How Do I Know When the Hot Water Has Run Out?

GrahamF · 2025-08-29T08:15:03Z

This is an apparently simple question: how do I know when the hot water has run out? The hot water thermostat is measuring a temperature about 300l below the top of the tank, so it reports colder than what comes out of the top. Of course, I could run the hot tap and see how hot it gets, but that is a rather crude measure and wastes water. Below is some background. Our Grant Aerona 290 15.5kW heat pump was installed last week. We have 2x200l cylinders plumbed in series. Cold water flows into the bottom of the first tank, hotter water comes out of the top and goes into the bottom of the second tank. Hot water for the house comes out of the top of the second tank. Both tanks are heated by a single plate heat exchanger, which works well. We also have a hot water loop with circulation pump that comes on at meal times for washing up. Almost all of the time, only two of us live in the house, so 400l should probably last us 2-3 days. We are on the Intelligent Octopus Go tariff of about 7p per kWh overnight and 29p during the day. In order to minimise the cost and avoid cycling the heat pump, I have set the water to run for an hour each night, plus a weekly Legionella cycle a couple of hours later. The hot water thermostat is located at the bottom of the first tank. I assume the water stratifies, with the hottest water at the top and the coldest water at the bottom, although the plate heat exchange and hot water loop must interfere with this to some extent. The reported temperature gradually declines from about 47C after heating to about 35C by the following evening. At tea time, it briefly dropped to 30C, but recovered to 35C without the heating being turned on. I was concerned that the water might be too cold, but my wife and I both had showers and the water was nice and hot. Of course, it came out of the top of the second tank, which must have been considerably hotter than 35C. Any suggestions on how to track or predict the output temperature?

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Domestic Hot Water & Cylinders

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GrahamF

(@grahamf)

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Topic starter 30/08/2025 7:38 pm

@transparent that is a very interesting suggestion, thanks.

Grant Aerona 290 15.5kW, Grant Smart Controller, 2 x 200l cylinders, hot water plate heat exchanger, Single zone open loop system with TRVs for bedrooms & one sunny living room, Weather compensation with set back by room thermostat based load compensation

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