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The Economics of our SunAmps


TerryE

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We use 2 × SunAmp PVs for our HW system in a household of 3 people.

  • According to our water bills, our consumption is about 83 ltr per person per day. Our pattern of use is pretty even across the year: more showers in the summer; an occasional shared bath in the winter.
     
  • The year round average temperature of our rising main is 11.3 °C (Oh, the wonders of logging everything in a DB and knowing how to do SQL subqueries).
     
  • The H/W manifold is mixed to 53°C (perhaps a little too hot for kiddies but we are an adult household).
     
  • I estimate that ~40% of our water is run as hot. (The washing machine and dishwasher, bogs, etc. are cold fill.)
     
  • Cranking these number into a heat calculator, this gives a total heating requirement of just under 5 kWh / day + another 1 kWh / day heat loss as the SunAmps are tight side-by-side and amazingly insulated.  (I don't separately meter the SunAmps, but a quick sanity check of my actual half-hourly electricity meter readings would indicate this figure is about 20% too high, but let us stick with this figure for estimating purposes. 
     
  • All heating is done at cheap rate tariff ( fixed at 9.66 p / kWh inc VAT) so this costs us ~ £211 p.a.
     
  • Using an ASHP to supply the SunAmps at 40 °C, say,  would drop this to 2.5 + 1 kWh saving us less than £100 p.a. or about £1K over 10 years.

So in our case if  we decide to install an ASHP, there aren't enough savings to make it worth installing an extra pump,  a buffer tank and a two temperature ASHP to use it to (part) heat the DHW. 

 

We will stick to Keep It Simple Stupid.

 

A couple of caveats here: 

  • I think our pattern of water use would be very different with children in the household.
  • We have a fixed price deal until end 2022.  We are going to see a big hike in our next tariff, but I feel that this will settle down in the longer term, so I am ignoring this for now.
  • Thanks 4

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Have you done the same calculation using an ordinary E7 hot water cylinder?

May find that the lower initial cash outlay easily offsets the thermal losses (which do not have to be high in an insulated cupboard).

 

Excel does it for me.

 

image.thumb.png.878efff4d6a7e93fcfd14e0696153814.png

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Isn't the economic calculation you should be doing to work out if a sun amp is viable, is compare the running cost of a resistance heated sun amp, with a resistance heated unvented hot water tank.  Work out how much cheaper the sun amp is (due to lower heat losses) and work out the payback time of the extra cost of sun amp vs UVC

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Dragons everywhere in this conundrum. Although the immediate financials are compelling I guess we (the collective planetary population) need to see this as a full cost exercise across the whole systems lifecycle - including recycling / re-circling.

 

@SteamyTea can you plot draw off from the tank on the chart so as to see how that affects the temperatures - or is the above just a no draw off day! 

 

I also guess we need to see the whole dwelling energy demand in the round. We know that @TerryEuses good old Willis heaters for his UFH, we will probably also go the Sunamp-Willis way at the outset but have put the pipework and other infrastructure in for an ASHP in as well, and that an ASHP might well be able to do the UFH and the DHW so would pay back in both areas, while also costing in both. I naturally recall your work on looking at Willis vs ASHP. I also feel that any PV must also be brought into the equation and, looking forward - but not much, the massive amount of electric storage we will have in our cars. I appreciate that governments, the grid people and the generators are looking longingly at these as buffers for the grid but they would also work as buffers for us  - assuming we are allowed to use them as such. Imagine being able to charge your car at 9.66p (as you say other higher rates can confidently and imminently be expected) and then put it back into the house during the normal rate time even allowing for the losses both ways.

 

A copper tank, excluding its insulation - which if not sprayed onto the tank can house the next cylinder, is almost fully recyclable at very low environmental cost. Is this the same / different / better / worse for the Sunamp, the ASHP, the UVC and their associated sub assemblies I wonder?

