On-demand heat pump water heater

1. A fluid heating device comprising:

a fluid inlet;

a fluid outlet;

a heating chamber disposed in fluid communication between the fluid inlet and the fluid outlet, the heating chamber configured to hold a fluid and having a fluid capacity of approximately ten gallons or less;

a heat pump configured to heat the fluid in the heating chamber;

a controller configured to control a temperature of the fluid based on a predetermined temperature setting by controlling an output of the heat pump;

a supplemental heat source disposed in fluid communication with the fluid outlet and configured to heat the fluid, wherein the controller is configured to control the supplemental heat source; and

a fluid mixing valve in fluid communication with the supplemental heat source and the fluid inlet and configured to reduce the temperature of a fluid heated by the supplemental heat source.

2. The fluid heating device of claim 1 , wherein the fluid capacity is no more than 5 gallons.

3. The fluid heating device of claim 1 , wherein the fluid capacity is no more than 2 gallons.

4. The fluid heating device of claim 1 further comprising a temperature sensor configured to detect a temperature of the fluid.

5. The fluid heating device of claim 1 , further comprising a flow sensor configured to detect a flow of fluid downstream of the fluid inlet.

6. The fluid heating device of claim 1 , wherein the fluid mixing valve is controlled by the controller.

7. The fluid heating device of claim 1 , wherein the controller is a first controller, the fluid heating device further comprising a second controller configured to control the fluid mixing valve.

8. A method of controlling a fluid heating system, the method comprising:

detecting, by a flow sensor, a flow of a fluid and outputting flow data;

detecting, by a temperature sensor, a temperature of the fluid and outputting temperature data;

in response to determining that the flow data indicates positive flow of the fluid, outputting, by a controller and to a heat pump, instructions to heat the fluid in a low fluid capacity heating chamber, the low fluid capacity heating chamber configured to hold a fluid and having a fluid capacity of approximately ten gallons or less;

in response to determining that the temperature data indicates fluid temperature is less than a predetermined temperature setting, outputting, by the controller and to the heat pump, instructions to heat the fluid in the low fluid capacity heating chamber;

outputting, by the controller and to a supplemental heat source disposed in fluid communication with a fluid outlet of the low fluid capacity heating chamber, instructions to heat fluid output from the low fluid capacity heating chamber; and

outputting, by the controller and to a fluid mixing valve disposed in fluid communication with the supplemental heat source and a fluid inlet of the low fluid capacity heating chamber, instructions to reduce a temperature of a fluid heated by the supplemental heating source.

9. The method of claim 8 , further comprising:

in response to determining that the fluid temperature cannot be maintained at a predetermined temperature setting for a predetermined amount of time based on the flow data and the temperature data, outputting, by the controller and to the heat pump, instructions to heat the fluid in the low fluid capacity heating chamber.

10. The method of claim 8 , further comprising:

in response to determining that a fluid temperature is outside of a predetermined temperature range, output, by the controller, adjustment instructions to a fluid mixing valve.

11. The method of claim 8 , further comprising:

in response to determining that the fluid temperature is less than a predetermined temperature setting, outputting, by a controller, instructions to a supplemental heat source to heat the fluid.

12. A system comprising:

a low fluid capacity heating chamber configured to hold a fluid and having a fluid capacity of approximately ten gallons or less;

a controller; and

a memory having instructions stored thereon that, when executed by the controller, directs the controller to:

receive, at the controller and from a flow sensor, flow data indicative of a detected flow rate associated with the fluid;

receive, at the controller and from a temperature sensor, temperature data indicative of a detected temperature of the fluid;

in response to determining that (i) the flow data indicates a positive flow of the fluid or (ii) the temperature data indicates a fluid temperature that is less than a predetermined temperature setting, output a heat pump control signal to heat the fluid in the low fluid capacity heating chamber;

output, by the controller and to a supplemental heat source disposed in fluid communication with a fluid outlet of the low fluid capacity heating chamber, instructions to heat fluid output from the low fluid capacity heating chamber; and

output, by the controller and to a fluid mixing valve disposed in fluid communication with the supplemental heat source and a fluid inlet of the low fluid capacity heating chamber, instructions to reduce a temperature of a fluid heated by the supplemental heating source.

13. The system of claim 12 , wherein the heat pump control signal is a first heat pump control signal, and

wherein the instructions, when executed by the controller, further direct the controller to:

output a second heat pump control signal by the controller in response to determining, based on the flow data and the temperature data, that a detected fluid temperature cannot be maintained by the system for a predetermined amount of time.

14. The system of claim 12 , wherein the instructions, when executed by the controller, further direct the controller to:

output a fluid mixing valve control signal by the controller in response to determining that the temperature data indicates a detected fluid temperature that is outside of a predetermined temperature range.

15. The fluid heating device of claim 5 , wherein the controller is further configured to: receive flow data from the flow sensor; and

in response to determining, based on the flow data, that the heat pump is unable to heat the fluid to a temperature greater than or equal to the predetermined temperature, output a control signal to the supplemental heat source to heat the fluid.

16. The fluid heating device of claim 1 , further comprising a condenser within the heating chamber.