Tankless water heaters are terrible....

[cartwright]

For instance, ones like this that tie together your hot and cold supply in order to recirculate until a thermostatic control stops the pump @nwdiver. It's been educational.

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[Merrill]

How much hot water would you like? Sink level, shower level, other?

We only use hot water for showers and the dishwasher.

 

[McRat]

My motivation for starting this thread is that people often choose tankless because they want a more efficient water heater. The improvement over a regular water heater is a <10% reduction in energy use vs >70% reduction with a heat pump.

In addition to the higher maintenance costs there's a loss of demand response potential. The ability to control demand is going to get more critical as wind and solar generation expand. Few people would be willing to time their showers around wind generation but with a large tanked heater you sync its heat cycle to when surplus wind or solar is available without any inconvenience. The worst thing for the grid would be a consistent spike every evening and morning when people with tankless electric heaters take showers....

Just timed it. It takes 2m50s for hot water to reach our furthest tap, which is just over 3 gallons. This is due to mandatory water restrictors. We live in a desert and almost all our water comes from wells which are electrically powered. I will assume an efficiency of 90% for the electric motors and wire losses.

Once I turn off that hot water spigot, all that 3 gallons of heated water is lost. That is just under 1 kWh or what a 310w Solar Panel produces in 24hr in my array in winter on a clear day.

It's sort of like battery storage and Bitcoin which are both CO2 generators. Folk don't understand how power never goes away. We don't teach science anymore.

Turn on that spigot 10 times a work day with 4 family members and cooking, it's destroys the output of a 3,500 kW solar array completely due to inverter losses.

 

[BrettS]

There seems to be lots of folk on this forum that are very informed, I have a question that maybe someone could answer. I’m all electric and need to replace my hot water heater (50 gal). I actually have had it on a timer for 30 years and have it come on in the morning for 2 hours and in the evening for 2 hours. There is only two of us and it has seemed to work and save us some electricity. As I read this thread it looks like an on demand electric hot water heater is not very efficient or reliable, it would be nice to have instant hot water but not sure I want to deal with a pump. I’m thinking of just replacing the old heater with the most efficient unit on the market. Any input is welcome.
Merrill

I won’t rehash it here, but as I’ve said earlier in this post, tankless water heaters do have their place and can save energy over traditional water heaters, but really the ideal use case for them is infrequent high demand and it sounds like that’s probably not what you need.

It’s also worth noting that replacing a traditional water heater with a tankless one will not give you instant hot water. For some reason this seems to be a common misconception. But assuming the tankless unit gets installed in the same location that the traditional one was in then it will still need to send water down the same pipes to your sinks and showers and it will take the same amount of time to get hot water out of the taps.

There are options to install point of use tankless units in your bathroom or under your sink and those can provide instant hot water, but they will only feed one sink or one bathroom, not the whole house.

As others have said, it certainly sounds like a heat pump water heater would be the best option for you. The downside there is a long recovery time, but if you only have your existing water on for 4 hours a day as it is it doesn’t seem like that would be an issue for you.

 

[mongo]

Once I turn off that hot water spigot, all that 3 gallons of heated water is lost. That is just under 1 kWh or what a 310w Solar Panel produces in 24hr in my array in winter on a clear day.

If your water temp is 120 F leaving and 50F entering, that is 70×3×8.34=1,751 BTU, about half a kWh. For a single cycle, the initial 3 gallon and then stagnant 3 gallons get you to 1kWh, but for the repeated usage case, the stagnant of one is the pre use of the next. So 10 uses is 5kWh of waste domestic, still not good.

A few ideas: Can you add a solar collector/ water heater to the roof with a thermal storage tank? With your reduced flow fixtures, can you reduce the supply piping size to increase responsiveness and minimize waste? Depending on climate, an external water to air heat exchanger before the WH could help too.

 

[ohmman]

Just timed it. It takes 2m50s for hot water to reach our furthest tap, which is just over 3 gallons. This is due to mandatory water restrictors. We live in a desert and almost all our water comes from wells which are electrically powered. I will assume an efficiency of 90% for the electric motors and wire losses.

To be clear, you're just saying that the time it takes is a result of the water restrictors. The volume and energy differences should be negligible, correct?

 

[mongo]

We only use hot water for showers and the dishwasher.

Gotcha. Here are some ideas or questionable value...
If the dishwasher has a water heat feature, then the temp to it can be ignored. For kitchen, a small 120V electric under sink unit in the hot water supply can provide instant hot water in small quantities and buffer the colder water in the lines.

For a shower worth of water you'll either need an instant heater or a fully heated tank. For quick reaction at the bath, without a recirculation pump you might concider:
A small unit plumbed into the hot water supply near to the bath for hot water right away. This could also offset the cold periods of your timer setup if showers are predictable.
Minimally sized water lines from the main tank to reduce reaction time and losses.
Main water heater sizing will have a small impact on standby losses based on surface area and insulation value.