 

Systems thinking 101 tells us that everything is connected to everything else and also, among many other things, that investment challenges change. I think that means that this whole field is best by causal and influence like connections which increase complexity, and hence optionality / decision risk, but also that self builders tend to be in a position to invest for the long term and are not sitting on a raft of investment others made previously as far as the house is concerned anyway.

 

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44 minutes ago, MikeSharp01 said:

@SteamyTea can you plot draw off from the tank on the chart so as to see how that affects the temperatures - or is the above just a no draw off day! 

I can probably do more than that as  have been away quite a bit, so can see when the tank is on and off, no draw days which are also no heat input days), normal days, and my general usage.

The chart above is all usage between the stated dates, usually had a bath by 10 AM when I am there.

Shall see if I can have a play with the data later, got niece and her children visiting (actually visiting my Mother) soon, so that is my day buggered.

Edited by SteamyTea
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@SteamyTea Nick, if you bring the tank to temp say up to midnight, turn off electricity and don't draw any HW until the morning,  then you could use the sort of curves above to estimate the daily heat loss and do the sums and trade-offs on insulation options.  I suspect that that its doing to be a few kWh of heating being dumped into the general internal heating.

 

I haven't done this calc for a conventional cylinder because I don't have one, but I do know that the one in our old house lost a lot of heat -- we used it as a drying / airing cupboard because of this.  The choice of SunAmp for us was one of those informed coin tosses that you have to make when finalising the house design. It's also one that I think experience has shown to be a good one for us. Why?

  • Form factor. The 2 units are the size of a large PC / small server and are cuboid rather than cylindrical.  We use a small cupboard off our downstairs toilet for all of our services: DHW, CH and water filter.  This form factor means that we could stand them on a shelf against the walls, and so be very space efficient.
  • Low heat losses. The thermal density of PCM means that the units have a small external area, and the flat surfaces means allows for the use of vacuum insulated panels which have an R-value maybe 5-7 × greater then equivalent foam insulation.  This means that the service area doesn't overheat, and we can do all of our heating overnight.
  • Redundancy. Controller failures (see below) have meant that we've been on one SunAmp a couple times, but one SunAmp still gives enough HW even if I needed a midday heat top-up -- a lot better than no HW.
  • No CITB training requirement / G3 certification needed for installation.  I installed and plumbed in the SunAmps myself. (The electrician wired them in and his Part P certificate covered this.)  My BCO did as for the G3 certificated, but accepted the manufacturers statement that there was only ~1 litre internal H/W at 3 bar pressure, there wasn't a safety issue as with an UVC.  Being able to do all the plumbing ourselves saved us a lot more than the price premium for the SunAmps.

The general build quality of the SunAmpPV units is excellent, though I notice that some members have reported quality issues with the next generation Uniq  units.  I also personally view the controller board design as overly complicated and poorly implemented (used in both types) .  For example there is no proper separation of the 5V logic and the 240V power and the 240V tracking is totally inadequate.

 

Also, even though the PCM-base units are more thermal dense and hence kWh for kWr they are a lot lighter than the equivalent full water cylinder; however unlike a conventional cylinder, their empty weight is still basically the same as their full weight so they aren't easily movable for maintenance. For this reason, I view this  "all in 1 box" approach as unwise because of long-term maintenance issues, though to be fair I can see why this is a better option for the vendor and for most customers and installers.  It's a pity that you can't just get a mini dumb thermal store in this cuboid + PCM form-factor.

 

Edited by TerryE
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@MikeSharp01

 

Right, hope this is understandable.

This year, I have always been away on a Sunday and Monday (get back late Monday or more usually a Tuesday), usually travel up on a Saturday, so turn the DHW off, but leave the heating on.

Now as I am on E7, there is a time shift of a few hours, but my E7 is limited to 3 AM to 7AM (to get the most out of LC generation and less standing losses).

So a chart of all energy usage and top of cylinder temperature (water is probably higher temperature as I have only taped a sensor to the output pipe).

