[Greenbuilding] Max Temp Storage Water Heaters+Temperature Sensors

[Haudy Kazemi]

On 2/7/2011 5:36 PM, Lawrence Lile wrote:
>
> >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.
>

There will still be a minor savings from turning off the heater 
overnight, however this savings is unlikely to outweigh the 
inconvenience and family hassles involved.

The savings come from the change in the rate of heat loss between 
maintaining a higher temperature tank over the time period vs the lower 
rate of heat loss in any relatively cooler tank.  Heat loss (and heat 
gain) increases in proportion to the temperature differential between 
the two locations.  (A temperature gradient forms between the two).  
Larger temperature differentials lead to significantly higher heat 
loss.  By decreasing the temperature gradients, you can decrease energy 
losses in a heat storage system.  This is part of the concept behind 
turning down the heat in a conventional forced air house overnight.

Another factor overlooked in nearly all water heater comparisons is 
where the 'lost' heat actually goes.  If the water heater is in the 
living space/finished basement/etc any heat that escapes from it goes to 
heat the house.  This is NOT a problem in heating dominated northern 
climates.  It is only a problem in cooling dominated climates.  By 
similar measure, air source heat pump water heaters are a mixed blessing 
in heating dominated climates because they will make your furnace work 
harder because they cool the area they are in.  Measuring how much 
harder may be difficult because the water heater load is small compared 
the whole home heating load in most homes.

-hk


> 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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