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probably if a scientist with sophisticated measuring equipment was able to study this he might find a small difference due to heat radiating from the supply pipes or some other factor, but it would be so small as to be entirely inconsequential and have no impact on your bill. Basically, you're heating a certain amount of water (the water which actually exits the shower head) to a certain temperature, and that will take a certain amount of energy.
 
I've considered that the end result of energy use of higher flow at lower temp vs. lower flow at higher temp may be comparable. However, there's another aspect to keep in mind. Tankless requires a minimum flow rate to activate and stay running. This is to prevent the unit from turning on in response to a dripping faucet or water leak (depending on how fast is the drip or leak, of course). Depending on the particulars, at high temperature output for showering the hot water flow rate could conceivably be turned so low that the tankless turns off completely resulting in a sudden cold shower, the showeree turns the hot water higher, it warms, then he/she turns it back down, the tankless turns off again ....

Just conjecture on my part, but I'm wondering if this may be the source of many of the complaints I've read about tankless in regards to temperature fluctuations. Some tankless units require a relatively high flow rate to activate, 0.66 to 0.80 gallons-per-minute. I've *never* had a problem with temperature fluctations.

I'm heading off to take a shower, and will do an experiment. I'll raise the temp to the maximum 140°F, adjust the shower flow for what feels like my usual temperature, and check what is the power consumption the unit reports.
 
One thing a tankless heater does that a tanked heater does (gas or electric) is vary the amount of input heat. This is how the units adjust the temperature when you tweak it on the remote control. If you are taking a straight hot-water shower with the 'stat at 100 degrees, you are flowing lots of hot water through the heater, but the flame, or electrical element is running on a lower setting. This also gives you extra capacity for multiple people to take showers, because if another shower or faucet is turned on, the only thing necessary to maintain the temperature is for the heater to increase the amount of heat applied to maintain the equalibrium at the set temperature. The user of the first shower may feel a slight, momentary cooler change in temp until the heater re-adjusts itself.

Now, if you set the water heater to 140 degrees, and then the shower user adds cold water for a comfortable temp, you are subsequently flowing less hot water, but the flame (or heating element) is running at a higher rate. Although this method would also allow multiple users to operate showers at the same time, if another shower or faucet IS turned on, the 140 degree temperature won't be held...the user of the first shower will get a blast of cooler water, and would be required to adjust the cold water valve to re-balance the mix (just like in an old tanked system)

I would say that you get the most energy savings by using the lower temperature and running straight hot water because less fuel will be used when the shower is operated...and it would make the system more hospitable for multiple users.
 
Results --

I turned the shower on to my usual flow, with the tankless set at 103°F. Flow-rate through the tankless was 1.5 to 1.4 GPM. Tankless capacity initially at 30% then dropped in a few minutes to 19% after the elements got heated and the unit adjusted itself to the conditions.

I raised the setpoint to 140°F, adjusted the shower mix to what felt like the normal temp. Tankless reported a flow rate of 0.8 GPM and 22% of capacity. Note that this flow rate is just within the higher trigger range of some units on the market.

I left the water running after finishing the shower so I could check the numbers again. I typically run the shower flow lower than most people, both from a economic standpoint and from force of habit -- my dad taught us kids years ago not to run the shower at full faucet flow. The shower temp as checked by an instant-read kitchen thermometer was 99°F, which corresponds to what I typically get at 102°F-103°F setpoint without mixing in any cold ... so I had gotten the faucet hot/cold mix on target. By the end, the flow rate through the tankless had dropped to 0.4 GPM (the shower was not running that slow!) and output capacity to 13%. 0.4 GPM is *below* the minimum trigger flow rate of many tankless units. However, my tankless activates at 0.4 GPM flow rate, and will stay on at 0.3 GPM.

Note that the input temperature initially was about 81°F (this is Texas, getting close to June), dropped to about 75°F after the lines from the water well and through the attic were purged with fresh flow.

Conclusion: A modern tankless water heater modulates its power consumption according to the flow rate and setpoint temperature (which I already knew). The mix of hot/cold at the usage point has a direct effect. Higher output temperature and mixing in less hot and more cold water at the usage point will reduce the flow rate through the tankless and result in largely the same effective energy consumption as a higher flow rate at lower temperature with less cold water mixed in at the usage point. However, the flow rate in some instances may drop below the minimum needed to keep the tankless unit running.