 

image.thumb.png.1c86455e1095c6231afbd6e80469beab.png

 

On a Monday I have not been at home, so there has been no E7 recharge (heating is on though). Generally the same on a Tuesday, think I have had only 1 Tuesday at home.  This is when I turn the DHW back on, but it does not recharge till Wednesday morning (3AM to 7AM, or when enough is in the tank).

Wednesday though to Friday I am at home as normal.  I usually leave Saturday morning, really early, so DHW is used first thing, but I turn it of so there s no recharging on a Sunday.

Now you would think that the usage and temperatures would be more in sync, but I suspect that when I return, the DHW is at a lower temperature, and my 4 hour window is not enough to heat it up fully.  So maybe I need to look a bit more closely at what is happening when I first turn the DHW back on i.e. how low has the temperature dropped, how much extra energy have I used.

Off to cook some lunch now.

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38 minutes ago, TerryE said:

if you bring the tank to temp say up to midnight, turn off electricity and don't draw any HW until the morning

Can't really do that, but it is possible to see the temperature loss over the day, bearing in mind that from 10AM to about 9PM (short after work shower, there is a steady loss.

That loss is around 5°C.

Taking the median temperature (only have two probes on it), the losses are less than 1°C.

May seem odd that, but it s probably down to turbulence caused by the bottom element heating, hot water usage and being replaced with cold water base temperature can actually increase by 3°C or so.

So how much energy is actually lost.

4.18 [J.kg-1.K-1] x 200 [kg] x 1 [K] = 836 kJ [0.23 kWh]

Now I am probably at the lower end of energy usage (my water is not that hot as I only need enough for me, though my system can do 4 people i needed)

So call it 0.3 kWh.day-1 losses.

At 15p.kWh-1 (about what E7 is now), that is £16.50 a year.

I replaced my cylinder after 32 years, cost less than £300.

 

Oh, forgot the chart of a days cooling off.

 

image.thumb.png.8965d24e4287a463fb6d500a5b77700b.png

 

 

 

Edited by SteamyTea
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9 minutes ago, SteamyTea said:

That loss is around 5°C.

 

Nick, doesn't this 5°C refer to the top of the tank?  You have a thermal gradient throughout the tank, with the corresponding number at the bottom over 25°C and a tank average some 15°C so surely the losses are ~ 3× what you say.  But still a good number. ?

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1 hour ago, TerryE said:

You have a thermal gradient throughout the tank, with the corresponding number at the bottom over 25°C and a tank average some 15°C so surely the losses are ~ 3× what you say

Not sure, I was doing it in a hurry, but I think the overall temperature is okay to use.  There will be more losses at the top than the bottom, but on average it evens out.

It should be the mean temperature I think.  As the probes are on the outlet pipe at the top (as close to the tank as I can get, and under pipe insulation) and the lower probe is on a bare patch at the base, the median temperature is the same as the mean temperature (technically it should be the geometric median, but treating it as a 2D problem, our CET weather temperatures use the median temperature at 9AM and 9PM I think).

Any water I draw off is from the top half of the cylinder, so the hotter part, that will reduce the overall losses thereafter.

But even of the losses where up to a kWh a day, still pretty low in reality.  Shows that insulating the cupboard around the cylinder helps.

What is interesting to me, is that the cylinder has fully stabilised after 1.5 hours after taking, the the majority of the turbulence being in the lower half of the tank, which isn't a surprise as that is where the element is.

Edited by SteamyTea
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@SteamyTea, Note that I've added an extra SumAmp advantage above.  Unlike a UVC, the SunAmps don't require a G3 certificate, so Jan and I could do the entire plumbing installation ourselves.  This saved us maybe 3× the cost premium of the SunAmps over the UVC alternative, and we also got the installation that we wanted and not one suited for a conventional new build. 

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16 minutes ago, TerryE said:

@SteamyTea, Note that I've added an extra SumAmp advantage above.  Unlike a UVC, the SunAmps don't require a G3 certificate, so Jan and I could do the entire plumbing installation ourselves.  This saved us maybe 3× the cost premium of the SunAmps over the UVC alternative, and we also got the installation that we wanted and not one suited for a conventional new build. 

