Tankless water heaters are terrible....

[mongo]

No, which is why the solution is bigger tanks

Sort of.
Total water output can't exceed production over a long time span. A large hot tank gives increased instant capacity; however, a large depleted tank also takes a long time to recover (temperature wise).
That's why series connected/ cascade water heaters are handy, they combine their heat output and provide surge capacity along with improved near term recovery.

Heat pump heaters are going to be slower. Just due to the design choices they have made towards efficiency.

For normal single unit sized setups. Gas is higher BTU than electric, and heat pump is lower BTU than electric.
Rheem has 4,500 W backup element which is 15k BTU. Heat pump is 4.2k BTU/hr.
For reference, if incoming water is 60F and hot is 120F, 1k BTU/hr provides just over 2 gallons per hour.

One could stack three 40 gallon units in series and get >150 gallons first hour and 25 gallons an hour recovery using the heat pump only or 80 gallons HP+element (extra 15kW).

Roof solar water heaters and heat recovery from dryer and waste water would go a long way towards reducing energy needs (but adds complexity).
Roof solar to large holding tank(s) with indirect heat exchanger might be interesting .

 

[SageBrush]

That's why series connected/ cascade water heaters are handy

Interesting idea, but I think OP is power supply limited

 

[mongo]

Interesting idea, but I think OP is power supply limited

Oh, they definitely are, even with ipgrades meter, 30A * 220 * 3 is only 20kW. Heat pumps could give 4x the heat for the same power vs resistance. Otherwise, propane...

 

[STS-134]

Besides low efficiency... tankless heaters are BRUTLE on the grid.

Peak usage doesn't really matter in the grand scheme of things. It doesn't matter, for example, if you charge your car at 48A, or at 24A for twice as long, or at 12A for 4 times as long. Over a large number of cars, it's going to average out to the same thing in the end.

What I don't understand is why they don't have dual fuel water heaters and more powerful heat pump water heaters with the compressor and evaporator coil outside. So something like 100k BTU/hr under ideal conditions and if the heat pump can't deliver, kick in the gas burner to assist.

 

[nwdiver]

Peak usage doesn't really matter in the grand scheme of things. It doesn't matter, for example, if you charge your car at 48A, or at 24A for twice as long, or at 12A for 4 times as long.

…. um…. what? It matters enormously. Imagine the consequences if every shower hit the grid with 10kW of demand. People generally shower in roughly the same time frame.

For a good real world example look at TV pickup. A flatter demand profile is always better.

 

[STS-134]

…. um…. what? It matters enormously. Imagine the consequences if every shower hit the grid with 10kW of demand. People generally shower in roughly the same time frame.

For a good real world example look at TV pickup. A flatter demand profile is always better.

In the case of a tankless heater, all of the usage comes only when the water is running. It's a lot of usage but it ceases immediately after water usage stops. In the case of a tank heater using resistance heating, the usage is during the shower and probably for 30 minutes after it ends. Yes, people generally shower in the mornings or evenings but the peaks really don't matter. There will be some percentage of people who are taking a shower at any given time; this translates to a certain number of watts of usage. Whether it's 10% of houses all using tankless heaters consuming all of that energy at once and then ceasing to do so as soon as the person gets out of the shower or 30% of houses using tank heaters consuming 1/3 the power for 3 times as long, the load on the grid is the same. This is why we don't worry about everyone's AC units all cycling on at the same time; over a very large number of houses, it all averages out and having them all cycle on at the same time is unlikely (just like it's unlikely that all of the air molecules in the room you're sitting in will spontaneously go to the other side and make you suffocate).

 

[phigment]

In the case of a tankless heater, all of the usage comes only when the water is running. It's a lot of usage but it ceases immediately after water usage stops. In the case of a tank heater using resistance heating, the usage is during the shower and probably for 30 minutes after it ends. Yes, people generally shower in the mornings or evenings but the peaks really don't matter. There will be some percentage of people who are taking a shower at any given time; this translates to a certain number of watts of usage. Whether it's 10% of houses all using tankless heaters consuming all of that energy at once and then ceasing to do so as soon as the person gets out of the shower or 30% of houses using tank heaters consuming 1/3 the power for 3 times as long, the load on the grid is the same. This is why we don't worry about everyone's AC units all cycling on at the same time; over a very large number of houses, it all averages out and having them all cycle on at the same time is unlikely (just like it's unlikely that all of the air molecules in the room you're sitting in will spontaneously go to the other side and make you suffocate).

