mod/con excessive fuel usage----Why

Brad White

Hi Bob-

Happy to oblige at least here for now...very quick lesson:

I've made bold the factors in the equation below but will start with the explanation before setting it up.

Start with your known heat loss in BTU's per hour <b>(BTUH)</b> and at a known indoor-outdoor temperature difference <b>(TD)</b>. Know your degree-days <b>(DD)</b> for the period you are looking at, be it a year or month, whatever. Don't forget that there are 24 hours in a day <b>(24)</b>

Know the fuel value of the fuel you are using, 92,500 BTU's per gallon of propane, 140,000 per gallon of #2 oil, 100,000 BTU's per therm of gas, etc. (Tweak the oil and propane as you need to, some folks use slightly different numbers but a therm is a therm when it comes to gas. Call this factor <b>(FUEL)</b>

The last two parts are guesses but you have to start somewhere: The assumed actual annual efficiency of your heating system <b>(Eff)</b>, expressed as a decimal and the Cd factor <b>(Cd)</b>which varies with your degree days. This last one, I will have to post the graph but in my 6,000 degree-day range, it is about 0.60. It can vary from 0.4 to 0.7 if memory serves. Hold on to that for a moment.

Here is how I set up the equation:

<u><b>(BTUH)</b> x <b>(DD)</b> x <b>(24)</b></u>

<b>(TD)</b> x <b>(Eff)</b> x <b>(Fuel)</b>

All of this would be multiplied by <b>(Cd)</b> to get net fuel usage. The Cd factor seems to account for empirical differences of solar gain, internal gains, things not otherwise explained.

Setting the above up as a test run, here is an example:

<u>100,000 BTUH x 6,000 Degree Days x 24 Hours</u>

68 TD x 0.80 efficiency x 140,000 BTU's per gallon of oil.

From this I get 1,890.75 gallons gross. With the Cd factor, say 0.60, I would have burned 1,134.5 gallons over that period.

TK03

Excessive fuel usage

New to mod/cons. I have been called in on a weil Mclain Ultra which is using an excessive amount of fuel. I figured it to about 8 gallons a day. Home is about 1800 sq ft, brand new this summer. What I have found so far and am going back tomorrow. The heat loss which I just finished is about 68k. The boiler is about 150k. The home is broken down to 7 zones. Half the house is radiant and the other half is baseboard. No domestic demand. The installers did not hook up the ODR. I can’t find the sensor to install so will order a new one.
Piping looks good and is p/s.
The radiant has 3” foam insulation under slab, walls R-10, R30 in ceiling and R19 in floors. All windows are double payne and insulated doors. I was thinking buffer tank due to seven small zones, but I hated to make a suggestion without being sure. The burner is set to spec.
This guy is pi$$ed.

Brad White

Back of a Napkin

If the house has a 68K heat loss and say the past month was 1000 degree-days, just a guess, I have no idea where you are or what the weather was like. Figure 68 degrees indoor design at zero degrees outside, 90% efficiency as optimistic and a 0.6 factor.

I would figure maybe 173 gallons of propane I assume you mean. If over 30 days, that might be 5.75 gallons per day.

The variables I see are:

The 90% efficiency with baseboard and no ODR is a fantasy. Without ODR your modulation has little room.

Micro-loading is short-cycling the boiler....

If I back-calculate from 8 gallons per day, I get in the 70's percentile efficiency.

Add ODR and a buffer.

kevin coppinger_15

you could...

adjust the parameters to slow the fan speed down to lower the boiler output...
But Start at the begining... heatloss, what are the loop lengths,etc.
kpc

GW

how's the house?

Before he gets his bonnet all bunched up maybe the HO should take a peak at the thermal envelope. A blower door will do wonders showing where all that heat is goind.

Has anyone looked at the combustion? Maybe the o2 numbers are high?

I dig Brad's answer; the ODR and buffer should help, but it's hard to believe it will be the magic bullet.

Gary

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GW

Fuel

I'm wondering how important those items are aat this point; the fuel usage says the house is losing lots of heat. If the loop lenghts were too long, one would think thaat the heat won't flow. No flow, no heat. No heat, no fuel usage. I bet the house sucks.

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TK03

Southern VA. 400 gallons in 50 days. Sounds excessive. Not sure of loop lengths. Job was installed by someone else. If the boiler modulates firing does it even fire at 100%? O2 is 5.6%

Brad White

Modulation

Modulation will only take effect when there is a given target water temperature below the boiler's firing rate. As for that target temperature and how it was set, and we do not know that that is.

If the heat loss is below the boiler output, yes it will modulate. During start-up and until the space reaches target temperature, the boiler may well run to limit whether the high limit or a manually set operating limit. Once there, the burner will modulate to maintain that supply water temperature in general terms.

With ODR that target temperature will float with outside temperature by definition. The burner will try to track that target temperature by modulating over a wider range if that makes sense.


If you set the boiler target temperature manually at 180F or anything above 150F, you can pretty much forget condensing. You will not be above 90% until you get your return water temperature below 125F or so. Right off the bat you are running probably 85% efficiency before micro-loading takes a toll.

