Stiebel Eltron Eltron WWK 302 H Questions & Answers

I don’t know the technical answer that you are looking for but even with lots of guests staying here we have never run the hot water down to a temperature that you couldn’t shower with

Thanks Tony
Being a technical person I am looking for heat transfer data for instance to raise the temperature of 300l of supply water say at 15 deg in through to 65 deg out you need to have 300x50kilo calories of heat energy transferred

1 kcal is equivalent to 0.0012 kw
If you know the time taken you can work out the heat transfer rate and get an idea of the the continuous flow rate achievable and the running costs

Warning, it gets a bit technical. Here is the short answer:
A traditional single phase hot water system that turns electric energy into heat (hot water) will in theory take a bit over 7 hours to heat those 300 litres from 15 - 65 degrees or in other words it can do about 41 litres per hour. For that is uses about 17.4kWh which will cost you roughly $4.20 in electricity.

Heat pumps are way smarter than to just turn electricity into heat. Through clever engineering they can heat up about 3.5 times as much water using the same amount of energy. The way they are engineered, they still take about the same time (so roughly 41 litres per hour), but use WAY less energy (cost) to do so.

Hint: Put the heat pump hot water system in the warmest possible spot outside, the outside temperature makes a big difference in the running cost for air to water heat pumps

All numbers are theoretical numbers and do not include any losses. Real numbers will be smaller for the amount of water heated per hour and higher for the costs to do so.
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Here is how I came up with my numbers. Please let me know if anybody finds a mistake.

In theory 1 kWh (that's what you pay for on your electricity bill) can heat up just under 860 litres of water by 1 degree. Or, in reverse, for 1 litre to be heated up by 1 degree you need 0.001163 kWh (1/860 = 0.001163). Source: Play around here a bit: https://elementsofheating.wordpress.com/2012/09/26/how-to-calculate-the-kw-required-to-heat-a-volume-of-water-in-a-particular-time/

If you want to go from 15 degrees (inflow temperature) to 65 (in the tank), that is a 50 degree difference. Hence 300 litres to be heated up by 50 degrees takes 300 (litres) x 50 (degrees) x 0.001163 kWh (per degrees per litre) = 17.44 kWh. Even of you don't want to do all the math, just take this number. It takes 17.44 kWh to heat your 300 litre water tank.

Your typical electric hot water system can run on a 10A circuit, or say 2400Watt (2.4kW). It will take that system 17.44 (kWh) / 2.4 (kW) = 7.26 hours to heat up that 300 llitre tank. And it can hence produce 300 (litres) / 7.26 (hours) = 41.32 Litres of how water per hour.

What does that mean cost wise? Well, if you use a typical old fashioned electric water heater, it will take 17.44 kWh to heat up the 300 litre tank, at current prices and if you don't run the heater on a timer late at night, that will cost you 17.44 x 0.24 ($/kWh) = $4.18 in electricity. (Way more if you run it in peak time, half of that if you run it off peak, but it depends whether you are on a time of use bill or not). If you do that every day, that is about $377 per quarter.

Plus all the losses incurred, so your bill will be way more than just this theoretical value.

Now let's use some smarts and some engineering and move away from "making" the heat from electricity (as per above) and use a heat pump. A heat pump does not turn electric energy into heat energy, it rather just moves it from outside of your tank to the inside. Since the natural heat flow would be from the warmer (inside of the lank) to the colder (outside of the tank), it takes energy to turn this flow around (reverse it). The good thing about it is, that it takes way (!) less energy to do that than to turn electricity into heat.

How much more efficient? Well, that depends on the system you buy and other environmental parameters, mainly the humidity of the air surrounding the heat pump (the pump is trying to take away heat from the air, remember) and also of course the temperature of the air. The warmer the air, the less energy is needed to get it into the tank.

How good a heat pump is at doing this reversing of the natural flow (of heat) is given by the Coefficient of Performance (or COP). They vary from system to system and also vary with humidity and temperature (I am sure if we go through the manual for the Stiebel, we will find those charts showing the COPs). But the COP for water heat pumps is roughly somewhere between 3 and 4 - ish. So let's make the an average COP of 3.5.

In layman's terms that means that if you give a heat pump 1kWh of electric energy, it can heat up between 3.5 times as much water as through a conventional electric to heat conversion heater.

Back up to the calculations and in short: it will take 3.5 times less energy (cost) to heat up your 300 litres by 50 degrees (plus, you can easily run them on a timer and never allow them to run when electricity is expensive - which makes a huge difference). Nevertheless, the Stiebel unit to my best knowledge when running uses around 0.7kW (compared to the 2.4kW for a typical non-heat pump electric water system, which is, who would have thought, 3.4 times less). So, if I understand all this correctly, the heat pump uses about 3.5 times less power when running but can heat up about 3.5 times more water using that power through smart engineering - and that results on about the same time it takes for the heat pump to heat the 300 litres (or roughly the same amount of hot water it can produce per hour).

Boy, that was way longer than I anticipated, but I hope this makes sense.

Yes, but the Return cannot be fed into ANY heat pump. A smaller recovery heater will need to be used for reheating. if is is a single dwelling, it is likely that a single plugin heater could be used. For larger commercial applications with a bank of Hotty's, you may need a 3.6kw quick recovery units. - Ultimately you will still have large savings over electric storage.