Just how much do you expect G3 sign off to cost?  I am a little unusual that I know a lot of trades, but one plumber just came and checked my pipework, connected the cold feed and tested it all and signed it off for not very much money at all.

 

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I also have a SunAmp for DHW and Wills heater for Heating (decision influenced by this forum and TerryE in particular). I've a water meter and electrical meter on the SunAmp so these might help you @TerryE 

 

Recently the SunAmp provided 1,647 litres of hot water and used 175 kWh. The hot water meter was only installed in December but have the electrical data for three years now and it's very consistent. I know the incoming water would be slightly colder over the winter but looking at the electrical data it can't impact much.

 

I've a water meter on the incoming mains, grey water, hot water and washing machine and think you could be over estimating the hot water usage. We've two adults and a newborn and use 150 litres per day in total. Differences would be we run the washing machine more , newborn in nappies don't flush the toilet and their bath is like a basin so uses very little hot water but I still think your circa 40% for DHW is very high.

 

 

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@ProDave, what really triggered this was a comment by @Bramco on another blog post where he said that he'd ended up configuring his UFH system as multi-zone to keep his plumber happy.  Another friend did a self build locally and his plumbing was just amazing -  the amount of copper involved and HW cycles, etc.  Again he did it that what because his plumber insisted he need this to have rapid HW.  As @Nickfromwales would point out: use a manifold system on run most of basin hots in 10mm -- the whole installations would have been a lot simpler and more energy efficient.  So I adopted an either / or approach: I either gave the job to a tradesman / sub (e.g. tiling, electrics, PBoarding and plastering) or did it myself (all joinery, plumbing, MVHR, home automation, ...).  In our area when the build taking place there was lots of construction going on, so finding decent tradesmen for small piecework was fraught.

 

@Dudda, I think that you are right: the 40% "back of the fag packet" is probably to high.  I am going to do a reinstall of the control and power switching in the summer to make it easier for my son / son-in-law / 3-rd party to take over maintenance if I pop my clogs.  This will include metered switching of the SunAmps, so I will have a better idea. However this only serves to strengthen my overall conclusion of this topic: the running cost savings gained by using external ASHP, etc., to preheat water going into the SunAmps is far too small to justify the extra installation complexity.

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20 minutes ago, TerryE said:

 

@Dudda, I think that you are right: the 40% "back of the fag packet" is probably to high.  I am going to do a reinstall of the control and power switching in the summer to make it easier for my son / son-in-law / 3-rd party to take over maintenance if I pop my clogs.  This will include metered switching of the SunAmps, so I will have a better idea. However this only serves to strengthen my overall conclusion of this topic: the running cost savings gained by using external ASHP, etc., to preheat water going into the SunAmps is far too small to justify the extra installation complexity.

I agree. In my case for DHW this is the same. Where I might use an ASHP (which I've already fitted the plumbing for) is in heating. I've had a Wills heater for three years now with an electrical meter fitted and know exactly the real word amount of energy required. It was 3750 kWh for last year. With rising energy costs I'm now at the stage where over 10 years an ASHP would be a slight saving so I will fit one at some point particularly with energy costs set to rise further.

 

Along with the metered switching you should fit a water meter on the SunAmp feed to give more accurate results in your data.

Edited by Dudda
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2 hours ago, TerryE said:

Unlike a UVC, the SunAmps don't require a G3 certificate

A vented cylinder does not require a certificate either, though if you fit a pump for showering it may well require a Part P sign off.

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1 hour ago, Dudda said:

It was 3750 kWh for last year.

 

An ASHP running at an output of 30°C will have a CoP over 4, so  will save around £300-600 p.a., say, depending on tariffs, say.  Probably not worth investing £5K, but £2-3K is a goer, which is why I would only consider a simple approach of directly plumbing an ASHP into the stab loop, albeit with a Y-plan valve so the ASHP can be bypassed when doing hourly recirculation and the  ASHP is off.

