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EEVblog 1517 - Heat Pump Hot Water Install & Analysis

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EEVblog:
Installation of a Reclaim Heat Pump hot water system replacing a high efficiency gas tank hot water system. And anlysis of the results. Is a heat pump hot water system more efficienct than a gas hot water system?

thm_w:
Interesting to see all the data.
I think there is one logical misstep though. Just because the 175L tank never ran out, does not mean you are using less than 175L of hot water per day.

If you use an average ambient temp of 15C and a water temp of 50C, dT = 35C, with your 9kWh adjusted gas energy calculation.
Pt = (4.2 * L * dT) / 3600

220L hot water per day
Average in AUS is 75L/day per person.

EEVblog:

--- Quote from: thm_w on December 02, 2022, 06:08:57 am ---Interesting to see all the data.
I think there is one logical misstep though. Just because the 175L tank never ran out, does not mean you are using less than 175L of hot water per day.

If you use an average ambient temp of 15C and a water temp of 50C, dT = 35C, with your 9kWh adjusted gas energy calculation.
Pt = (4.2 * L * dT) / 3600

220L hot water per day
Average in AUS is 75L/day per person.

--- End quote ---

I don't think so. We can very plainly feel when the hot water "runs out". It's a constant temperature and then womp, it gets cold very quickly.
Obviously there is a thermocline in the tank that rises and when the last of the hot water (presumably the full 175L) gets used, you hit the cold water layer.
We have low flow shower heads and I in particular am very frugal with my shower.

BrianHG:

--- Quote from: EEVblog on December 02, 2022, 06:40:52 am ---
--- Quote from: thm_w on December 02, 2022, 06:08:57 am ---Interesting to see all the data.
I think there is one logical misstep though. Just because the 175L tank never ran out, does not mean you are using less than 175L of hot water per day.

If you use an average ambient temp of 15C and a water temp of 50C, dT = 35C, with your 9kWh adjusted gas energy calculation.
Pt = (4.2 * L * dT) / 3600

220L hot water per day
Average in AUS is 75L/day per person.

--- End quote ---

I don't think so. We can very plainly feel when the hot water "runs out". It's a constant temperature and then womp, it gets cold very quickly.
Obviously there is a thermocline in the tank that rises and when the last of the hot water (presumably the full 175L) gets used, you hit the cold water layer.
We have low flow shower heads and I in particular am very frugal with my shower.

--- End quote ---

Same for my Gas and electric tanks.
Hint, if you want to go from hot to an almost instant cold, before use, cut the power or gas to your tank.  Once you go through the tank, that cold layer hits you like a rock.

I know that turning on my kitchen sink's hot water for only ~3 minutes and the gas burner will turn on.

I guess the incoming and outgoing water doesn't stir the water within the tank.

thm_w:

--- Quote from: EEVblog on December 02, 2022, 06:40:52 am ---I don't think so. We can very plainly feel when the hot water "runs out". It's a constant temperature and then womp, it gets cold very quickly.
Obviously there is a thermocline in the tank that rises and when the last of the hot water (presumably the full 175L) gets used, you hit the cold water layer.
We have low flow shower heads and I in particular am very frugal with my shower.

--- End quote ---

Yes, but the previous gas burner was capable of putting out 4-10kW+ of equivalent thermal power. As soon as that temperature drops below ~50C, the burner fires up and won't stop until people have stopped using water.
Its different with the new tank, because its only heating during some portion of the day, so if you are using it outside of that time zone, you will be limited to the ~300L in the tank. Whereas before, 175L was never a true limit.

Also, the new tank is using even more (10-11kW vs 9kW you calculated with gas). Either thats some measurement anomaly or the hot water use has gone up a little bit (from ~220L to ~245L per day).

edit: 220L is top end estimate though, would be a bit lower due to heat loss in the pipes.