Water heater design oriented towards heat pumps

[jg167]

A heat pump water heater has a longer recharge time then gas or electric water heaters due to their lower heater capacity (likely not the proper term but the supply of heat). All the tank hot water heaters (i.e. not tankless) I am aware of follow the same design. Hot water is supplied as needed and as the level of the water is drawn down water from the line is used to keep the tank full which of course immediately cools the water in the tank down bringing the heating method online to bring it back up to temp. All this is ongoing as hot water is being used.

Consider another possibility which would be two (A and B) smaller water heaters in tandem with a common control system. Hot water would be taken from A withOUT refilling it with line water until perhaps 90% of its (fully hot) water is gone. Then the active tank is switched to tank B and tank A is filled back up engaging its heat source if not already active. At all times the heating element in each tank monitors the water temp and comes on accordingly (so it has to work even if the tank is only partially full). It is unlikely that tank A when newly refilled with cold line water could be brought up to temperature in the time it would take to drain tank B if at maximum output, so you still could run out. But this seems unlikely and while you had hot water it would be fully hot. Such a design might replace a single 40g unit with two 30g units and use 2/3's of each until the the active unit is swapped . So you would get a full 40g of fully hot water before it suddenly got much cooler instead of perhaps using 20g form a standard unit until you noticed the cool down, especially with a heat pump.

Has anyone heard of such a design? If would of course be more expensive but also offer better functionality.

 

[GReynolds929]

Not a good idea to run a tank controlled by a thermostat to near empty. Electric elements powered on outside of water will burn up in seconds. Preheating steel water and air with a gas burner and introducing cold water will cause the water to flash steam and stress the metal.
It would be better to pipe two water heaters in parallel or series, or turn up the thermostat and use a TMV, to do what you want.
You're overthinking a problem that has already been solved.

 

[Sponsor]

 

[Fitter30]

Heatpump heater 15k btu 200-300% efficiency based on space temp
4500 watt electric 15,345 btu's 100% efficiency
Gas 40k btu's. 85% efficiency
A pump and air would have to enter the tanks in your scenario for steady water pressure since there is no added pressurised water entering the tanks. Doesn't matter if X gallons are used it still take 1 btu to raise 1 lb of water 1°

 

[Bannerman]

Hot water is supplied as needed and as the level of the water is drawn down water from the line is used to keep the tank full which of course immediately cools the water in the tank down

Hot water is less dense than cold water so hot water will rise to the top of the tank where it will exit the WH to supply faucets and appliances.

The cold water entering the tank is delivered to the bottom of the tank, either by locating the water inlet connection through the side near the bottom of the tank, or when the inlet connection is located at the top of the tank, an internal dip tube is utilized to route the cold water to the bottom without mixing with the hot.

Since hot water rises to the top and as the cold water is delivered directly to the bottom of the tank which remains pressurized, there will be minimal turbulence or mixing, resulting in the hot water temperature to remain fairly consistent until there is minimal hot water remaining within the tank.

Hot water would be taken from A withOUT refilling it with line water until perhaps 90% of its (fully hot) water is gone.

How do you propose hot water will flow out from the tank under pressure with no water entering to replace the hot water exiting the tank?

A heat pump water heater has a longer recharge time then gas or electric water heaters due to their lower heater capacity (likely not the proper term but the supply of heat).

What leads you to this conclusion?

 

[Taylorjm]

Heatpump heater 15k btu 200-300% efficiency based on space temp
4500 watt electric 15,345 btu's 100% efficiency
Gas 40k btu's. 85% efficiency
A pump and air would have to enter the tanks in your scenario for steady water pressure since there is no added pressurised water entering the tanks. Doesn't matter if X gallons are used it still take 1 btu to raise 1 lb of water 1°

How is a heatpump heater 200-300% efficiency? Doesn't that mean it's actually producing more electricity than it uses?

 

[Bannerman]

Doesn't that mean it's actually producing more electricity than it uses?

Takes considerably less energy to move heat compared to producing heat.

Just as your air conditioner will cool your home by removing the heat from the air, moving it outdoors, a heat pump WH will transfer heat from outside of the unit, moving it to heat the water inside the WH tank.

