OpenEnergyMonitor Community

Best ASHP+solar thermal cylinder for monitoring DHW usage/volume


I am in the process of removing my LPG tank and installing an ASHP in my house (the whole process feels like opening a can of worms).

I currently have 2 solar thermal panels which I am planning to keep as they provide all hot water demand for 5-6 months of the year.

I would like to monitor hot water demand and, most importantly, estimate how much hot water there is in the tank.

I really like the idea of the Mixergy tanks and all the data I may be able to get from the cylinder. However, they are significantly more expensive and, for ASHP + solar thermal versions, I cannot heat by volume (I’ve been told by Mixergy).

I am thinking of having a heat meter on the cold water inlet and temperature sensors on the cold water inlet and hot water outlet (which I believe would give me data to implement similar functionality to the Mixergy tanks).

Would people suggest going with Mixergy as I’d have access to all the data I’d possibly need, or could I achieve the same level of monitoring and control using another manufacturer/model?

I appreciate there is a large number of variables that play a role in selecting DHW cylinders, but since this forum is about energy monitoring, I’m focussing on that for now.


you could just use 3 temperature sensor ( top middle bottom) on any water tank to estimate the temperature gradient with in the tank and then just calculate the amount of stored energy within it

for me I have solar water heating as well and GSHP for household heating and hotwater production as well and I just use ordinary hotwater tanks . I found it more efficient to to use instant water heater ( 4.5 kw ) to just top up when I actually need the hot water instead of the heat pump pulsing every 1 to 1-1/2hrs to maintain it at temp 24/7 . and in the winter time when the heat pump runs more often anyways the instant water heater barely has to run. it only use alot when very cloudy for extend period generally fall and spring

Thanks @stephen, it’s good to hear how people handle solar thermal and heat pumps working together and how water heating is scheduled - great insight!

When you suggested using 3 temperature sensors, how would you go about attaching the sensors to the tank? Should I look for one with enough sensor pockets (if that’s an option at all)? Not sure I’d be able to easily tamper with the insulation of a tank (I’m a newbie on this topic).

Some manufacturers of ASHPs will promote their own cylinders, or point you towards a cylinder design they prefer. Your ASHP installer should double check which ever cylinder is being chosen, has a suitable square meter surface area coil to match the heat pump requirements and suitable coil for the solar thermal connection.

Monitoring, if the cylinder has multiple temperature pockets top, middle and bottom, that’s ideal to keep an eye on it. Some cylinders may have spare spaces where additional immersions can go, so it might be possible to get a pocket on a blank plate made up rather if more sensor locations are needed. I would not go gouging out insulation to place a probe against the cylinder body as that will likely void cylinder manufacturer warranty as well as cause extra thermal losses.

I would guess right now, you already have a solar thermal controller with one temperature probe on the roof panels and one, maybe two temperature probes for the cylinder? If your existing solar controller has 2 tank probes, you’ll want to check the replacement cylinder has positions to place those probes. Also remember that most modern ASHPs will use a temperature probe of their own to judge the tank temperature, so check where that will go also.

I know of installations with twin coil cylinders, for HP + solar and to keep things simple, the users put a timer block on the heat pump from running during the day for DHW, then only allow heating in the evening if there was not sufficient hot water provided by the solar thermal. It also depends to a certain extent when that hot water is going to be used.

does your current tank have 3/4" top sensor pocket or spare port on the top of the tank you can make it 3 in one sensor pocket using 3 acr copper tubing in varying length for the top middle and bottom sensor or if it has sensor pockets already then you can use it for the ones that are missing

@stephen I will need a new cylinder and that is what I am trying to spec right now.

You suggestion makes me think I should make sure there are enough sensor pockets for the solar thermal + ASHP + the monitoring I’m after.

in Canada we general do not use specialized cylinders so much (but i guess it getting more common now) - most places just use normal hotwater tank and attach as side arm . some are passive and other are not there a version that you take out the bottom heating element and you attach a flatplate exchanger - but that one uses a small pump(20w) to drive the circulation ( i’ve used those but they are a little expensive to do but you can uses on any tank you want) . I myself I build hotwater tank inserts they work much better they generally not that expensive to build ~$50 and a couple hours of time and are passive exchange

While i don’t have solar DHW yet, I spend some time looking for a suitable tank for similar reasons.

In case helpful, the Tempest Heat Pump Solar tank provides a number of pockets. which i believe would take multiple sensors in each. The Gledhill Stainless Heat Pump Solar ranked looked similar.

Tempest Heat Pump Solar Data Sheet.pdf (91.6 KB)