My personal choice is to stick with a lower setpoint. This will also help prevent "accidents" and "overrun" situations such as turning on a faucet for a bit of warm water for washing hands and getting more than what's really needed. I also wonder about the additional stress on the tankless unit's components when running at a high setpoint. The water coming out of the showerhead may be 99°F, but the water coming out of the tankless is 140°F.
 
Here's a question I haven't seen addressed here yet:

I live out in the country, and I have a private well with a pressure tank. This means that both the temperature and pressure of the water going into the water heater vary quite a bit.

I'm wondering whether this would cause shower-temperature problems.

Between these factors, the cost of the unit, and the cost of getting enough gas or electricity hooked to the thing, I've stayed with the old tank-type heater.

-kevin
 
I hadn't noticed that you had a well

Around here, well water is around 50 degrees.

I was wondering because the few websites I've done research on all mentioned the maximum temperature increase for a given flow rate. It seemed to me that the ratings they gave wouldn't allow for a hot shower with 50-degree water going in.

It doesn't matter as much for other purposes, but I can't stand cold showers.

-kevin
 
Incoming water temp varies greatly from summer to winter here in TX. Summer it can be high as 80°F or a little more, winter 40°F or a bit less. I've not ever had any trouble getting water heated enough for a shower, but any tankless sized correctly for the job should be able to do 102°F to 105°F under almost any condition. I do recall one occasion during a cold snap that lasted for several days, the unit had a little trouble reaching 140°F (I raised it to blast a load of whites). IIRC, it managed ~135°F. Simple way around that is to turn the faucet slower to compensate. Since the unit had obviously maxed-out on how much heat it could produce, slowing the flow-rate would give the water more time to get heated.
 
Thanks for the info!

Much as I love my house, it was built in 1968 and 1972, when the pundits were promising that with the coming nuclear power plants, electricity would soon be so cheap that they'd no longer be metering it -- you'd just pay a flat monthly rate.

As a result, I have an all-electric home. I'm doing what I can to reduce the obscene bills I'm paying, and tankless water heat is one approach I'm considering. Unfortunately the layout of my home makes venting a gas water heater a bit tricky.

The electric ones are cheaper to run than the standard water heater, but they do suck quite a bit of power, and they seem to be able to provide less heat rise and lower flow rates than gas ones.

Gas heat is definitely a higher priority. Unfortunately I don't see any way to install it for less than $15,000, and that's doing the work myself.

-kevin
 
only trick is trasportigg the oil

I have something like this in mind for my next home.

Lube and brake joints pay to haul away their water oil. Not to mention the stuff in the fryers in fast food joints. They woeuld be happy to gicve ti away free or even PAY you to take it.

These may not be allowed in the house, (Fear of back-firing) but in an attached garage or in a shed for the purpose and it may work.

Many car repair joints in CT seem to have these and similar.

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~Unfortunately I don't see any way to install it for less than $15,000.

Propane tank and a between-the-studs direct-vent gas wall-furnace? These are milivolt operated gravity furnaces. Works with no external electric. Even one centrally located heater of this type will save you plenty. Some models go between rooms and can heat multiple rooms.



 
I am contemplating the above for a friend's heat-pump forced-air system. It works well till about 15 to 20*F when it can't keep up with the demand for heat.

I'd like to see the above type gas furnce kick in intead of expensive resistance electric coils. Done with finesse, I believe the lower electic rate for an all-electric house can be kept.
 
I'll have to look into some of these a little further.

My favorite concept is in-floor water heat, but the heaters in your link would be able to heat most of my home for under $7500, plus the LP tank and a fill.

Of course with a system like the in-wall furnaces, you can do one room at a time and spread the cost out.

Hmmm.

-kevin
 
Please don't fart in bed.

Some jurisdictions may not allow gas in bedrooms. Just Something to check so you dont get your insuarance cancelled.

Feel free to write with regards to what you have in terms of heat and layout of house.

Everyone is entitled to my opinion (ducks and runs).

HVAC is one of my passions.
 

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