I’m having a new system installed from scratch on eco4. I’m able to specify things to an extent. I’m still waiting for them to get back to me with a defined spec, but in the meantime I thought I’d be proactive and look at my options.
So, in my mind the heatgeek cylinder seems to be the best option as it has been designed by them specifically for heat pumps whilst taking measure to address the flaws with other cylinders. I especially like that it has a longer coil than other cylinders. To me this means a much quicker reheat time.
Mixergy also gets a lot of praise, but i’ve seen one or two negative reviews.
I don’t know of any other cylinders which compete on the same level as the two above. Is the heatgeek cylinder the best of the bunch, or would you choose something else?
The Mixergy heats very small volumes of water, because the secondary cylinder inlet & outlet to/from the PHEX are right in the bottom of the cylinder?, if a very small vol of HW, is drawn off then its just heating this very small vol of water starting with a very low HP discharge temperature which slowly increases as that small vol of water heats up but the temperature of the HW above it is unafected until the lower vol exceeds the temperature above it, The coil method heats from the top down. so irrespective of the coil size then the coil inlet, (HP outlet), temperature must allways be kept at say 48C or so otherwise the remaining hot water in the cylinder will start cooling down. I suppose the trick is to use up as much of the available HW as possible before reheating the HW cylinder (coil), you can then start off with a lower HP outlet temperature, gradually increasing as the cylinder heats up.
For the same cost you can get a standard tank that will hold a day’s hot water so reheat on cheap rate with the coil totally covered in cool water at the start of the reheat.
(Small heat pump tanks do have the issue if coil coming 2/3 of the way up the tank.)
I’m surprised no one has developed a small inline electric heater - a bit like an electric shower, that can boost warm water to a ‘normal’ DHW temperature. End of the day, tank depleted, but you need a shower’s worth of HW. It has to be more efficient than running a DHW cycle (and quicker too).
My solar (2xFlat Plates) HW cylinder has 3 very closely wound coils, in parallel, with the bottom coil sittinging in the bottom of the cylinder so is heating the coldest water first, its 1.2m2,
I have a Newark heat pump cylinder 150L with 3m2 coil (twin coil) and Im very happy with it: Newark Cylinders - Vented Heat Pump Cylinder (I get SPF 3.8 from hot water over the year, I do only heat to 42-45C most of the time, but I do also have a 2013 Mitsi EcoDan so would expect a little better from a newer heat pump).
I believe Newark make the HeatGeek cylinders. I’m not totally convinced by the longer 6m2 coils. I think it makes more sense for the hot water cycle to only be activated when the stratification layer rises above the top of the coil and for there to be still half a tank left of hot water above the coil with the coil itself is sitting the cold layer (this gives plenty of time before the hot water runs out and provides a super efficient from cold hot water heat up cycle). That said @glyn.hudson is getting better results than I’m getting with his HeatGeek cylinder with the 6m2 coil - though he does have a new Grant R290 and seems to be heating to lower maximum flow temps than I am (COP 4.4 vs 4.0 in DHW in September)
I have a Mixergy and find the real benefit is the level of data it gives you about the amount of hot water you have left meaning the family can easily gauge if there is enough hot water for a shower etc without having to ask who has used what hot water today. This means we avoid unnecessary hot water boosts as the most efficient water heating is only heating the water you actually use
At the risk of irritating some folks, the idea of running out of hot water - for whatever use but presumably showers - is completely unacceptable. With three children, the grief ensuing from a cold shower is not imaginable.
The energy required is a small fraction of the household heat requirement and an unnecessary distraction from the larger picture.
Apart from convenience, presume there must be some (mixergy) energy saving in heating say oneX150L than 3X50L batches of water from cold, the charge level is a nice bit of info, wonder how it’s monitored/calculated.
Hot water was 32% of our total heat output from the heat pump in both 2025 and 2024. Hardly a ‘small fraction’ to us.
I personally love our Mixergy cylinder.
Not having to mess about with hysteresis and us knowing exactly how much water is left are key features for us.
Also, if you have a 250L Mixergy, you get all 250L usable at your target temperature. This is certainly advantage over a conventional cylinder where mixing comes into play and you won’t get all the 250L usable.
Agreed. We’ve got one and it’s great to be able to access it remotely to check levels, or to quickly see (via the LEDS on the side) how full it is if you open your airing cupboard or plant room door, but I think even Heat Geeks might suggest the cost v benefits are not enormous. With hindsight, I’d get a “normal“ tank, or one with longer coils.
I’ve recently had a Zip InLine CEXU instantaneous electric water heater installed in a workshop building where it doesn’t make sense to pre-heat and store hot water. I’m only ever feeding that with cold but the manual makes it clear it’s happy with a variable inlet water temperature anything up to 70C. Basically you tell it what temperature you want on the outlet and it works as a ‘top-up’ heater to the extent required to deliver that. It will restrict the flow if 8.8kW isn’t enough power to achieve the target temperature at full flow.
So such devices are available - and there are even more powerful models in the Zip InLine range (but those need 3-Phase electrical connections). I suppose if a household regularly runs out of hot water and it’s not practical to install a larger cylinder one of these heaters could protect against the cold shower scenario but a heatpump would surely always win from a running-cost standpoint.
Most electric showers explicitly state you must not do this. It is likely the water isn’t cold enough for the electric shower to modulate low enough.
I had seen these previously but I’m sure they used to not like really hot water on the inlet.
Perhaps, there are lots of variables and you could have a smaller tank if this was available as assurance. Depending on the circumstances, it might be the right tool and the cheapest one overall.
I believe i’ll have to pay the difference for any extras. I doubt a mixergy or super cylinder will be covered. Which is why i’m trying to get an idea of which is the better option. Nobody here seems to be discussing the super cylinder, in fact they’ve gone completely off topic.
So far i’m leaning towards the super cylinder has it has a long coil and should reheat quicker than the others. I don’t see what benefit a mixergy offers other than being able to see the level?
