See @zapaman’s post above.
When used only to soak up spare PV at possibly low amounts it is suitable as a pre-heater and if it should reach usable temperature then the boiler shouldn’t ignite. In both cases saving gas and thereby gas emissions.
The Sunamp can also be charged during off peak to provide (a contribution to) hot water and/or heating and a further reduction in gas use.
There is no sensible reason why you’d use one for a gas boiler. With PV and an air sourced heat pump, I’m much more sympathetic.
The problem with combi boilers is that they cannot provide the power output or responsiveness of a direct hot water cylinder.
The limited power output requires using only one shower at a time, which needs a set of valves and a controller to manage which one gets priority etc. Or to be grossly oversized for 99% of typical loads.
Gas boilers also need to operate at two different temperatures, one for domestic hot water, one for central heating, to be maximally efficient.
Combi boilers struggle to do this well. They end up being less efficient and much higher maintenance as a result.
Sunamp uses paraffin to store heat, which has a phase change at low temperatures to provide a higher volumetric storage capacity than water (about double) at a low temperature delta.
The problem with all paraffin based PCMs is that they’re very bad at heat exchange due to the low thermal conductivity of the storage medium. Which is exactly what is required with a combi boiler. So it just does not make sense.
Water on the other hand, is fantastically well suited. Just put in a 150L water tank and convert your heating system to an S plan with hot water priority. Some boilers are terrible and won’t allow setting separate flow rates and temperatures to deliver the highest efficiencies. But we can’t keep using them anyway, so there’s little point going into how best to use gas.
Our Sunamp iPV (with PV obviously) as pre-heat worked superbly when we had the Combi. The Sunamp was handling 95% of our hot water from April to Sept months and about 5% during the least sunny months of Dec to Jan. Our Sunamp iPV has a mixer valve on the output limiting the flow temp to 45C; still plenty hot enough for all our HW uses.
During the very sunniest weeks of the year we simply turned off the combi bolier completely and let the Sunamp handle 100% of the DHW duties.
The flow rate from the Sunamp is at mains water pressure that there is no loss of pressure when someone is using hot water tap at the same time as someone else having a shower.
NB I am not referring to using the Sunamp to store heat FROM a gas boiler, but rather the opposite to avoid using gas altogether (or as much as possible).
Cylinders are cheaper and it you don’t need the space saving feature of the Sunamp then more often than not a cylinder will be the natural choice.
Sorry. I think I’ve misunderstood. I’m assuming it’s only for a combi with no PV.
Even then, I would argue that they’re going to play a small role. But it’s a much more interesting debate than using them to replace standard DHW cylinders!
Here are the Sunamp stats since installing our ASHP. I could do a similar set of stats going back to when we had the combi but it would show a similar picture with the Sunamp ASHP P50 being replaced by the Combi. The summary would be that the iPV feature is/was providing ~42% of our our total DHW direct from the solar PV.
NB all of these figures are in kWh-thermal, the COP on the ASHP during the hot water cycle is ~2.0 so the elec input on the Sunamp ASHP P50 device is ~0.5x times the kWh-thermal quote here.
See this link / thread for more info on the ASHP performance more generally. Understanding heat pumps! - #35 by zapaman
Sunamp certainly can be used as an alternative to a standard cylinder in many circumstance.
But I am not suggesting here that a Sunamp would be used to swap out an existing cylinder that was already working and was well insulated.
You would, however ,use a Sunamp if you needed its particular set of features and benefits: small footprint and low heat loss etc.
Cost/kwh of paraffin based TES is too high. It’s rare the volume is so valuable that the better volumetric density compared to sensible heat storage in water is worth it. You’d have to go into high density urban environments to get real estate that is that expensive, but those are where the system would be inadequate to begin with.
I had hoped that salt based TES would be more practical, but they have a bunch of other issues associated with them. I’ll spare you the details.
Fortunately, we have ice when it comes to AC.
Unfortunately a lot of modern homes are designed on the assumption that there will be instant hot water available, whether by Combi boiler or other means. This means that even if space is available for a water cylinder and expansion vessel the internal layout would need redesigning to fit them. This is particularly so for the rabbit hutches currently sold as “trendy apartments” or similar. In such properties some reduced volume solution is an obvious solution, if not the only one. Sunamp and similar products are mainly aimed at that market.