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5 minutes ago, SteamyTea said:

A vented cylinder does not require a certificate either

 

Yup, but I would never go back to having the CW @ 3 bar and the HW @ ~0.5 bar.  All the mixers just work so much better on a 3+3 configuration.

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3 minutes ago, TerryE said:

 

Yup, but I would never go back to having the CW @ 3 bar and the HW @ ~0.5 bar.  All the mixers just work so much better on a 3+3 configuration.

Not how I plumbed in my system.

I pump from they cylinder on the hot side, and from the F&E on the cold side.

Pump was 100 quid in 2007 when I fitted it, still going.

No need for thermal mixers, just set the 'tap' to same place (easy as it is a lever) and scrub away.

Using a relatively low DHW temperature gets rid of the scolding risk at root as well.

 

I like to keep things simple.

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17 minutes ago, TerryE said:

albeit with a Y-plan valve so the ASHP can be bypassed when doing hourly recirculation and the  ASHP is off.

What is this hourly recirculation?

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1 hour ago, TerryE said:

An ASHP running at an output of 30°C will have a CoP over 4, so  will save around £300-600 p.a., say, depending on tariffs, say.  Probably not worth investing £5K, but £2-3K is a goer, which is why I would only consider a simple approach of directly plumbing an ASHP into the stab loop, albeit with a Y-plan valve so the ASHP can be bypassed when doing hourly recirculation and the  ASHP is off.

Exactly, I'm approaching that break point. I'd be of the same opinion of just directly plumbing the ASHP into the slab too. I'd have enough buffer in the slab.

 

I find that as my house is very open plan with windows facing east and west (not south) running the pump on it's own like you have made very little difference although I think I only tried it for a few days. Very easy to do as it's only a dumb timer. I don't have a particular build up of heat in any area so it's not really beneficial in my case. Do you find this makes a considerable difference or what does it impact or effect? Do you find it more useful in very hot / very cold periods, etc?

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2 hours ago, ProDave said:

What is this hourly recirculation?

Sorry, I discussed this on my previous blog entry at this comment.  My control system circulates water through the slab for  8 mins every hour throughout the year to redistribute heating from direct sunlight, etc., and the averaged UFH return flow temp at the end of 8 min window is an extremely accurate measure of the overall slab temperature.

 

Flow immediately loops back from the slab return through the Willis and the pump back into the slab input.  If I were to hook in the ASHP then I'd only want to do this when the ASHP is running (with maybe 10 mins bracket either side.)  I certainly would want the water circulating through the external loop to the ASHP when it is powered off.

 

@Dudda, I've always done it. My house front faces SSE so we do get a bit of solar gain through to ~2PM.  I also use it for the slab temperature.  This is less that 0.2 kWh / day so the cost is a few pence and ends up heating the slab anyway.

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6 hours ago, TerryE said:

Again he did it that what because his plumber insisted he need this to have rapid HW.

 

On that front, our plumber said don't worry about recycling pump for anything under 12m. Fortunately ours aren't.   ?

 

Simon

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On 29/01/2022 at 21:05, ProDave said:

Isn't the economic calculation you should be doing to work out if a sun amp is viable, is compare the running cost of a resistance heated sun amp, with a resistance heated unvented hot water tank.  Work out how much cheaper the sun amp is (due to lower heat losses) and work out the payback time of the extra cost of sun amp vs UVC

Did anyone pic up on this cost comparison? It would be interesting to know. This seems like a relatively cost effective system to install. Are people able to charge their sunamp using the Willis heater as well as their ufh or would this not be the correct application? 

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4 hours ago, Rishard said:

Did anyone pic up  ...  It would be interesting to know.  Are people able to charge their sunamp. .. would this not be the correct application? 

 

Rishard, These sorts of Q aren't needed if you can use the search facilities, on forum and Google.  30s to ask this sort Q and how much time do expend other members to spend answering?

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I will do just this. Was looking for a direct link maybe to the question as some people have discussed this in depth and may already know or have a link. There is a vast amount of content on here. Which is great. Not enough hours in my day clearly…

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