Modulating Boilers

Xc8p2dC_2

I recently ask this type of question, trying to get the same info you seek, but formated my question improperly and got the response from the HVAC gods, that it was almost impossible to determine as there are to many variables.

The direction I was going was based upon info on my current oversized system, that I do have run times on. I will repost and let the experts decifer if it a feasible way to look at it, or correct it.

My current On/off natgas boiler is 140,000 BTUh INput[confirmed by clocking the gas meter]and water temp is 195/180 > The burner runs for 8 mins, 2 times an hour at 25 deg outside temp>> if I divide 140K by 60 mins I get 2,333 btu per min,, times 16 mins [37,328 btu] times 24 hrs a day[895,872btu],, times 30 days a month [26,876,160btu] divided by the proposed 1040btu avail per CF of natgas or 104,000 per CCF,, I get 258 CCF of natgas consumed a month [This is coinciding with my bill]

NOW, I was projecting if I used a Modulating boiler, that say runs at 30,000 btuh BUT runs a lower water temp and runs for [say] 24hrs a day,, would equal 720,000 btu,, times
30 days = 21,600,000,, divided by the 104,000per CCf = 208 CCF a month

I know this subject to many vaviables like not 24 a day, not 25 deg out Not 30k all the time, but isn't there something realistic to it??

But there is also the added cost of the electricity to run constant circ and for me a .20 per KWH would add to heating cost

Let the arrows fly, again>>
Rich

Tim Doran

Modulating vs. On/Off

I have been trying to work out a calculation routine for comparing modulating boilers to on-off boilers and truthfully I am stumped. Has anyone ever seen anything like this? The efficiency vs. efficiency stuff is easy but figuring out the actual benefit of modulation in terms of fuel savings is a bit tougher. Any thoughts?

Tim D.

jwade55_3

In Elite software EZDOE energy analysis program, you have the ability to enter modulating parameters, seems to show a good comparision between fixed fire and modulating, from the standpoint of steady state efficiency vs short cycling.

J

Mike T., Swampeast MO

I know this subject to many vaviables like not 24 a day, not 25 deg out Not 30k all the time, but isn't there something realistic to it??

Sort of, but in all honesty still many other missing variables:

I get 258 CCF of natgas consumed a month [This is coinciding with my bill] Won't disagree at all, but unfortunately this say nothing about how much of that 258 CCF actually went to the system itself--in other words you only know consumption, not demand. Perhaps the current boiler has 80% efficiency (common). You can't just take 80% of the consumption and assume that such is the actual demand. You need a carefully conducted heat loss calculation to estimate demand--and even then such calculation is almost always an overstatement. How much? Seems to vary significantly with the structure and the form of heating.

After you have a reasonable estimate for actual demand, you can get a decent idea of the total system efficiency with the current boiler--but with a simple calculation like above it's based on some sort of "steady" conditions that don't occur in real life. Total system efficiency is dynamic--with conventional boilers the closer the load is to the output the better the efficiency. As the weather gets colder your system as a whole becomes somewhat more efficient.

So, you really need to attempt to determine the total system efficiency over a complete heating season to get a reasonable number for comparison.

It gets even more complicated when you try to predict the system efficiency of a modulating/condensing boiler.

You can't just assume "lower temperature"--you have to attempt to determine how much lower depending on the weather--in other words you have to calculate an assumed reset curve. While there are some general "rules of thumb" for the ratio (e.g. how much warmer the water gets with 1° drop in outside temp), you also have to know the shift of the curve.

You can only get an idea of the shift by carefully calculating the output ability of the emitters at various average temperatures. Presuming yours is one-pipe fin baseboard, you essentially have to re-calculate each loop each time you change the temperature.

All doable, but THEN you still have to try to figure out how well the condensing/modulating boiler will perform. Some energy reduction comes from modulation (no cycle loss), but with anything but a fully proportional system (TRVs or pneumatic) you'll still be getting cycles. How long or short those cycles depends on how well the supply temperature matches the actual load. Also, whenever the total load is lower than minimum modulation you will always get cycles--with ANY system. How the system is used complicates this even further--with an ideal curve and no thermostat changes you could get pretty close to 100% burner operation in colder weather. Problem is that this seeming "perfection" makes it very difficult to raise thermostat settings as there's only enough energy being produced to maintain temperature, not raise temperature! (I know this problem ALL TOO WELL.)

Then you have to get an idea of the savings from condensation (some manufacturers post efficiency ratings for their condensing/modulating boilers at various input/output temp combination). You can't just say "forget about condensation--I'm interested only in modulation". Why? In most cases if you don't lower water temperature to suit the load at the time you won't get modulation to begin with! (The exception here is a basic Munchkin or Trinity which modulates on delta-t across the boiler. In this case however you can't take full advantage of lowered temperatures and condensation because you're firing to a fixed temp regardless of the weather. Add simple reset and you no longer get modulation!)