Clearly there are other applications for the technology for those with the initiative to utilise them.
For those curious about what this actually looks like here are a couple of photos showing the Sunamp iPV in our kitchen cupboard. This is the smallest Sunamp size and provides similar hot water storage to a notional 70L cylinder. NB we have a 2nd unit which stores nominally the same as an 80-90L cylinder as well giving us a total of ~150L equivalent cylinder size. This is quite a modest amount of hot water storage but the re-heat time for the ASHP connected Sunamp is ~25mins so we generally have ample quantities of HW at all times (for 3 people). Oh and I also have access to the temperature data inside the Sunamp units thanks to help from @ben at Carbon Coop (see last image for details).
You can clearly see why those without a cylinder, and no practical space for one, could opt for a Sunamp.
I have a cylinder with solar thermal coil but am still interested in getting one for a distant hot water supply instead of “miles” of pipe through the length and height of the house.
I see that the illustrated unit is larger than revealed at first sight. It is sitting on the floor and protruding into the kitchen unit rather than sitting on the bottom shelf. Much as I would like a thinner wider Sunamp, such as might fit entirely under the kitchen units, I understand the practical restrictions on shape.
Yes our only option if we wanted a cylinder would have been to locate it in the loft (which is already converted into a room apart from some crawl spaces in the eaves). However, doing all the plumbing associated with running pipes from the ASHP (outside at the back of the house) up 2 floors into the loft would have been costly and disruptive. The additional cost of the Sunamp set up was offset by these practical considerations.
Re-igniting (!) this conversation… I’m still wondering why teh Sunamp can’t supply all the hot water needs for a house.
We have an ASHP that doesn’t do hot water 
so we still ahve a combi boiler for that. I’d like to replace the combi (and therefore gas) completely; I don’t easily have anywhere to put a hot water cylinder and like the claim that the Sunamp only loses about 0.5kw/h per day.
Can someone persuade me for or against?
Meanwhile, when I got a quite the electrician wanted £1500 to wire in a spur via an Eddi to run the Sunamp. From what I can see it is just a 13amp feed, and there was one of these already adjacent to teh target Sunamp location - why would I need an extra feed?
My caveat is that I have no experience of Sunamp but have read numerous posts on the subject.
That depends on your hot water requirements. If you need a mega sized one consider 2 smaller ones located nearer to their outlets.
If it only needs a 13 amp feed the existing cable might be ok but if it will be drawing full power over extended periods you would be better off with more robust cabling.
If you want to stick with the existing cable it just needs diverting via the Eddi.
Fining some to install the Sunamp who understand how it works will be your biggest challenge.
As for the Eddi, it may be that they misunderstood that it has 2 outputs (so can connect to a tank with two seperate immersions heaters), it sounds like you only need one connected?
I’d only be rehashing my comments above, but in summary:
Problem with sunamp is that it uses paraffin, which does not conduct heat readily and requires a large heat sink for a given amount of power, so it’s an expensive form of storage /kwh, with limited output. I also deeply suspect their fossil fuel funding.
The storage only gains value if you can load shift when you use the electricity, so you will need a variable tariff or economy 7. In which case it will save operational costs compared to producing DHW using ohmic/resistive heat on demand.
However, on demand heat requires powerful electric showers (40A!), and there’s often load limits if you have multiple electric showers, which can be managed with a controller only allowing one on at a time. It’s expensive too, so there’s that to consider.
Other options are to use a water cylinder with a dedicated air source high temperature heat pump. They can draw heat from inside, outside, or the exhaust on your ventilation system, if you have one. The water cylinder is there to provide peak power, and doesn’t need to be large. This would be my choice.
I’m not sure what you would like to be persuaded “against”?
A suitably sized Sunamp unit (eg the Thermino 210 size perhaps) can easily supply all your hot water. This is 87cm tall with a footprint of 57.5 x 36.5cm. Some space around this will be needed for pipework connections, but all in all it works out very compact. NB they are heavy so lugging the unit upstairs will take some logistics (a stair climbing system can be used).
You will of course need to consider where the electricity is coming from (primarily to assess running costs). So if you have solar PV you could get a lot of free electricity from that source.
Also if you on a tariff like Octopus Go then you could top up the Sunamp using over-night cheap electricity, especially during the winter months. I hop that helps.
