Energy Consumption Puzzler

gasfolk

Mike, do you know

1) How many degrees cooler are your supply temps now compared to before your reset curve adjustments?

2) Any idea what is the smallest change in ceiling temperature that might be perceptible as a component of MRT?

gf

Mike T., Swampeast MO

Installed a new boiler (Viessmann Vitodens) for the last heating season. Cast iron rads, gravity conversion system. TRVs, no low-loss header. Was very happy with about 43% fuel reduction for the season as a whole. Occupied spaces were kept at about 66-68F, unoccupied 58-60F.

Actual fuel consumption during modulation (determined by clocking meter) was quite close to my estimated loss of 85% of the value produced by HVAC-Calc. Had originally set the boiler curve to my calculations. Late in the season I experimented with lower curves and warm-weather shutdown and found significant fuel savings in moderate weather. There was however a "price" in comfort. For the same indoor temp, my subjective opinion is that the space felt cooler.

This year I was determined to find out just how "low I could go". Have been maintaining occupied spaces at about 60-63F and unoccupied at about 52-55F. Have also been experimenting with the reset curve to find the "perfect" curve that it just adequate to maintain the highest space temp desired. Even before the adjustments boiler curve hit 141F at design (8F).

Fuel reduction this year (including a week-long period of well below normal temps including below design 3F with 8F design) has been astounding. At least 50% and that's a VERY conservative estimate.

Curiously, during the VERY few periods of sustained modulation, the boiler has been consuming little more than half the amount predicted by HVAC-Calc.

I can understand some reasonable fuel reductions from the generally lowered temp (5-6F) and from the significantly lower boiler operating curve. But the actual reduction seems FAR greater than can be explained by these changes.

Since the Vitodens doesn't break thermodynamic laws and I don't believe that my house recently entered an energy vortex, there MUST be some reasonable explanation.

Is it correct to take a clue from operation at the end of last heating season when for the same space temp, the comfort level seemed reduced? Perhaps mean radiant temperature is falling faster than the space temp reduction? Would this explain the BIG predicted vs actual loss change?

In support of this theory, I offer observations well-documented here last year. With the original reset curve, the radiators were running cooler than with the previous boiler for the same indoor conditions. Yet, the boiler was consuming nearly what was predicted. There was a significant amount of "missing" heat. I suggested the possibility that the "missing" heat was radiation that seemed to pass directly from the radiant burner of the Vitodens to the house itself without appearing in the radiation.

Now, with the "barely adequate" curve, this does not seem to be happening as radiator outputs are within reasonable bounds. (About 2 btu per sqft EDR per degree of temperature difference compared to 4+ btu per sf in some cases.)

Please offer ideas and comments! I'm stumped!

S Ebels

Mike

What does HVAC-Calc say the heat loss for your house is at design outdoor and 68 indoor? Then punch in a 60* indoor using all the same parameters for design conditions. What's the percentage reduction in heat loss?

Joel_3

zactly

Your winning on two fronts , lowered temps = less energy consumed ,they also = lower heat loss as delta t to outside is lowered . Then combine that with lower water temps and lower modulation on th eburner and you've got your extra savings

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bob_50

Mike

have you ever repaired the damper in the fireplace? bob

Constantin

Some wild thoughts...

... my guess is that you've hit on the true value of modulation, i.e. the ability to resize the heating plant to reach nearly 98% seasonal efficiency almost all of the time. Meanwhile, the low output and low water temperatures maximize the energy uptake from the flame whenever the Vitodens fires.

I would be surprised if your older boiler was not a lot less efficient over the course of the shoulder season than you gave it credit for. Steady-state efficiency numbers

Mike T., Swampeast MO

Nope. Completely jammed and will take custom made castings to repair the mechanism.

Mike T., Swampeast MO

I would be surprised if your older boiler was not a lot less efficient over the course of the shoulder season than you gave it credit for.

Didn't give it much credit in that regard but you're probably right!

And yes, certainly agree that the Vitodens is spending the vast majority of its time operating at conditions nearly ideal for combustion efficiency--including that "pulse mode".

