EEVblog 1517 - Heat Pump Hot Water Install & Analysis - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: December 01, 2022, 12:37:16 am
content: 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?

author: thm_w
author_rank: Super Contributor
timestamp: December 02, 2022, 06:08:57 am
content: 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) / 3600220L hot water per dayAverage in AUS is 75L/day per person.

author: EEVblog
author_rank: Unknown
timestamp: December 02, 2022, 06:40:52 am
content: Quote from: thm_w on December 02, 2022, 06:08:57 amI 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.

author: BrianHG
author_rank: Super Contributor
timestamp: December 02, 2022, 08:56:11 pm
content: Quote from: EEVblog on December 02, 2022, 06:40:52 amSame 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.

author: thm_w
author_rank: Super Contributor
timestamp: December 02, 2022, 09:44:52 pm
content: Quote from: EEVblog on December 02, 2022, 06:40:52 amYes, 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 02, 2022, 10:01:23 pm
content: Mixing the hot and cold water in the tank effectively reduces the useful capacity. There is not much use of tank full of 30 C water, even though there is still some energy in it. A little hot water on the top and 15 C for the rest is more useful. So to make the tank work well they want to avoid the mixing, e.g. by careful design how the water flows in (not to small a diameter). A rather slow use of the water helps a little as with slow flow less mixing is expected. The stratification is also a reason why the tanks are relatively high and not lower and slightly larger diameter, even though this would mean less surface area.The data from the very first day and heat up cycle may give a reasonable guess for the heat needed: That tanks started all cold and than when to hot for the upper 70% and the rest somewhat warmed.I would expect a slight correlation in the power consumption to the outside temperature, though the change in COP with temperature is not that great.3 kwh per day is not much electricity used. It would not be bad even if you have to pay the regular price.

author: EEVblog
author_rank: Unknown
timestamp: December 02, 2022, 11:16:49 pm
content: Quote from: Kleinstein on December 02, 2022, 10:01:23 pmThere is no data for the first day. All data is regular usage cycle.QuoteYeah, it's fine.

author: EEVblog
author_rank: Unknown
timestamp: December 02, 2022, 11:19:18 pm
content: Quote from: BrianHG on December 02, 2022, 08:56:11 pmEven with the gas heater on it still hits you pretty quick. It goes from normal hot temperture to fairly cold in maybe 30 seconds. That flame doesn't heat up much instantly.

author: thm_w
author_rank: Super Contributor
timestamp: December 03, 2022, 12:52:34 am
content: The average gas hot water tank takes 30-60mins to heat up. So, theoretically in a 12hr period with a 175L tank you could use 2,000L of hot water.Just looking up the actual numbers now, a Rheem 50gal (190L) is rated at 36k BTU. That is 10kW minus losses (maybe 8kW).For comparison, an on demand water heater that can handle continuous 8GPM (30 L/min) is rated at <20kW.