.

Ron

Mike T., Swampeast MO

Have all zones/TRVs/etc. been maintained at the same level for at least 24 hours? At what temp? Lower than "normal"? Surely you adjusted the heat loss for the actual indoor temp and not the typical 70° assumption...

gasfolk

Glad for your advice Ron and Mike.

Except for radiant floors in the kitchen and a small bathroom, the house is a single-zone, gravity conversion with outdoor setback and primary-secondary piping. A triple aquastat switches between boiler firing (with boiler loop circulator) and system circulator (also with boiler loop circ) in the boiler temp range of 115 to 145, and this has lengthened burns from 30 sec before to 4 1/2 minutes since.

The house was maintained at 67 F for several weeks prior, and the results were adjusted for that 67 F even though the thermostat was increased to (and maintained at) 70 F throughout the clocking (in an attempt to simulate a recovery from setback).

I appreciate your suggestions and would love not to change the boiler. Is anything else worth trying? If replacement becomes necessary, would these numbers convince you to size below Manual J's 90 to 120 MBH estimated heat loss?

Thanks for your comments.

gasfolk

Correction...

Sorry, was reading the wrong number off my spreadsheet.

The heat loss adjusted to indoor 70 F and outdoor 10 F was 41.9 MBH. Adjusting only to indoor 70 F gives 42.9 MBH. All during "simulated recovery from setback."

What magnitudes might any other contributing errors have? How far off from the "true" heat loss could these numbers be?

Mike T., Swampeast MO

Might be an error in your graph.

350,000 btu/hr input, correct?

During the period you averaged 22½% firing time.

So, for an hour you input 78,500 btu.

At gross efficiency of 80%, that means 63,000 btu were available for the load and remaining inefficiencies. Sure sounds like a boiler with an I=B=R rating of 60,000 would be quite adequate. In the current condensing/modulating world that would mean the smallest unit available in any line.

Was the test overnight? If during daytime was the sky clear or overcast? How about wind? If bright and sunny during a good portion and little or no wind, then such could definitely account for a decent-sized chunk of the "missing" heat loss. Of course the fact that you were raising the space temp points in the opposite direction...

I'm certainly no expert on this, but considering the extreme boiler oversizing, your 4½ minute burn times seem quite good. BTW, does the burn time stay fairly consistent with rising outdoor temp?

As long as the gas bills aren't eating you up in moderate weather, it wouldn't seem prudent to change boilers until the current one dies...

Mike T., Swampeast MO

BIG difference there!

Was having a hard time believing such a massively oversized boiler could be operating so near its optimum efficiency.

See later message. I have no idea how to increase your efficiency without changing boilers as again, your burn times seem good. If you do decide to change boilers a modulating-condensing model [should] be able to pay back the price difference with reasonable speed.

gasfolk

Does boiler efficiency decrease as load drops?

> Might be an error in your graph.

>

> 350,000

> btu/hr input, correct?

>

> During the period you

> averaged 22½% firing time.

>

> So, for an hour you

> input 78,500 btu.

>

> At gross efficiency of 80%,

> that means 63,000 btu were available for the load

> and remaining inefficiencies. Sure sounds like a

> boiler with an I=B=R rating of 60,000 would be

> quite adequate. In the current

> condensing/modulating world that would mean the

> smallest unit available in any line.

>

> Was the

> test overnight? If during daytime was the sky

> clear or overcast? How about wind? If bright

> and sunny during a good portion and little or no

> wind, then such could definitely account for a

> decent-sized chunk of the "missing" heat loss.

> Of course the fact that you were raising the

> space temp points in the opposite

> direction...

>

> I'm certainly no expert on this,

> but considering the extreme boiler oversizing,

> your 4½ minute burn times seem quite good. BTW,

> does the burn time stay fairly consistent with

> rising outdoor temp?

>

> As long as the gas bills

> aren't eating you up in moderate weather, it

> wouldn't seem prudent to change boilers until the

> current one dies...



The 42.9 MBH load is A.G.A. gross (equivalent Net I=B=R value of 37.3 MBH) WITH assumption of reduced boiler efficiency to about 50% at an OBSERVED load of 22.5% on the boiler.

Last year, on a 37 F morning, the boiler clocked at 18.1% run time. The calculated low-load efficiency was 44.5%, giving an output AT 37 DEGREES F of 28.2 MBH (A.G.A. gross) and a calculated design value for 10 DEGREES F of 66.2 MBH (A.G.A. gross). If true, declining efficiency at the low end (and a shallowing of the "Calculated % Run Time at Load" curve above) could explain boiler run times varying in a narrow band and the difficulty in extrapolating to design temp from low loads.

How is steady state efficiency determined? Can a 4 1/2 minute burn achieve the published "steady state" efficiency of 80%? Is it a myth that on/off (non-modulating) boilers lose efficiency at low outputs?

"Sure sounds like a boiler with an I=B=R rating of 60,000 would be quite adequate. In the current condensing/modulating world that would mean the smallest unit available in any line."

Can you suggest anything else to improve the heat loss estimate and make it more convincing.

gasfolk

Does boiler efficiency decrease as load drops?

