Returning from the overly-warmed cavernous mechanical room, I was once again invited to join my new old friends at the kitchen table. One option always suggested is solar thermal (reminder: it's not my money), which was rejected due to the up-front costs.
"If we were younger, but at our age we don't expect many more years," said the customers.
So, the options were quickly narrowed down to adding an indirect water heater and a zone relay controller with an isolated end switch to turn on the boiler only when there's a demand for heating the home or DHW.
The customers asked, "What's the installed cost, and how much fuel will we save?"
Energy Conservation Value (ECV) represents the dollar value of avoided-fuel costs and is a term that my customers have readily accepted. The challenge we've all faced: how to calculate the reduction in fuel by making the changes we're suggesting. In order to more accurately predict a reduction in gallons of fuel oil burned, I needed some help and guidance. I don't have a test lab, but my source does!
The DOE/AHRI/ASHRAE test parameters are as follows for appliances making DHW: 58°F cold water inlet; 135°F average storage temperature; and 66-GPD usage. 66 x 8.33lbs/gal. x (135-58) x 365 days ÷ 140,000 Btus per gallon of oil = 110.4 gallons of oil per year if the appliance is 100% efficient.
Energy to heat DHW
For the exercise below, the installed cost for a 65-gal. indirect water heater, zone relay controller, and a scald-guard thermostatic mixing valve will be $2,800.
Cold start boiler with indirect water heater:
- 85.1% rated efficiency
- 4% jacket loss during off cycles
- 2% piping loss
- 65-gal. indirect water heater loss of 350-Btus per hour = 21.9 gallons of oil per year
110.4 ÷ (85.1% -4% -2%) x 100 = 139.6 + 21.9 = 161.5 GPY
Overall system efficiency: 110.4 ÷ 161.5 = 68.4%
Boiler with tankless coil:
- 85.1% rated efficiency
- 32% jacket loss during cycles
110.4 ÷ (85.1% -32%) x 100 = 207.9 GPY
Overall system efficiency: 110.4 ÷ 207.9 = 53.1%
We're looking strictly at production of DHW and we see a reduction of 46.4 gallons in fuel usage. At $3.99 per gallon, the ECV = $185.14. The ROI is 6.6%, which isn't great, but that's also not bad either. However, both boilers and indirects have long lives and looking down the road 20 years with a 5% per year increase for fuel will realize an ECV of $6,121.83, which more than returns the initial investment. Conserve fuel. Keep a bit more of your hard-earned cash. Reduce the cooling bills. Provide safer hot water at a more stabilized outlet temp while also ensuring the occupants can draw higher volume without affecting the delivery temp. And in addition, there's the liming issues for coils that can be negated by using an indirect that continually sheds hard-water deposits before they can grow. All in all, it's still a great move from my perspective.
However, we’re missing some real-world conditions. If the DHW coil in the boiler is still working, we will allow this to pre-heat/temper the DHW incoming cold-water flow. Looking at it backwards while using the DOE/ASHRAE/AHRI factors: 140,000 x .6836-efficiency = 95,704 net Btus per gallon of oil ÷ 641.41 Btus required to raise one gallon to 135°F from 58°F. Therefore, 66-GPD DHW = .442 GPD of oil. 95,704 x .442 = 42,301.17 ÷ 641.41 = 65.95 GPD DHW. 365 x .442 = 161.33 gallons of oil per year. So far, so good and we’re right on target with the laboratory results.
During the heating season, there would need to be an adjustment to account for the boiler's water already being warm to hot dependent upon when DHW production and calls for heating would cross over. But, even in the dead of winter, I've personally witnessed oil-burners running to meet DHW demand while the heating zones are OFF and cycles just to maintain their hot status. Based on that, I'd have to give each month a weighted average based on heating degree day info. Gut feeling is we're looking at something around 60% of that 161.33 for a total of 96.8 gallons of oil consumed to produce DHW. ECV = 111.1 gallons of fuel x $3.99 = $443.29
If that's true, then the ROI jumps to 15.8%, which is a much more exciting sales tool and the 20-year ECV becomes $14,657.81.
Annual maintenance is required for all fossil-fuel heating appliances (including gas). A cold-start chimney-vented boiler needs to warm up quickly, so that the exhaust temperature rises above the dew point in the boiler, flue and the chimney. A bypass line needs to be installed between the boiler’s supply/return piping and the indirect should be set up to ensure a longer run cycle. This can be accomplished by increasing the tank volume and/or setting a wider delta-T for targeted storage temperature. Removal of flue piping to inspect the chimney must be incorporated during annual service. Last, but not least, ensure the home has smoke and CO detector(s) to CYA (cover your ass-sets).
Dave Yates owns F.W. Behler, a contracting company in York, Pa. He can be reached by phone at 717/843-4920 or by e-mail at: [email protected].
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