I have a Buderus very efficient oil fired boiler 92%. I have used this for the past 2 years but I still have my electric hot water heater still connected as a back-up system. My question is with the risinf cost of fuel oil here in CT would it be cheaper to turn off the boiler in the summer and use the electric hot water heater?

thanks
gary

Posted By gew3201 on 24 Apr 2011 08:23 AM
I have a Buderus very efficient oil fired boiler 92%. I have used this for the past 2 years but I still have my electric hot water heater still connected as a back-up system. My question is with the risinf cost of fuel oil here in CT would it be cheaper to turn off the boiler in the summer and use the electric hot water heater?

thanks
gary

Maybe, maybe not- depends on the volume of hot water use and your oil pricing & electric rates.  I suspect your condensing Buderus is system #4 in this test report (or at least isn't likely to perform a lot  better or worse.) See table 2, and also Appendix 4:

 http://www.nora-oilheat.org/site20/...cyTest.pdf

The system measured at Brookhaven Nat'l labs gets ~55% efficiency in summertime water-heating mode, vs. ~90% efficiency with an electric tank on standard EF-test volumes..

Assuming $4 oil @ 55% efficiency you're getting 138000BTU x 0.55=75.9KBTU per gallon showing up in the hot water distribution plumbing for $4, which is 75.9K/4=  ~19KBTU/$ or ...


~190BTU per penny

Assuming 18cent/kwh electricity @ 90%, is 3412BTU x .9=3071BTU/kwh showing up in the hot water.  At 18cents/kwh that's 3071/18=...

~170BTU per penny, which is not as good a deal as with $4 oil @ 55% efficiency.

But at 15cents/kwh electricity starts looking pretty good against $4 oil, (but not against $3 oil.)

With either system you're WAY ahead of the game (and cost-effective in your case) if you insulate all of the near-boiler and near-tank plumbing with at least R4 pipe insulation (if the boiler loop  to the indirect ever exceeds ~160F you'll have to use fiberglass pipe insulation, but otherwise 5/8" walled closed cell foam is the ticket.)  This goes for any accessible hot water distribution plumbing, as well as the nearest 6-8' of the cold feed and temperature/pressure valve outflow plumbing to/from the indirect. (Box stores tend to only carry R2 foam pipe insulation- you may have to order some online or find some at a better plumbing supply distributor, or maybe Grainger.) 

By insulating the near boiler & near-tank pipes you cut down on standby idle losses, and by insulating the distribution plumbing you abandon less heat in the plumbing on intermittent short-draws (less than 30 minutes apart). It adds up.  See: http://www.leaningpinesoftware.com/...ipes.shtml

If your family spends a lot of time in the shower (20+ minutes/day total shower operation) there's a solid financial rationale for drainwater heat recovery a heat exchanger too.  It's only effective for simultaneous flows like showers, not batch draws like tub baths, but getting 50% of the heat back on 40-60% of the total hot water use adds up too.

You could replace the water heater with a heat pump model. They are well suited for summer use.

Heat pump water heaters are lot of money to spend for at most 3-4 months of annual use, but they'd cut summertime water heating costs nearly in half.

A showering family would get a bigger annualized ROI out of drainwater heat recovery though. The present-value economics in that Minnesota study was based on 36 minutes/day of shower use with 7.5 cent electricty @ 92% efficiency- water heating costs less than HALF that of 15 cent electricity @ 90% efficiency or $4 oil @ 55% efficiency. Average incoming water temps in MN are somewhat lower than in CT but not by a huge amount- not more than 10F colder- not nearly enough to make drainwater heat recovery un-economic at 20 minutes/day with CT utility or heating oil pricing.