Comments on: The 3 Types of Energy Efficiency Losses in Water Heating

By: David Butler

David Butler — Fri, 11 Nov 2011 23:44:30 +0000

Sorry, I meant @Richard.
Sorry, I meant @Richard.

By: David Butler

David Butler — Fri, 11 Nov 2011 23:43:06 +0000

@Robert, good comments. I've
@Robert, good comments. I've found having on-demand pump for dhw makes you think twice about drawing hot water. After living with one for several years, we now only use hot water for shaving, showering and occasionally for washing dishes (if you don't eat greasy food, you don't need to use hot water). And my wife claims washing laundry in cold water is a no-brainer. YMMV if you have kids!

BTW, there's another reason not to draw hot water from the tap for consumption. Water heaters collect mineral sediment and grow various bacteria that thrive in warm, stagnant water. It might or might not be harmful but why risk it? Also, there's a certain yuk factor. And homes built before 1990 are likely to have lead solder. Hot water leaches lead from the solder.

By: Richard Patterman

Richard Patterman — Fri, 11 Nov 2011 19:52:00 +0000

Good article on DHW
Good article on DHW efficiency. Someone mentioned the inefficiency of heating 3 gallons of water to get one cup of hot water. This is both a distribution inefficency and a User IQ inefficency. People that do not have point of use installations need to give some thought to their use patterns. If my bathroom is on the second floor and my DHW heater is in the basement, does it make sense to heat 3-5 gallons of hot water to wash my hands? At what point does it make sense to heat a cup of water in the microwave instead of running gallons of hot water to the kitchen to get that one cup.

A friend washes every dish as he uses it because he thinks it is more efficent than the dishwasher.

But a cold rinse and only using dishwasher when full is more efficent.

Another thing rarely discussed about DHW is a preheat system or buffer before the DHW heater to raise the temperature of the incoming water and decrease the energy load. Here in Colorado the incoming water is 45 to 50 degrees.

If that could be passively raised to house temperature (70deg) the heating load would be decreased by 1/3. This could be accomplished by putting a tank or pipe loop in the warmest room of the house (utility room or sunroom).

By: David Butler

David Butler — Fri, 11 Nov 2011 13:22:32 +0000

As stated previously,
As stated previously, recirculation pumps waste energy, even if operated via timer and hw pipes insulated. A lot of heat still escapes. Gary Klein advocates high speed on-demand pumps that only operate 10 or 20 seconds at the time... just long enough to get hot water to the fixture. Also, button activation system is best. Motion activated pumps will operate unnecessarily.

On-demand pumps cost more than conventional circulator pumps, but since they don't require a return loop, the overall cost (in new construction) is actually less. For the same reason, on-demand can be installed in an existing home.

@M.Johnson, whether or not a solar thermal water heater is a better option than PV + electric water heater depends on dhw usage, climate, energy prices and local incentives. I recently installed PV and switched from gas to electric dwh. My net cost for a regular electric water heater plus additional PV required to offset dwh energy was actually less than a solar water heater. Keep in mind that a properly sized solar water heater only supplies 75% to 85% of dwh requirements.

By: lnthomp

lnthomp — Tue, 08 Nov 2011 10:12:45 +0000

So now are you going to get
So now are you going to get sued by the water heater manufacturers too? :-P

By: Eric

Eric — Sat, 05 Nov 2011 08:45:32 +0000

Are "short" tanks
Are "short" tanks less efficient overall compared to standard-sized tanks? Or was that just the selection I was looking at...

I saw one 38-gal tank at a local big orange hardware store and it said 0.69EF, I was pretty surprised - just a standard draft hood model, no forced exhaust... how'd that get so high, I wonder.

By: John Brooks

John Brooks — Sat, 05 Nov 2011 04:20:16 +0000

Hey Allison, sorry to go off
Hey Allison, sorry to go off topic…just thought this might interest others

http://www.greenbuildingadvisor.com/blogs/dept/green-building-news/guardian-fiberglass-threatens-blogger-legal-action

By: Mike Rogers

Mike Rogers — Sat, 05 Nov 2011 02:10:06 +0000

Thanks for including
Thanks for including distribution loss--Gary Klein is my hero when he talks about this. And distribution loss can often dwarf the other losses (think about heating three gallons of water to get one cup of hot water).

I'll add another loss. Down the drain loss. Take a shower and use that hot water for a couple of seconds before letting it run down the drain. I've been happy with drainwater heat recovery.

Thanks,

Mike

By: Harris

Harris — Sat, 05 Nov 2011 01:47:31 +0000

M. Johnson - the irony is
M. Johnson - the irony is certainly there. Until you site the house on the lot for solar, design the house floorplan, systems, and roof structure, and max out Fed and local Tax incentives.

In our case (custom green homes) this is what we must run through. In the case of retrofit, most of the same rules apply... if you're roof is "this" big, can it sustain both solar PV and thermal? Then, can you afford a solar thermal system at $7000 (starting) or $700 (for TWO small electric tankless heaters) self-installed?

By: M. Johnson

M. Johnson — Sat, 05 Nov 2011 01:36:06 +0000

It seems ironic to me to have
It seems ironic to me to have a PV system turn sunlight into electricity and then use that electricity to heat water. Perhaps I am wrong but would it not be more efficient to have a solar hot water heater? Would appreciate some discussion of this.

It seems to me what solar yields easily is low-grade BTUs, that is gross heat energy without high temperatures. Which is more useful for hot water than for many other applications.