[Greenbuilding] Max Temp Storage Water Heaters+Temperature Sensors

[Lawrence Lile]

>We have been in the practice of turning the heater on when we are getting ready for a bath to avoid the standby heat losses
(Energy $$) from continually heating the water.



Eli, let's think about this assertion.   How much energy do you actually save by turning off the water heater?

OK, one second before you turn it off, the losses are X.  This is a function of insulation, temperature of water, temperature of the space, but it is going to be a pretty constant number for a given installation.


One second after you turn it off, you are saving a bunch of electricity!  Great!  But the losses remain the same - the temperature of the tank hasn't changed significantly, you are still losing X amount of energy that you already paid for, driven by temperature difference.  Are you saving any money yet?  Nope, you are still losing energy that you've already paid for.

Let's say you leave it off for an hour.  In that time, the tank will lose enough heat to drop in temperature a little, but not much.  Your losses, driven by temperature difference, are still approximately X.  You already paid for that heat, so you still aren't saving any energy.  Same for two hours, three hours, and beyond.

Then you turn it on.  The thermostat kicks in, and the heating element replaces all of the heat that just leaked out.  How much energy did you save?  NONE.   You put back in energy that was leaking out in losses during the whole night. The losses never stopped.  You put the energy that was lost back in the morning. You didn't save a watt or a red cent.

If you let the water heater cool all the way off, to room temperature, then finally the losses stop altogether.  That will take a couple of days with a well insulated water heater, not overnight.   In this case you really are saving energy by turning off the water heater. But that isn't overnight, that is a week of vacation.

If your water heater cools off a lot during the night, then the losses, driven by temperature difference, will be a little smaller than X.  But if this is happening, you need insulation, not a switch.  Wrap that tank up!  A good tank ought to lose 5F or less overnight with no power.  Wrap up your pipes, especially near the water heater where they stay constantly warm, as they are heat leakers.  Put those little heat trap thingies in the pipes to prevent thermosyphoning.

You won't save any significant energy by turning your water heater off at night.  You will suffer the scorn of a wife who has taken a cold shower, which is much worse than any energy bill let me tell you.

A modern, high efficiency electric water heater, such as the best model Whirlpool available at a lot of hardware stores, has so much insulation as to make the standby losses almost negligible.  You can add more if you wish.  Compare the price of one of these well-made units with a tankless and you'll realize the tankless is a pretty bad deal.

Tankless can save 50% over a crappy 1950's water heater with ½" of bad insulation, no heat traps, and bare pipes right next to the heater.  It cannot save much at all over a modern, good quality tank water heater, not the cheapest model but the best models, especially with an extra wrap of insulation.  It will cost you a lot more to buy and install a tankless, it will take a bigger and more expensive electrical circuit, and you'll be frustrated with limited water flow and sometimes variable temperatures (Brrr).

I've been through the whole tankless craze, put several of them in various houses, and began to realize I'd been sold a bill of goods when I did the math on newer water heaters.  Keep It Simple, Simon and put in a good quality tank.



--Lawrence





From: greenbuilding-bounces at lists.bioenergylists.org [mailto:greenbuilding-bounces at lists.bioenergylists.org] On Behalf Of elitalking
Sent: Wednesday, February 02, 2011 9:41 AM
To: Green Building
Subject: [Greenbuilding] Max Temp Storage Water Heaters+Temperature Sensors

I am replacing a water heater that is in a location that is inaccessible for servicing.  We have been in the practice of turning the heater on when we are getting ready for a bath to avoid the standby heat losses
(Energy $$) from continually heating the water.

I want to know the maximum temp that typical storage electric water heater can safely accommodate.  Since we are not storing the heat, the higher temperature is not an energy penalty.  The higher temp allows for a smaller tank.  We are separating the kitchen use from the bathroom.  Tank will located next to the bathtub.  We will have an on off switch at the tank.

Does anybody know of a heat sensor and or sound indicator when the water reaches a set temperature? Currently we allow more time than is necessary because we want to avoid being too cool.  It sure would be nice to know what is going on.

We could use a 20 gal tank for 20 gal bath and set at temp for bath.  However, we use a cistern.  During dry spells (most of the time) we mix creek water in bath to reduce water usage off of cistern.  Results of my spread sheet below show that 20gal bath can be supplied with 10gal HW tank at 185F.  Can a normal temperature relief valve store that kind of pressure-temperature?

Bath (gal) 20.0
Bath Weight (#)          160.0
Bath Temp (F)  120.0
Cold Water (gal) 10.0
Cold Water (#)  80.0
Cold Water Temp (F) 55.0
Hot Water (gal) 10.0
Hot Water (#)    80.0
Hot Water Temp (F) 185.0

        Thanks for your consideration.
        Eli
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