Haven't done much tightening this year. Mainly just insulation of window weight pockets with 1" Styrofoam against the exterior casing and a bunch of foam injected in the walk-in bay (E-facing). Downstairs windows are still not weatherstripped, but do have good storms. Heat loss calculation conducted as if place is finished--guess the discrepancy will get even bigger!

I also second the notion that the lower internal temperatures are significantly reducing the heat flux because the

jp_2

mrt?

what is MRT? how do you measure it?

Mike T., Swampeast MO

80 mbh at 70° in; 8° out.

67 mbh at 60° in; 8° out.

Approximately a 17% reduction. And this for 10° and the general setback amount has been 5-6. Manual J produces linear (as related to temperature difference) values.

Mike T., Swampeast MO

MRT = Mean Radiant Temperature

Essentially the average temp of the walls/floor/ceiling/windows/heat emitters/physical contents in a space.

Entire pages of formulas devoted to an attemp to estimate in idealized situations. MRT sensors are available, but unless things have changed they're not very accurate and are mainly used to detect gross changes. The little hemispherical black domes on the dash of GM cars with climate control are a form of MRT sensor.

jp_2

so mrt is temp?

so this is just a fancy way of calling temperature a differnet name. after all temperature of a body is the temperature of the body, why get fancy? I think i read where einstein said one time, if you can not explain it in simple language you do not understand it, either that or I said that.

remember air transfers heat by convection AND conduction, when a molecule slams into the wall,floor,ceiling, etc it either picks up or gives up energy(heat) by "Conduction".

remember too, even in the stillest of rooms the molecules are moving extremely fast! several hundered mph! I don't remember exact average speed.

I still question your software and the exact methods used to model heat loss.

Mike T., Swampeast MO

Mean Radiant Temperature is a well-known and simple concept–it's the calculation (and worse prediction) of MRT that gets hideously complicated. Entire books are devoted to the subject.

MRT is not the same as the average air temperature in a space--not by a long shot.

I certainly question the software as well--even if it is based on the "standard" used for heat loss calculations...

Will attempt a hand heat loss calculation but I'm not seeing how the end result will be much different unless I intentionally start manipulating "known" values. Not fair to do that because if a generally accepted standard is not followed the results are just as suspect and nearly as worthless as an Enron balance sheet.

gasfolk

Man-made Convection?

Is it possible the air temperature has fallen slightly?

"Operating the panel at lower water temperatures increases the percentage of heat output by thermal radiation [versus convection]." Source: "Panel Discussions" By John Siegenthaler, P.E

http://www.pmmag.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2379,86893,00.html

How much hysteresis is built into your TRVs? With a more-nearly-ideal reset curve, could your system flow be more nearly continuous, avoiding on-off cycles in the TRVs? Reducing even minimal over-shooting? Allowing every btu supplied to get out into the emitters?

In a perfect world, (sounds like you are tweaking toward perfection) would flow be truly continuous through all emitters, at a temperature that each living space could take heat from as needed but never more than needed? At such a reset temperature, could rooms over-heated by solar gain actually give heat back, to be moved to other areas, balancing the system through "man-made" (i.e. hydronic) convection.

Perhaps a little far-fetched. But, you are doing a very nice experiment. Interesting. Thanks.

gf

gasfolk

A couple of follow-up questions:

How fast do TRVs close? At higher reset curves (and higher circulating temperatures), how many excess BTUs can get into a room's emitter(s) before the TRVs close? How much temperature over-shoot could occur in the room (probably not much, but less and less as the curve gets tweaked)?

Best,

gf

Boilerpro_3

some thoughts for you

If your rads run hotter they will produce hot air heading to the ceiling. This hot air increases stack losses through the house. However, this hot air can also increase MRT because the ceiling surface is heated by the warm air and becomes a low temperature radiator...increasing comfort.

Heat loss is NOT linear with change in Delta tee from inside to outside. The amount of air leakage in volume increases as the delta tee increases (nonlinearly, just like convection from radiators)and the air temperature delta tee increases. This causes heat loss to accelerate as it gets colder creating a curved upward line on a graph of heat loss vs. delta tee.

