typical cycling of an dormant water heat
I am curious to know if anyone has seen a study of typical cycling of an electric water heater if left unused.
I realize that there are a number of variables involved – external temp, insulation etc, but I was interested in just a standard test performed by EPRI or some one of that league.
The focus of the question is that I have two heaters – one tankless and the other conventional. I am not impressed with the tankless, and the regular heater is one that I am thinking of putting a timer on to let it go inactive during most of the day and energizing shortly in the morning for showers (it only purpose) then letting it go dormant again.
After installing the tankless a few years ago I have become become wary of great ideas and now research prior to working.
So my reason for the question is if the heater only cycles once during the day (if unused) why bother.
Thanks
Replies
boag, I doubt you will save much if anything by turning a tanked heater on and off each day. When you turn it on, you have to spend energy to bring the water up to temp. Sure you save a tiny bit by maintaning a slighty lower overall average temperature, but the average would not be much lower than your usage temperature. Also think about this. I have a gas tank heater. When I have been on vacation for one month my gas bill (water only) is about $15. Therefore the cost of stand-by water is $15. How much lower could that be if the temperature were ten degrees or twenty degrees lower (which is likely what your tank would drop over night in your senerio)? I would bet a buck or two.
Oh, I noticed you have electric not gas. Well that depends where you live and what you pay for electric. I know that gas and electric is about a breakeven push at 7 cents per kilo watt. So if you pay 7 cents, you might say a buck or two. If you pay 14 cents, two or four bucks. But what a bother.
I'm in the TVA system and power is around 7.8 cents/kwh. I didn't expect to save any substantial amount of money, however I became curious after the tankless water heater fiasco.
Depends a bit on the age of the water heater, and, of course, the quality. If you want a really well-insulated unit look up the Marathon units. But any electric water heater is pretty well insulated, so you don't save a lot by having it switch on and off.
I like that Question.
Lot's of thoughts come to mind. Not saying I'm the one to answer them but I do like good questions.
Hopefully some great mathematicians can come by and give us some calculus of the variable lapse rate formula.
I 'm also thinking to be able to cycle it off before it reheats after the use. So I think maybe calculating heating ? x gallons x time and amp x kWh =s daily cost. Check the hwh recovery data would give rough gallons per hour at ? temp. The data tag on the hwh should have that.
You never lose on the deal, assuming that electricity is billed at a constant rate.The energy saved is the "energy under the curve" (or perhaps "over the curve" in this case) -- the difference, summed over time, in heat loss at a constant temp and heat loss as the temp slowly decreases due to heat loss. The enery required to produce this heat is almost exactly the delta in KWH input, for a standard electric resistance unit -- 100% efficient.An important point is that the advantage of a time switch decreases as the quality of tank insulation improves, so you're generally better off spending the money on more tank insulation.
If your view never changes you're following the wrong leader
Even more complex than you state. Don't forget that the rate of heat loss declines as the temperature increases. Not sure if a heater is more efficient at making small heating adjustments, the kind necessary to maintain stand-by, vs heating the water up from a prolonged cooling period. If you get my point.
Yes. That's why it's to bad I majored in hooky. Now if I had majored in Hooke I would be able to tell you.
noun: English scientist who formulated the law of elasticity and proposed a wave theory of light and formulated a theory of planetary motion and proposed the inverse square law of gravitational attraction and discovered the cellular structure of cork and introduced the term `cell' into biology and invented a balance spring for watches (1635-1703)
I don't think that average HWH has variable input? I think when it calls for heat it's on.
And I under stand a sliding scale of loss, higher the standing temp of the hwh the faster the BTU loss down to ambient temp.
Ok, sure, I see that, sure, yeah, ok fine.