See how complicated this becomes?

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All that said, I do have an idea for a relatively easy comparison. You must first however estimate the total system efficiency of the current system over an entire heating season. Note this is NOT the AFUE of the boiler! To do this you will need reasonably accurate weather data--hourly is better than a daily average. You also need an accurate heat loss calculation PLUS a reasonable estimate of how much this estimate is typically overstated. Trust me when I say that Manual-J calculation WILL be overstated to some degree. Then you have to relate the weather data to the heat loss and actual indoor temps in a reasonably accurate way--this will likely not be the "standard" base 65°F degree day data by the way...

Once you have this you still need to equate such to operating temperature of the system. You'll need to carefully assess the output of the emitters related to your computed load. To save time and trouble, I'd suggest doing so at the "average" outdoor temp of a given heating season. Don't forget that you're working with "average" emitter temps so you must make assumptions regarding delta-t...

THEN find the net boiler efficiency at this "average" requirement--again most manufacturers should provide a simple graph.

FINALLY you can reasonably assume that your seasonal system efficiency with the condensing/modulating boiler will be quite close to the net boiler efficiency in your calculated conditions. Say you calculated a system efficiency of 65% for your current system and some condensing/modulating boiler has 92% net efficiency at your average temperature requirement. Your fuel savings would be close to 92-65 = 27% provided that you use a nearly idealized reset curve.

Was that any easier???

Mike T., Swampeast MO

Tim: Believe I've done this with reasonable accuracy with one system, but it has taken YEARS of data collection. Also, the system is not typical by American means--fully TRVd, standing iron, gravity conversion with Vitodens and no low-loss header. The modulating nature of the system removes a number of variables.

Previous boiler (conventional cast iron around 2x oversized) operated the exact same system for a number of years. Prior to replacing the boiler, I had calculated that the net annual efficiency of the system was around 47%.

After more data and fine-tuning found about 45% efficiency for the old system.

By carefully adjusting the Vitodens reset curve (based on data collected last year) was able to achieve about 97.5% net annual efficiency so far this season. Also, have kept indoor temps quite low this season--while I factored in corrections for this, the generally lower supply/return temps are obviously part of the efficiency increase as I'm certainly getting more condensation. I'm essentially running the system at a curve much more common with radiant floor systems using good conduction methods. Also, this heating season has been more consistent and mild than most--again boding for higher efficiency based on lower temp requirement.

BUT, the boiler has achieved true modulation very little of the time compared to last season. Instead, it's either producing "bursts" or "pulses". So, even the mighty Vito must take some sort of efficiency hit when the load is so low that it rarely modulates...

If my numbers are anywhere near accurate, I should be able to achieve something like 94% seasonal system efficiency in more "normal" weather and warmer room temps. BUT, to achieve this the reset curve will be cut so close that raising space temp is torturously slow unless you adjust both boiler and TRV(s).

Xc8p2dC_2

expected

Thanks again Mike T,, I knew from your reply to my last topic, you would go to great detail, but luckily for me, [thanks to the wall], I understood everthing you explained!!

It just dosen't make for an easy sell, if a customer ask's
"how much savings would I get from a ModCon">>
Rich

Mike T., Swampeast MO

Glad if that helped and I'm honestly trying to simplify...

It just dosen't make for an easy sell, if a customer ask's "how much savings would I get from a ModCon"

I cannot agree with that statement.

Modulating/condensing boilers offer a selling point unavailable with other boilers. "The more you consciously try to save, the greater the return".

Again, I know ALL TOO WELL that trying to save with my old traditional boiler was almost fruitless. Reducing the indoor temp only made the boiler less efficient! While fuel consumption would drop slightly, comfort dropped MUCH faster--plus I seeming hit a point where no further fuel reduction was possible no matter how low I tried to keep the place!

Not all homes/structures are well-suited to modulating/condensing boilers. If they are not reasonably insulated and weatherized, then forget it--just use a traditional boiler as if they're trying to "save" they just use deep daily setback and accept the resulting discomfort.

Another "red flag" is highly modified systems. For instance: say most of the place is heated by iron rads and a fin baseboard (or worse kickspace heater) has been added in a separate zone. These emitters will likely have VASTLY different temp requirements but the condensing/modulating boiler will only have the efficiency afforded by the one with the highest temp requirement.

If the house itself has been modified significantly and repeatedly with the heating system being "adapted" to the change, watch out. Balance is likely to be screwed and again, the mod/con is a slave to the least efficient element!