The 42.9 MBH load is A.G.A. gross (equivalent Net I=B=R value of 37.3 MBH) WITH assumption of reduced boiler efficiency to about 50% at an OBSERVED load of 22.5% on the boiler.

Last year, on a 37 F morning, the boiler clocked at 18.1% run time. The calculated low-load efficiency was 44.5%, giving an output AT 37 DEGREES F of 28.2 MBH (A.G.A. gross) and a calculated design value for 10 DEGREES F of 66.2 MBH (A.G.A. gross). If true, declining efficiency at the low end (and a shallowing of the "Calculated % Run Time at Load" curve above) could explain boiler run times varying in a narrow band and the difficulty in extrapolating to design temp from low loads.

How is steady state efficiency determined? Can a 4 1/2 minute burn achieve the published "steady state" efficiency of 80%? Is it a myth that on/off (non-modulating) boilers lose efficiency at low outputs?

"Sure sounds like a boiler with an I=B=R rating of 60,000 would be quite adequate. In the current condensing/modulating world that would mean the smallest unit available in any line." For a 3200 sq.ft., 1928 house. with leaky storm windows and esentially no wall insulation, that is stunning--but I agree. 60 MBH is the maximum. If the heat loss is lower still, we could make do until the next generation of technology?

Can you suggest anything else to firm up the heat loss estimate and make it more convincing? The above numbers were without an automatic damper and also had combustion air drawn from living space...

gasfolk

I agree, Ron.

Unfortunately, the "Adjusted Combustion Efficiency" and "Calculated Run Time" curves may be inappropriate.

To estimate combustion efficiency at partial load, we used the equation on page 29 (the 32nd page of the file) of Henry Manczyk's paper Optimal Boiler Size. A similar curve appears on page 6 of Condensing Technology, but we have found no supporting reference to the equation's validity or how to adapt it to our boiler.

If efficiency drop at partial load is real, then our 8.6 F heat loss estimate would be on the 22.5% line, somewhere to the left of 63 MBH. That 39.7 MBH figure was calculated using Manczyk's equation.

Any suggestions how to better estimate partial-load efficiency?

Thanks.

Ron Schroeder

Hi gf,

Brookhaven National Lab is currently doing a study of true operational efficiency compared to AFUE. I am watching the study closely. Previous studies at the Lab have shown significant reduction in efficiency with partial load and short cycling. The new study intends to develop accurate models of part load operation for both heating and DHW by identifying and quantifing all losses. Although the new study has just started, it shows significant effect on efficiency from control stratagy and utilization of thermal mass.

Yes, the efficiency does drop a lot on low % loads, especially with short run times. A major portion of a 4 1/2 minute run is not near steady state efficiency.

Ron

[Deleted User]

Hey Ron...

Any chance we can watch the results too? Got links?

Thanks!

ME

Ron Schroeder

Hi Mark,

I don't think anything will be online until the study is finished but I will try to post intermediate results.

Ron

gasfolk

> Hi gf,

>

> Brookhaven National Lab is currently

> doing a study of true operational efficiency

> compared to AFUE. I am watching the study

> closely. Previous studies at the Lab have shown

> significant reduction in efficiency with partial

> load and short cycling. The new study intends to

> develop accurate models of part load operation

> for both heating and DHW by identifying and

> quantifing all losses. Although the new study

> has just started, it shows significant effect on

> efficiency from control stratagy and utilization

> of thermal mass.

>

> Yes, the efficiency does drop

> a lot on low % loads, especially with short run

> times. A major portion of a 4 1/2 minute run is

> not near steady state efficiency.

>

> Ron

gasfolk

Ron,

"Previous studies at the Lab have shown significant reduction in efficiency with partial load and short cycling."

Are results of previous studies available? Online?

Many thanks.

gf

Mike T., Swampeast MO

Ron

Keep seeing mention of objective thermostat setback studies at Brookhaven but can only find vague references--not actual studies. Search at their website was fruitless. About all I can find is that there seems to be a flip-flop with daytime instead of nighttime setbacks recommended...

Any suggestions?

Mike T., Swampeast MO

If you check monthly gas use vs. degree days, I believe you'll find a substantial drop in efficiency with rising outdoor temps.

It really seems to me that attempting to estimate heat loss by the run time of a traditional boiler is a "chicken and egg" situation. Without an estimate of heat loss you can't estimate efficiency; without an estimate of efficiency you can't estimate heat loss.

Rodney Summers

Buffer Tank?

1. Volume of boiler and boiler-loop piping is 28 gallons.

2. A 280 MBH (output) boiler at steady state (80% efficiency) should take 1 1/2 minutes to raise 28 gallons by 30 degrees?

3. Current run times are 4 1/2 minutes (extra Btus reheating the boiler each cycle?).

4. Current efficency is 1 1/2 over 4 1/2 = 33 1/3% of 80% or just 27% overall?

5. So, an 80 to 100 gallon buffer tank in the primary loop should increase run times by 3 to 4 minutes, substantially improving overall efficiency of the existing system?