I guess you've never heard of peaker plants, or demand pricing?

 

[mongo]

This is why we don't worry about everyone's AC units all cycling on at the same time; over a very large number of houses, it all averages out and having them all cycle on at the same time is unlikely (just like it's unlikely that all of the air molecules in the room you're sitting in will spontaneously go to the other side and make you suffocate).

Unless you are in a new development that overloads the local grid and all the houses have the same brand of AC unit...
Trip
Restore
Dim
Dimmer
Trip
Wait hours for power to come back..

 

[ZenRockGarden]

Peak usage doesn't really matter in the grand scheme of things. It doesn't matter, for example, if you charge your car at 48A, or at 24A for twice as long, or at 12A for 4 times as long. Over a large number of cars, it's going to average out to the same thing in the end.

What I don't understand is why they don't have dual fuel water heaters and more powerful heat pump water heaters with the compressor and evaporator coil outside. So something like 100k BTU/hr under ideal conditions and if the heat pump can't deliver, kick in the gas burner to assist.

I think there's a market for a completely unified HVAC+Tankless heat pump system - like a great big version of what's in Tesla cars it heats or cools the interior, and also heats or cools the liquid for the battery pack. Scale it up, maybe put a powerwall next to it to handle the occasional surge in

 

[nwdiver]

In the case of a tankless heater, all of the usage comes only when the water is running. It's a lot of usage but it ceases immediately after water usage stops. In the case of a tank heater using resistance heating, the usage is during the shower and probably for 30 minutes after it ends. Yes, people generally shower in the mornings or evenings but the peaks really don't matter. There will be some percentage of people who are taking a shower at any given time; this translates to a certain number of watts of usage. Whether it's 10% of houses all using tankless heaters consuming all of that energy at once and then ceasing to do so as soon as the person gets out of the shower or 30% of houses using tank heaters consuming 1/3 the power for 3 times as long, the load on the grid is the same. This is why we don't worry about everyone's AC units all cycling on at the same time; over a very large number of houses, it all averages out and having them all cycle on at the same time is unlikely (just like it's unlikely that all of the air molecules in the room you're sitting in will spontaneously go to the other side and make you suffocate).

As @phigment pointed out if the law of large numbers applied here TOU wouldn’t be a thing.

Tankless heaters are a double hit. Not only is the peak demand ~20x higher than a heat pump but you also can also schedule a heat pump to use energy off-peak.

 

[STS-134]

I think there's a market for a completely unified HVAC+Tankless heat pump system - like a great big version of what's in Tesla cars it heats or cools the interior, and also heats or cools the liquid for the battery pack. Scale it up, maybe put a powerwall next to it to handle the occasional surge in

Yeah, and this would be a big advantage in the summer when you want to cool the house and dump the waste heat into your water heater tank instead of dumping it outdoors and using additional energy to heat the water.

As @phigment pointed out if the law of large numbers applied here TOU wouldn’t be a thing.

Tankless heaters are a double hit. Not only is the peak demand ~20x higher than a heat pump but you also can also schedule a heat pump to use energy off-peak.

Peak vs off-peak demand doesn't matter with the law of large numbers. But I was only comparing tankless to tank resistive heaters. Heat pumps are better for the grid because they consume less energy overall since a lot of the heat energy used to heat the water comes from the ambient air.

 

[nwdiver]

Yeah, and this would be a big advantage in the summer when you want to cool the house and dump the waste heat into your water heater tank instead of dumping it outdoors and using additional energy to heat the water.

Peak vs off-peak demand doesn't matter with the law of large numbers. But I was only comparing tankless to tank resistive heaters. Heat pumps are better for the grid because they consume less energy overall since a lot of the heat energy used to heat the water comes from the ambient air.