That rather slight change in interior temperature is probably greatly decreasing you air leakage (especially since it sounds like you still have a fair amount of tightening to do). Cutting down the radiator temps is decreasing the stack effect in the spaces, also reducing air leakage. Also, since the ceiling is not acting as a radiant panel any more, your MRT has dropped, and comfort suffered.

Just some thoughts

Boilerpro

Gordy

I have been following this thread. very interesting sperimentin Mike. Trying to nail down all the variables to bring them as close to constants is the hardest part. I believe Boiler Pro has hit the nail on the head with his theory.

Boiler Pro I understand you are from the Amboy area. I know a few people from the area. King,Liebing,Motter,Skulley ring any bells?

Gordy

Mike T., Swampeast MO

gf:

When maintaining space temp, TRVs don't open and close--they "open more" or "open less" depending on the load.

In rooms with little or no occupancy load temp variance is about ±1°F; this the same as years before.

With occupancy variance is more like ±2°F; again very similar.

The amount of overshoot seems related to the size of the emitter. Crank a TRV connected to a really oversized iron rad and you'll get some significant overshoot above the new setpoint.

Mike T., Swampeast MO

Boilerpro:

That's probably a good chunk of it. Still amazing me just how much Manual J loss calculations seem to be overstated.

The only time I've felt noticeable convection over any rads this year was with outside temp in the single digits. 92° supply water available to the rads at the time.

THANKS!

jp_2

measurement error

remember too, with all the different temperatures you are measuring, inside, outside, water temps, there's a good chance you have a 10% error within all those measurements- all instruments have error assoicated with them.

good thread though

Mike T., Swampeast MO

This year at design condition (8°F) I had 92° supply available to the rads.

Last year at design condition I had 124° supply available to the rads.

This year with outside temp hovering near freezing I have intermitting boiler burns averaging around 80°F available to the rads.

Last year with outside temp hovering near freezing I had full modulation with 112° supply available to the rads.

These are all measured temps, not calculated.

gasfolk

Mike,

If you calculate the change in BTUs sitting in the volume of your supply piping, hour by hour, how does that come compare to the drop you have seen in your MBH?

gf

Mike T., Swampeast MO

Interesting...

580 pounds of water (estimated) kept 30° warmer (on average) over 32 days...

IF the heat is consumed each hour that's a whopping 13,363 mbh or about 134 therms. Gross consumption difference (not counting for degree days) was about 60 therms.

With the exception of the garage, the mains and branch piping in the basement are quite well insulated.

For similar weather and time period basement is about 2° cooler this year than last.

Will dig to see if I can find the area (square footage) estimate for the basement piping.

gasfolk

Also,

how much has the surface temp of your rads changed? If not much, then could convection explain the magnitude of your energy savings?

Could you test the relative contribution of convection-induced losses, versus standby losses in the supply piping, by turning up your TRVs by, say, 5 degrees to bring your radiator surface temps to what you measured last year but using your current reset supply temps?

gf

gasfolk

Based on your room air temps,

how much more drafty would your house have had to become to lose those 60 therms?

Mike T., Swampeast MO

Certainly realize that temp instruments aren't perfect--particularly the ones I can afford...

Specs for all the temp sensors are ±2°F.

Did continue to collect data all year when the boiler was off and system drained for radiator relocation. Supply & return temps were always within ±.01F. Flue temp sensor generally identical as well excepting periods of rapid outdoor temp change with high wind. Did put all of the room air temp sensors side-by-side in the same room and found similar relative accuracy. I've never tried to measure the absolute accuracy as I don't have a lab-grade thermometer for reference.

The room air sensors are mounted at similar heights above the floor, and on interior walls that never get direct sunlight. The exterior temp sensor for the datalogger is on the South side of the house--the only place the darned solar powered thing will transmit reliably. Two other sensors are also solar powered, but the temp sensor has VERY limited range and it's been replaced three times with similar results. Do have another outside temp sensor to the North, but it's not datalogging. Without solar influence the outside temp measurements are within ±1°F.