As long as the structure is reasonably suited to a mod/con (or is DEFINITELY being brought up to standard), you MUST start with a good heat loss calculation.

Compare loss to the current boiler. If the current boiler is grossly oversized, then you can honestly say, "You have potential for good savings.

Then assess the emitters relating their output to heat loss (for this step I would honestly knock of at least 10% from the heat loss calculation). The lower the temp required to MAINTAIN conditions the more potential for good savings.

If the current boiler is oversized and the MAINTENANCE temp required is significantly lower than the 180° "standard" then there is a potential for truly exceptional savings.

If on the other hand, this is a fin baseboard system with a well-sized conventional boiler and well-sized (to the 180° "standard" supply temp), then savings potential is likely just "fair". (Just wish I could define "fair", but I'm unable as MUCH will have to do with how the system is operated.)

Then, your next duty to your customer is to use a system that allows reasonably easy access to the reset curve. Adjusting a reset curve is not difficult but with a condensing/modulating boiler it takes TIME--LOTS OF TIME--WITH SMALL, INCREMENTAL CHANGES. This is why the HOMEOWNER NEEDS ACCESS. Even if they're willing to pay, do you really want to go back repeatedly for just a few minutes each time? (Plus, the homeowner will STILL have to provide you information on their perception of comfort for you to be able to make proper changes.) If this is a boiler like the Buderus GB with vector-based indoor reset, then I still SWEAR that you should attach the controller to a tether wire and let the homeowner work to find the "sweet spot". THEN install it permanently!

Tim Doran

Calculation

Is it possible to create a standardized calculation based on your lessons learned and those of others. We should be able to prove out the advantages of a modulating boiler using some straight forward math.

Tim D.

J.C.A._3

Tim,

Wish I had the answer on the numbers part for you....but alas, no dice there. However, I can offer what I hear from customers and see for myself.

I just finished a job that had an old 2 door W/M converted to gas about 20 years ago. While doing the install, the boiler would fire 4 minutes then go off for about 25 minutes. Input was set at 170,000 or so BTU/HR. Water temps ran to 160° and the circ would shut off at the same time as the burner.

Proper heatloss, and considered exsisting radiation made a "perfect" scenerio for a Mod/Con Euro style boiler. I fired and after testing, set the "limit" in heating mode to 150°. With all that cast iron my Delta T is hovering at right around 25° and the home is just as comfortable as it was with the bang/bang boiler. BTW, boiler only comes out of condensing mode when running for hot water. Customer complaints(?)/compliments so far....We don't hear it start or stop, the temperature swing is unnoticable.The gas bills will surely be dropping as this boiler settled right into low modulation after 2 hours running time and almost 7 hours off for the changeover and removal. (I refused to run it with all that dust flying around...even with a concentric vent kit)

Mr. Milne has a customers report from last year as to therms consumed VS. degree days on another one...maybe we can chide the owner into giving us a comparison with the price increases and the mild winter we're having so far.

Bottom line IMHO is customer comfort. From the ones we've done so far, we get nothing less than positive feedback. The lower gas bills are a bonus! Comfort is King! Chris

Mike T., Swampeast MO

Tim: Certainly possible, but perhaps not particularly practical.

BUT...

If I wrote the computer program to do this would you:

1) Have access to HOURLY temperature data from a nearby RELIABLE source? The computer savvy to put this into some sort of reasonably standard format?

2) Spend enough time with the homeowner to understand their habits enough to make a reasonable guess at the "average" indoor conditions in the home? And would you be willing to tell the homeowner that their current thermostat practices may not be well-suited to a mod-con?

2) Would you be willing to analyze fuel bills for at least one year--provided of course by the potential customer--AND making reasonable adjustment for DHW (if from the same fuel source)?

3) Would you be willing to CAREFULLY measure heating emitters and CAREFULLY relate such to your previously calculated heat loss for each space?

4) After doing that would you be willing to pay even a token amount for the program to put this all together?

In my opinion this could never be anywhere near a "free" estimate.

Assess the structure, the current boiler and the emitters. THAT you can do with reasonable speed based on your experience. Give a relative assessment to the homeowner saying that YOU HAVE THE POWER AND I WILL GIVE YOU THE SIMPLE ABILITY TO OPTIMIZE COMFORT WITH CONSUMPTION--NO MATTER YOUR DEFINITION OF COMFORT.

This is the ART and I suspect that NOBODY, ANYWHERE at ANY TIME can ever reduce art to pure numbers!!!!!

Tim Doran

Different direction

I am thinking about something a bit different. I am wondering about guys like the crew at Wrightsoft. Could some of these really smart guys in the commercial software game possibly incorporate a boiler selection tool and understand enough about the total system to make some sort of prediction? I think it would have value if it is possible.

Tim D.