 

[Merrill]

I won’t rehash it here, but as I’ve said earlier in this post, tankless water heaters do have their place and can save energy over traditional water heaters, but really the ideal use case for them is infrequent high demand and it sounds like that’s probably not what you need.

It’s also worth noting that replacing a traditional water heater with a tankless one will not give you instant hot water. For some reason this seems to be a common misconception. But assuming the tankless unit gets installed in the same location that the traditional one was in then it will still need to send water down the same pipes to your sinks and showers and it will take the same amount of time to get hot water out of the taps.

There are options to install point of use tankless units in your bathroom or under your sink and those can provide instant hot water, but they will only feed one sink or one bathroom, not the whole house.

As others have said, it certainly sounds like a heat pump water heater would be the best option for you. The downside there is a long recovery time, but if you only have your existing water on for 4 hours a day as it is it doesn’t seem like that would be an issue for you.

When you say heat pump is that what is see online they are all called hybrid heat pump electric hot water heater, also what brand is the best.

 

[mspohr]

When you say heat pump is that what is see online they are all called hybrid heat pump electric hot water heater, also what brand is the best.

Yes, they are all hybrid heat pumps since they have electrical resistance heating as backup. In the discussion here, Rheem seems to be well recommended.
GE did make a heat pump hot water heater but apparently stopped production due to lots of problems. I didn't know about these problems and got a good price (~$1000) on one as they were selling out the last of the inventory. I've installed it and it has been working well with no problems for 1.5 years... I'm keeping my fingers crossed. I've installed it to provide hot water to a detached garage apartment. It's located in the garage downstairs from the apartment which is good since it does make noise. The garage is unheated but doesn't get below freezing so the heat pump apparently works well.

 

[McRat]

To be clear, you're just saying that the time it takes is a result of the water restrictors. The volume and energy differences should be negligible, correct?

Water absorbs heat like a magnet. It's one of the best compounds for that. So the slower your water flows from a heater or cooler, the more energy is lost. But you also need to account for the energy lost for pumping and treating drinking water.

It's interesting to note that this comes into play in both cylinder head design and racing heat exchangers. Velocity must be controlled. This is why sometimes removing a thermostat in an engine makes it overheat sooner.

In much our state, we do not have basements, so the water pipes are often embedded in the ground. This is a significant heat loss with hot water since the ground under a home is never warm.

Ideally there would be building codes that would require all hot water lines to have serious insulation, much like we are required to have double layer windows and insulation in exterior walls and the attic. But we don't.

In an efficient home, hot water used for cooking should be pulled from the tap then heated to boiling. A water heater is more efficient than a pan and stove.

 

[mspohr]

Water absorbs heat like a magnet. It's one of the best compounds for that. So the slower your water flows from a heater or cooler, the more energy is lost. But you also need to account for the energy lost for pumping and treating drinking water.

It's interesting to note that this comes into play in both cylinder head design and racing heat exchangers. Velocity must be controlled. This is why sometimes removing a thermostat in an engine makes it overheat sooner.

In much our state, we do not have basements, so the water pipes are often embedded in the ground. This is a significant heat loss with hot water since the ground under a home is never warm.

Ideally there would be building codes that would require all hot water lines to have serious insulation, much like we are required to have double layer windows and insulation in exterior walls and the attic. But we don't.

In an efficient home, hot water used for cooking should be pulled from the tap then heated to boiling. A water heater is more efficient than a pan and stove.

This is a good argument for point of use water heaters for kitchen and bath sinks. You just need a little bit of water occasionally and it's inefficient to fill all the hot water lines every time you wash your hands, etc.

 

[ohmman]

Water absorbs heat like a magnet. It's one of the best compounds for that. So the slower your water flows from a heater or cooler, the more energy is lost. But you also need to account for the energy lost for pumping and treating drinking water.

Yes, and it's complicated because depending on where the pipes are, that heat is either dissipated into a conditioned space (which could require extra energy to cool, or less energy to heat), or into another space. But I would like to see your math on the difference between slower and faster hot water moving through your pipes to get to your faucet. I am familiar with the specific heat of water but remain unconvinced the difference is meaningful.

In much our state, we do not have basements, so the water pipes are often embedded in the ground. This is a significant heat loss with hot water since the ground under a home is never warm.

Ideally there would be building codes that would require all hot water lines to have serious insulation, much like we are required to have double layer windows and insulation in exterior walls and the attic. But we don't.

