This isn’t directly related to EmonCMS but I would be interested in your thoughts.
We have a Mixergy hot water cylinder, which we used to have connected to a gas boiler and last year we switched to a 7kW Vaillant aroTHERM plus Heat Pump.
In order to optimise the heat pump efficiency, we currently have our target temperature set to 48°C. We went for a 180 Litre Unvented Cylinder - as recommended to us by Mixergy - but a couple of heating engineers have told us that it is not big enough for a family of four. Very frustrating when a bigger tank would have cost only little more. As a result we run out of water quite quickly and typically do 1 or 2 additional re-heats during the day.
However when the Mixergy tank decides to run a sterilisation cycle (60°C), we have been very surprised by how much longer the water in the tank lasts. This makes sense, as we’re storing more thermal energy at the higher temperature - but it got me thinking…
We’re currently on Octopus Go, where the night rate is 8.5p/kWh and the day rate is 28.79p/kWh - more than three times higher.
So, I’m wondering:
Would it make sense to heat the tank to a higher temperature overnight (say, 60°C), and then top up during the day only if needed at a lower temperature (e.g. 48°C)?
Mixergy don’t currently support this but I think I could schedule it myself using the API.
Do any other hot water systems support different target temperatures at different times of day? Has anyone tried a similar approach?
Hi Yes this is exactly what I do. I have the 210L Mixergy tank and use node red to drive the API. As I am on intelligent Octopus go I also reheat if car is charging and tank is low. Again using the API I have also implemented a guest mode which again targets a higher temperature if we have extra people in the house to ensure we don’t run out of hot water
Fixed temperature on the Heatpump and let the Mixergy take care of it. To be honest do not really target SCOP as I feel it is a false measure. The priority is comfort (hot water when needed) and second is cost which is the true measure. A COP of 3 when electricity is 7p is a lot less than a COP of 5 when it costs 25p
Surely the issue is having an appropriate amount of hot water when you need it? Saving pence and not achieving that wouldn’t go down with either my missus or children!
I have a different system but with a similar idea and it has worked well for ten years. We have a thermal store and it’s fitted with two immersions: one at the base and one half-way up. I connected the bottom one to my PV panels via a diverter, and the top one is connected to the mains via a timeswitch. The thermostat on the bottom one is set to a higher value than that on the middle one. So in summer the PV heats a full tank (250 L) to a high temperature, while in winter and at other times when there isn’t enough sun we use imported mains to heat half the tank to a lower temperature. We’re on an E7 tariff so do that overnight.
Not quite sure how this works in reality. Most shower water exits the shower head between 38-42 degrees. So any system where the water is heated and stored always mixes down the heated water with mains cold to achieve the desired output temperature. So I thought the idea of heating the stored water to say 46 c meant less mixing down at out put.so your tank of water in theory should serve your needs at output.
I would think that heating to 60 c just means mixing down more at output. Additionally in a closed system there is surely premixing as cold enters the tank to replace hot output.
So by heating higher are you stretching the water out by adding more cold at output
Yes, that’s right. If the water in the tank is hotter then it gets mixed with more cold water when it is used, and consequently you use less of the hot water. Effectively it increases the size of your storage tank. Losses are higher too of course, so it pays to insulate the tank well.
Ours is one of those fully insulated tanks 250 l , doesn’t lose much. We are on Goodenergy cosy tariff so two 10p/ kWh periods a day. 0500-0900 and 1300-1600, so hot water is not usually a problem.
Do you use the thermal store to store the hot water you actually use at the taps. Or store hot water which is then used to heat up your mains cold water via a heat exchanger?
A thermal store is a bucket of water that never changes. Cold fresh water from the mains is passed through it, either via a coil in the tank or in an external plate heat exchanger, like ours. A tank where hot water is stored until used is called a cylinder rather than a thermal store. One difference is the lack of legionella requirements, and another is mains-pressure hot water without a requirement to have a pressurised tank (with resulting inspections required).