BTU Calculation Question

Steve Garson

This is a technical math question:

The scenario:

1. New addition to house with master suite and bathroom with radiant floor heat. Heat loss calculation: 12,000 BTU.

2. Radiant heated by Bradford White hot water heater designed with a heat exchanger separate from potable hot water. Specs say it is 82% efficient. Rest of house is steam.

3. Gas usage 75 Therms for 29 days for 1127 degrea days. So that's 7,500,000 BTUs of gas used or 6,654.6 BTU per degree day. If this were fuel oil, 7,500,000 divided by 140,000 = 53.57 gallons for a K-Factor of 21.

4. Now the question: A proper boiler is more efficient than a water heater, but with this amount of usage, would there be any truly measurable savings?

Does anyone know how to perform the calculations with, for instance, an assumed 85% effiency, 90% efficiency, etc.

Steve_35

If everything else is equal

You can just divide the increase in efficiency by the lower rated appliance's efficiency. Then multiply the result times the cost of fuel using the lower rated appliance to come up wiht your savings.

If your bill is $2000 a year with an 80% appliance and you're wondering how much a 90% unit will save:

10% / 80% = .125
.125 x $2000 = $250 approximate annual savings.

Constantin

Good points but...

... Someone posted a while back that GAMA or whatever has explicitly stated that their EF ratings used on water heaters allegedly have nothing to do with AFUE ratings used on boilers. As such, unless you actually measure the amount of gas going into a water heater, the stack temp, CO₂ readings, etc. I doubt you'll have even a starting point as far as comparing the two systems is concerned.

Nevermind complicating factors like our dear friends at the gas company changing the caloric value of the gas by injecting mystery gas/air/whatever into the NG system.

Further muddying the issue is adjacent zones scarfing BTU's from each other. As Mike T. recently posted in his ongoing Vitodens/TRV experiments, his TRV'd radiators are not all at the same temperature and seem to benefit from radiant circuits in rooms next door, etc.

Overall, I agree with your approach - to a point. For instance, AFUE doesn't tell the whole story WRT energy consumption in the summer months where tankless coils are energy hogs and condensing, modulating, low-mass boilers do really, really well. Thus, while a user may have expected a 14% improvement based on switching from 80% to 94% AFUE boilers, we've had savings reports ranging from 25-40% on the wall.

Granted, the controls were updated also (outdoor reset, modulating water temps, etc.) but the bottom line is that a modulating, condensing, low-mass boiler can take far more advantage of outdoor reset than a conventional boiler can. Less mass means less standby loss, modulating allows very efficient production in the predominat heating season (i.e. the shoulder months), and the condensing feature allows supply water temperature modulation down to ambient.

It's too bad that oil isn't there... yet. Hermann HL-70, where art thou?

Steve_35

Good points. Constantin

That's why I started off saying "if everything else is equal".

One other point. You can fairly easily find a WH's steady state efficiency by looking at the constant output. Most are rated at so many gph at a 70º or 90º rise. From that you can derive btu output and compare it to the input. It doesn't show the standby loss which can be considerable with an atmospheric water heater or boiler for that matter.

Weezbo

this likely wont answer the question....

have you considered using an electric water heater ? and if its steam nice heavy heat exchanger?

Steve Garson

Based on the calculations, my present heater uses 346 therms for the average 5200 degree day heating season. At $1.20 a therma, that's $415. If I saved 25%, that would be $100/year saved. It would take a lot of years to pay off.

Sounds best to wait until it springs a leak in ten years and then replace with a condensing boiler and indirect hotwater.

What was the Hermann link? I use both oil and gas, but the link was in German.

Constantin

It's a link to a 2-stage oil burner...

... that is small enough to fit on most residential boilers in the US. We have them over here for commercial applications, but nothing below 1.5gph, if memory serves me correctly.

Brookhaven national labs is currently testing a slew of foreign-made modulating oil burners. Here, the problem is that most of them are too small for the US market... nothing above .75gph.

All I want for Christmas is a 5-1 turndown 1gph-max oil burner. Anyone need a beta test site for a Vitola?