A traditional electric WH resistance element will consume $1.00 worth of electricity to produce $1.00 worth of heat.

A heat pump that is 200-300 efficient, will transfer $1.00 worth of heat into the water while consuming only $0.33 - $0.50 worth of electricity.

 

[Taylorjm]

Takes considerably less energy to move heat compared to producing heat.

Just as your air conditioner will cool your home by removing the heat from the air, moving it outdoors, a heat pump WH will transfer heat from outside of the unit, moving it to heat the water inside the WH tank.

A traditional electric WH resistance element will consume $1.00 worth of electricity to produce $1.00 worth of heat.

A heat pump that is 200-300 efficient, will transfer $1.00 worth of heat into the water while consuming only $0.33 - $0.50 worth of electricity.

I can understand that an electric water heater is 100% efficient, because there's no exhaust up the flue to lose anything, but now your saying because a heat pump is transferring heat from outside to inside, it's consuming less electricity to produce $1.00 worth of heat? I don't think that's correct. Efficiency isn't rated on producing $1.00 worth of heat. I think you've been reading too many promo brochures from heat pump manufacturers. lol I guess I'd have to see it to believe it. All the literature from manufacturers and the government say heat pumps are superior, but when you dig deeper into reviews by actual plumbers, they are saying the recovery time is much less than an traditional electric water heater. So not sure who to believe.

 

[Reach4]

How is a heatpump heater 200-300% efficiency? Doesn't that mean it's actually producing more electricity than it uses?

They get that by comparing electricity use. If we have a conventional electric WH that takes 2.5 KWH to do an amount of water heating, and the heat pump WH takes 1000 KWH, then we might call that 250% efficient.

 

[Taylorjm]

They get that by comparing electricity use. If we have a conventional electric WH that takes 2.5 KWH to do an amount of water heating, and the heat pump WH takes 1000 KWH, then we might call that 250% efficient.

Exactly. It's a play on numbers. They are using the electric water heater as a baseline to justify saying the heat pump is this unrealistic percent of efficiency. Where like you said, if it takes 2.5kwh to heat a 40gal electric WH, and the heat pump uses 1kwh to heat the same amount of water, it doesn't automatically make the heat pump 250x more efficient, but for marketing purposes, they will definitely play on that. It sounds better to say it's 250% efficient than to say it uses 2.5x less electricity than a typical electric water heater. They just have to add a bunch of asterisks after the 250% efficient and they are covered.

 

[wwhitney]

Exactly. It's a play on numbers.

You seem to imply that it's misleading or dishonest, but it's not.

The usual term that I've seen used in describing heat pumps is COP or coefficient of performance. A resistance electric heater would have a COP of 1.0. If a heat pump water heater has a COP of 3.0, that means it will use 1/3 of the electricity to produce the same amount of hot water as a resistance electric water heater. That's it.

Cheers, Wayne

 

[Reach4]

You seem to imply that it's misleading or dishonest, but it's not.

I agree. I do think that vacuum cleaners that say "6.0 Peak HP" are being dishonest.

 

[Taylorjm]

You seem to imply that it's misleading or dishonest, but it's not.

The usual term that I've seen used in describing heat pumps is COP or coefficient of performance. A resistance electric heater would have a COP of 1.0. If a heat pump water heater has a COP of 3.0, that means it will use 1/3 of the electricity to produce the same amount of hot water as a resistance electric water heater. That's it.

Cheers, Wayne

I agree with that. The play on words is when they say it's 250% efficient.

 

[wwhitney]

I agree with that. The play on words is when they say it's 250% efficient.

But what's the problem with using the word "efficiency" instead of "COP"?

If option A is 50% efficient, and option B is 100% efficient, then option A takes 100% / 50% = 2.0 times as much of the input as option B would take to get the same output.

Likewise, if option C is 250% efficient, then option B takes 250% / 100% = 2.5 times as much of the input as option C would take to get the same output. Which is what we agreed that COP means.

I don't see "efficiency" as misleading in this context.

Cheers, Wayne