Any tank is better than no tank. Nice to shower when you want then heat the water when energy is abundant at 2am instead of paying peak rates at 8pm if that’s when you wanted a shower…

My water is heated 100% from solar. My water heater only heats during the day then I shower in the evening. Kinda hard to pull that off with tankless.

 

[brkaus]

Peak usage doesn't really matter in the grand scheme of things. It doesn't matter, for example, if you charge your car at 48A, or at 24A for twice as long, or at 12A for 4 times as long. Over a large number of cars, it's going to average out to the same thing in the end.

What I don't understand is why they don't have dual fuel water heaters and more powerful heat pump water heaters with the compressor and evaporator coil outside. So something like 100k BTU/hr under ideal conditions and if the heat pump can't deliver, kick in the gas burner to assist.

They definitely make outdoor heat pump water heaters. They don’t have tanks, but they do make tanks with heat exchangers built in. If the gas burner is a traditional gas water heater with the flu up the middle, that’s adding a place for heat loss. So one would want some sort of tankless gas heater with a circulation pump. Or, I’ve been considering just putting the gas tankless on the output of the heat pump water heater for emergencies.

 

[STS-134]

They definitely make outdoor heat pump water heaters. They don’t have tanks, but they do make tanks with heat exchangers built in. If the gas burner is a traditional gas water heater with the flu up the middle, that’s adding a place for heat loss. So one would want some sort of tankless gas heater with a circulation pump. Or, I’ve been considering just putting the gas tankless on the output of the heat pump water heater for emergencies.

Tankless heat pump water heater would be nice. The benefits of a heat pump with no standby losses. Best scenario for me from an efficiency standpoint is probably tankless heat pump water heater for kitchen and showers and then tankless electric resistance water heater for bathroom sinks. It's not at all efficient for me to heat up the water in the entire recirculation loop just to use half a gallon of water to wash my hands.

 

[brkaus]

Tankless heat pump water heater would be nice. The benefits of a heat pump with no standby losses. Best scenario for me from an efficiency standpoint is probably tankless heat pump water heater for kitchen and showers and then tankless electric resistance water heater for bathroom sinks. It's not at all efficient for me to heat up the water in the entire recirculation loop just to use half a gallon of water to wash my hands.

Something like this paired with a buffer tank for hot water.
Arctic Heat Pump 035ZA/BE – 29,000 BTU

But the max outlet temp is 131 which might be a bit cool once you have heat loss in the pipes and efficiency drops at the higher temps.

Cost and complexity goes up with more components.

Probably would make sense for a small tankless or very small tank for sinks. But properly sized plumbing can get hot water to the point of use pretty fast.

Definitely agree recirc loop problems. My stupid builder didn't think to insulate it much under the slab.

 

[Jjrss]

I had a Rheem natural gas tankless tied into a Hydronic blower HVAC central air. "Worked" spendidly, and an additional chip allowed water to heat to 185 degrees in the blower loop. Open a tap, and water was actually cooled to ~120 degrees before it hit the tap...

All in all, "worked" splendidly and only needed 120V on a 15A circuit.

I should emphasize it "worked"... 2 years after installed the 2nd unit "worked" great after the 1st one leaked everywhere frying the control board and requiring a full unit replacement (luckily under warranty). I sold the house soon after, and have had Heat pump HVAC with backup propane in the new house... learned never to put all my hens in 1 basket...

 

[ohmman]

 

[bxr140]

Possibly already in this thread somewhere: Harvest Thermal

 

[mspohr]

Possibly already in this thread somewhere: Harvest Thermal

Looks good.
I believe I have mentioned this before that I have Chiltrix (chiltrix.com) units installed in both of my houses. They are similar to the Harvest units. They have an air to water heat pump, DHW tank and a buffer tank for hydronic heating. I use these for DHW and hydronic floor heating in both houses. They also have air handler units for heating and cooling. They have worked well for several years.

 

[SageBrush]

My stupid builder didn't think to insulate it much under the slab.

I rented an upscale home in Colorado with exactly the same issue. I spent months pulling my hair out wondering why the the boiler/hydronics was such an energy hog.