Say that this was Roanoke which had 1227 degree days for the 50 days preceding today. (12/5/07 until today). I get 238 gallons if burned at 80% efficiency. 400 gallons is a 68% increase. The heat loss is either 68% greater (114 MBH?) or the efficiency is much lower or a bit of both. I suspect a bit of both.

As Gary Wilson pointed out, the largest variable could be air leakage. In other words that 68 MBH heat loss could be off a good deal. Air leakage is the largest single heat loss line-item in most structures.

Say the actual heat loss is over 100 MBH. The calculated heat loss at 68 MBH indicates a 32 MBH shortfall. This, if additional infiltration would be about 430 CFM assuming a 68 degree rise. That could easily be lost up an open fireplace chimney. Any takers on that one?

Mitch_4

Heat loss sounds excessive to me

and the boiler is way to big, tack in 7 zones...

68k @ 1800 sq ft = about 38 btu/sq ft loss. My house is 2300-2400 35 year old all original (and in poor repair)and I am heating with 40k and it is too big in SW Ontario.


You need a btu storage / buffer tank IMO

With condensing equipment, you need to keep it inthe zcondensing zone. lets say it takes the boiler 3 minutes to get there (steady state) in those little micro zones, popping on and off lets say the boiler runs 6x per hour @ 5 minutes apiece total run time 30 minutes, total time in condensing mode (where it save the big $) 12 mins.

Not set up a tank, run time is 1 cycle per hour, 30 minutes. Same fuel usage, and run time, but the boiler is in condensing mode 27 minutes a 225% improvement.

And the boiler is WWWAAAYYY To friggin big...

Mitch

Dave Yates (GrandPAH)

depends

on the height of the chimney(G)

Bob Gagnon plumbing and heating

infiltration

Caused by an open fire place like Brad mentioned or an open whole house fan in the attic. Also check the gas supply, I know someone who lost 2 tanks of propane through nail holes in his new flexible gas piping. It was being removed from the home by a radon removal system below the cellar floor, luckily. A simple gas pressure test will tell you, and may prevent a tragedy. Thanks, Bob Gagnon

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Unknown

leaks

On that same note, a below grade heating system leak will consume extra fuel, as well as extra water.

It's a stretch, but a possibility worth ruling out.

Noel

Mad Dog_2

I'm sure you'll eventually figure it out........................

BUT! I hope you get paid for your time, dedication and expertise. Keep us posted. Mad Dog

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kamney

Who knows...

8 gallons @ 90,000 BTUs is 720,000 or a steady 30 MBH.

How cold has it been? If the owner is that concerned about energy efficiency why in this day and age of rapidly rising energy prices would he build/buy a house with such low energy standards? R10 walls? R30 Ceiling? And then there is a 3x oversized boiler without ODR to balance it all out?

Weezbo

two things that i notice ..

the insulation is weak in the ceilings and walls and with lots of baseboard with no modulation you are having unrealistic expectations. If the idea is to get best efficiency out of the system and zones you might consider the Honeywell AQ 2000 series controller for the baseboard with some "float" or temp reset on the supply water temps to that side of the equation.

i think the out door reset and a buffer sounds like a definite plan also.

hot_rod

Is the slab well insulated?

below and the edges. any chance of ground water under the slab? Those are some common energy suckers.

hr

Bob Forand

Brad White-Degree Days

Hi Brad, It sounds as if you know this degree day thing pretty well. Any chance you would want to write a new thread in great detail on how to figure fuel usage based on degree days ? This is one area where I really want to learn more but don't know how to obtain the info. I am sure many of us would appreciate it. Thanks, Bob

TK03

No chimney or fireplaces. Was back and here is what I found. The home is built with a partial basement, 1/3 house slab on grade and 1/3 crawl space. In the crawl space there was some insulation falling down, open joist trusses, no plates and tubing hanging in a lot of the areas. The slab has good styrofoam on edge and beneath. LP pressure is good and boiler O2 @ 5.6. Within spec. Combined zones so instead of 7 I now have 4. Reduced fan speed to reduce input. This stopped short cycling. House is not fully furnished yet which will make a difference.

joel_19

listen to Gary!!!!!

# 1 have some one perform a blower door test and or infared test to dertimine what the actual infiltration is!! Your heat loss could be off 30% if you don't know the infiltration. Everybody else talking about the boiler etc. is probably right as well but start with getting your loads correct.

Steve_35

Burner cycle?

How is the burner cycling? I went on the exact same call recently with a Trinity. I found that some genius from a heating company/distributor had disconnected the primary pump and the OD sensor.

No surprise the first floor radiant wouldn't heat. And that the burner was cycling off on high limit in about 10-15 seconds after starting.

GW

wow

it seems like the boiler is running and running, but the sub-par heat transfer isn't moving the heat to the house. Are we ready to close this case?

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Plumdog_2

WOW

I think it is totally NOT OK to install gas lines below grade inside a building. (with the exception of inside a conduit vented to atmosphere at both ends). You should definitely have a pressure test done on your gas piping just to be certain.

Steve Ebels_3

But

If the boiler is running and running it's making heat. That means it's going somewhere. So where's the loss?

I agree on the paltry insulation comment. That house will be an energy pig in both summer and winter.

Unknown

bump

!!