On the question as to why the tankless water heater fails...I have a two zone house the secondary zone supplies a kitchen sink, dishwasher, washing machine and half bath - all which are seldom used for hot.The waterheater is ~4' directly below the sink/dishwasher. We only used the hot water for the dishwasher, washing machine may use hot every other week.The waterheater sat ther for year after year with minimal use. When if failed I replace it with a tankless wather heater. Being ~4 feet away from the dishwasher the water never heated adequately by the time the washer was filled. The seconady heater had to work to bring the temp up to design level in the washer (this caused an hour+ wash cycle). I devised a procedure for my wife (yea she really loves it) start the hot water on the sink - let it run for a couple of gallons till it heats up then, with out turning it off, start the dishwasher, waiting for the flow to decrease in the kitchen faucet before shutting off the faucet. For the washer - turn on the half bath hot faucet wait for a while and then turn on the hot water for the washing machine. Needless to say we only wash cold.That combined with the running two 8/3 lines to the TWH and the replumbing I did (which looks great)it turned it a real bust.
The problem was that you were trying to use an electric (limited output) water heater to supply multiple fixtures. In Europe, where point-of-use tankless heaters are common, they have a small heater at each fixture (and probably don't use them for large-volume appliances like washing machines).
A whole house electric tankless is close to worthless. The current generation of gas-fired tankless heaters are far more effective, though the best of them can supply only two fixtures at once. They have very large burners and can heat water much more instantly.
As for your tank heater, add an insulating blanket and keep it on. The recovery time is too slow to justify a timed cycle. You'd have to crank it up at least an hour before use. The amount of heat required to raise the water temperature is exactly the same as the amount lost during off-time. The only savings would be during any "coast" time once the water temp reached ambient and before it was turned on again.
This is exactly how a set-back thermostat saves energy. The quicker your house loses heat and consequently the longer it stays at the lower setting, the more savings there will be. But this works because you're dropping the heat perhaps 10 degrees, not the 70 degrees difference between water tank setting and ambient. Before the tank ever reached ambient temperature you'd be cranking it back up, so there would likely be no savings at all.
Where you DO save by cycling an electric water heater is if you can get differential off-peak electric rates and run the tank only during those times. But your electric rate is so ridiculously low that you'd be wasting your time and money installing a timer.Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
I was trying to get some info to gather a better answer but let's look at it like a on demand electric cept you have to lead it on by the recovery time. Say you use 30 gallons on a 30 gallon hwh. So the heater would run say 1 hour for full 30 gal to ? 110 degrees Then it's shut off. So the cost to run is 1 hour x ? 30 amp 240v and that's saying starting from say 60 to 110 rise. So the timer is set to 1 hour prior use and off. Say your late using it by ? 1hour no bigee, whatever the lapse rate on the temp? 3 degree an hour or ?. So it should work fine. Then the question is what's the # of cycles per day to maintain the 110, what's the duration of the cycles and does it total more cost or less then the timed option.
The water heater tank doesn't have to reach ambient for there to be a savings. It just has to dip below the setpoint. If you plot a graph of temperature vs time and draw a horizontal line at the setpoint temp, the area under that horizontal line but above the plot line is the energy savings.
If your view never changes you're following the wrong leader
I am afraid I lead you down the wrong path. The reason I used the multiple fixures was to prime the system - getting hot water in the line close to the point of need. After the hot water started to enter the needed fixture all secondary outlets are closed. The on;y reasin to do this was to ensure we didn't have 8 gallons of cold water and 1 gallon of hot water in a washer.
"I am afraid I lead you down the wrong path."
No, I understood that. I was speaking to the reason that you're unsatisfied with an electric demand heater that supplies multiple fixtures - it can't.
Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
"The water heater tank doesn't have to reach ambient for there to be a savings. It just has to dip below the setpoint."
The energy lost from the tank while the unit is turned off is exactly the same as the additional energy required to bring it back up to set-point temperature (if this energy is lost into the heated space, it's not completely wasted). The only savings in a temporary set-back of ANY thermostat - whether in a house or on a hot water tank - is during the time when the temperature is static, i.e. no additional losses that have to be replaced later.Riversong HouseWright
Design * * Build * * Renovate * * ConsultSolar & Super-Insulated Healthy Homes
Bullfeathers. The energy savings is due to the reduction in heat loss across the insulation due to the reduced delta T. It's real, just not especially significant.
If your view never changes you're following the wrong leader
So, Exactly why is it that the tankless fails you?