I am looking for some recommendations on indirect unvented 300l cylinders for a Vaillant Arotherm Plus 7kw???
Ideally a good performance with a sensible price tag.
I have looked at the Newark HG cylinders but they don’t seem to be financially viable when you do the math.
I have also been looking at Vaillants unistor 300l cylinder which might be an option but would really like to have some input on optimising the DHW side of things with a solution that would provide a viable return based on the investment.
We went with a Newark 300l standard ASHP cylinder which has a 3m2 corrugated coil. It was a custom built one as we wanted two immersions for a solar diverter as well as several sensor pockets.
We have had an issue with it though - this is from what is called dynamic resonance. It can sound like a jet engine. I had an email conversation with Newark about this and of course their attitude was that our plumber specced it and therefore it was our issue not theirs.
There is apparently a way round this which is to add a bypass valve in parallel with the coil to divert some of the flow - we’ve not done this yet as it means draining down the system.
The issue isn’t a real problem for us, as with our build, we only need to run the ASHP for heating for a short period every day, so we only use the ASHP for heating on the Octopus Go cheap night rate and the DHW immersions run at the same time on a schedule on the diverter. So we never use the ASHP on both heating and DHW. If we did have it on DHW as well, then that would take priority which is not how we want it to function.
Long story short, there are quite a lot of ASHP manufacturers that recommend straight rather than corrugated coils - and it’s probably for the dynamic resonance reason. So maybe look round for a simple cylinder with a straight coil about 3m2 and you should be OK.
Also, if you do in the end go for Newark, they may manufacture it with 2 coils - our UFH cylinder has 2. If this is the case, then they should be connected in parallel to the ASHP feed and return - this is not documented anywhere in the info you get with the cylinder. This came out in the emails with them about the noises from the tank.
PS - there’s been some discussion on other forums about ASHP guarantees and only using the cylinders that are recommended, so check the small print.
PPS, our 9kW Cool Energy ASHP is set to 35C for the UFH, it never reaches this, so runs continuously for the full 4 hour cheap Octopus Go window. That’s with a 200l buffer tank and the ASHP connected to the 3m2 coil. The reason it never reaches the setpoint is of course that the UFH is drawing a similar amount of heat from the tank as it heats the slab.
Your point about DHW only being about 10% is very valid and one of the reasons for me asking about other cylinder’s manufacturers options as the Newark HG cylinder just doesn’t make financial sense.
I feel it’s over priced in terms of the value it provides and would love to hear about some other more viable options.
The other option is to reduce hot water usage with WWHR and then you may be able to get away with a smaller cylinder (more space in the airing cupboard) and the COP will be even less relevant as it is an even smaller percentage of your total usage.
I am debating getting both WWHR and the 180l heat geek tank but it is not likely to get the all important wifey approval points until Octopus finish the replacement of the radiators and the reinstall is completed.
While it will cost more and it may never pay itself back just being able to run as green and cheap as possible would be enough for me (and it would keep my stats looking good .
regarding flow rate, I have a 16kw heat pump. with 26lpm I can load the cylinder at ~10kw. If I could get higher flow rate through it I could potentially load it faster. But 10kw is enough, its just an optimisation not a fundamental problem.
Another thing to consider is sensor pocket locations. The ones on our tank are abysmal for triggering reheats. One right at the bottom in the coil that gets cold as soon as there is any hot water use and one very near the top that when it goes cold is too late to reheat without the shower going cold.
I’ve added a couple of sensor pockets myself by (carefully) drilling through the outer skin of the cylinder and creating a hole in the insulation to push my temperature probe into. My Ecodan is set up with 2 sensors and it still uses the one in the proper sensor pocket in the coil to know it is up to temperature.
Obviously much less of a consideration if your tank is large enough to only be heated on a schedule.
Consider how quickly you need the reheat, as many heatpump have an eco dwh setting that that forces the heatpump to near lowest output so gives good (but slow) results without needing a very large coil