By "in the ground" I'm assuming you mean in a concrete slab, but maybe there are places where they're directly buried. Most houses in my area are perimeter pier-and-beam foundations and the hot water pipes run through the attics. The code requires insulation on all attic pipes unless they're buried by 4" of attic insulation. Insulation is not required for pipes going through insulated walls for the same reason, unless they contact metal braces. I don't know or understand the requirements for insulation on direct-bury or in-slab hot water plumbing.

 

[Merrill]

Yes, they are all hybrid heat pumps since they have electrical resistance heating as backup. In the discussion here, Rheem seems to be well recommended.
GE did make a heat pump hot water heater but apparently stopped production due to lots of problems. I didn't know about these problems and got a good price (~$1000) on one as they were selling out the last of the inventory. I've installed it and it has been working well with no problems for 1.5 years... I'm keeping my fingers crossed. I've installed it to provide hot water to a detached garage apartment. It's located in the garage downstairs from the apartment which is good since it does make noise. The garage is unheated but doesn't get below freezing so the heat pump apparently works well.

Thanks, Rheem does seem to get good reviews. You mention noise, my hot water heater is in the house in a closet but near the kitchen and tv so noise would be an issue.

 

[mongo]

But I would like to see your math on the difference between slower and faster hot water moving through your pipes to get to your faucet. I am familiar with the specific heat of water but remain unconvinced the difference is meaningful.

It is sort of like rockets and gravity, the longer you fight gravity, the more energy you expend.

Physics wise, if you have a fixed pipe size, the loss from that pipe is based on surface area, thermal resistance, and temperature differential. For a given temperature of water, there is a given loss over the pipe length. The faster the water moves, the less time each molecule loses energy. Twice as much water in the same time frame with the same total energy loss yields half the energy per volume. For a piping situation, this means the water arrives at a higher temperature. Since the final temp is usually a mix if hot and cold, you use less hot water overall. At the extremes, you can have a trickle of water at ambient temperature or a firehose at tank temp.

Just bumping up the flow rate only helps in the case of purging the stagnant water, since flowing more than is needed at point of use is a net loss of efficency. A higher flow fixture would use more water overall, but lesser percentage of the hot.

The optimized solution is insulated pipes and the smallest diameter that allows the desired flow (neglecting all other rules of plumbing).

Edit: typo

 

[tdjvfr]

I love my natural gas tankless water heater and will have one in any house I own. I would never want to put an electric tankless in. Uses way too much electric for me.

 

[Merrill]

Thanks for all the input, I do not think I can do a heat pump unit, not enough air space and no way to connect to the outside.

 

[nwdiver]

Here's an interesting article on how water heaters (Tanked & Electric) are 'hidden batteries'. The day is fast approaching that most of the energy used to heat the water in your home could be energy that would have been curtailed or wasted of you didn't have a place to put it...

 

[mspohr]

Here's an interesting article on how water heaters (Tanked & Electric) are 'hidden batteries'. The day is fast approaching that most of the energy used to heat the water in your home could be energy that would have been curtailed or wasted of you didn't have a place to put it...

Good to see that some utilities are exploring the possibilities of demand management. My small utility is conflicted. They are going through the motions but don't really get it. Just installed an extra set of solar panels with a generous incentive but am gobsmacked that they want me to install an old school plug in meter to monitor my production. The Enphase Envoy meter is too confusing to them and is not sufficient.

 

[nwdiver]

The Enphase Envoy meter is too confusing to them and is not sufficient.

There are specific accuracy standards that your Envoy meter probably doesn't meet. At my old job we had to weigh UF6 cylinders to verify contents. We had a 'WOHWA' gyroscopic scale that was extremely accurate and Load Cells that were much less accurate but met the QL1 standards so we always had to use the less accurate scales for quality control.

 

[Topher]

the only additional latent heat load is due to leakage/ respiration/perspiration.

You are neglecting cooking and showering moisture (plus a number of other sources: plants, clothes drying). A very rough estimate for those would be:
Respiration and perspiration: 4.2 lbs of H2O per person per day.
Shower: 0.6 lbs per person per day (5 minute shower)
Cooking: 2 lbs per person per day.
Each of those pounds of water involves 1075 BTU (0.32 kWh) of latent heat.

So it could be viewed at carbon negative overal (ignoring harvesting energy).

One key point is what replaced that tree? If you cut down a tree, put up a parking lot, that carbon is never sequestered again, so it is no different from fossil fuels. Also harvesting energy can be a significant portion of the total. If you are leaving carbon in your ash, you are burning horribly; but ash is not biochar, it gets converted to be plant available very quickly.

The day is fast approaching that most of the energy used to heat the water in your home could be energy that would have been curtailed or wasted of you didn't have a place to put it...

Yup. I am all set to add an electrical heater to my thermal storage tank, if my solar PV panels start producing more than I can monetize (i.e. net-metering). If my Model 3 doesn't arrive before July, I may need to do that this summer.

Thank you kindly.