10 years of solar hot water

[Wowbagger]

Hindsight is a wonderful thing.

In April 2005 I decided to spend some of my mother's legacy on a solar hot water panel. At the time, full domestic PV was so expensive it was pretty well non-existent, so I didn't bother with that. The system I went for was the aesthetically very pleasing Solartwin system. This takes cold water from the bottom of the hot water tank, pumps it up to the panel and then back into the top of the tank. The warm water floats on the cold and over an hour or two the entire tank is warm and ready to "go round again". Each cycle warms the water by up to 15°C, depending upon how high in the sky the sun is, and the electric pump is driven by a small pv cell, so the pump only operates when the sun shines, and of course that's at the same time as the water warms up. Very elegant. The only problem was that we live in a hard water area and in order to prevent the panel furring up it was necessary to install a softener. Never mind, soft water is very pleasant for showers and washing machines etc. For 75% of the year a sunny day will deliver huge quantities of free hot water. It's lovely.

After about 5 years it was clear that the pump wasn't working as it should and Solartwin replaced it under guarantee. Now, a further 5 years on, the second pump has packed. The trouble is, so has Solartwin. An innovative pioneering product has been overtaken by the mass market brigade, a bit like good old Solidlights.

Solartwin's website still exists, but the guy running it no longer sells or installs panels. He just flogs pumps to people like me. He's charging £380 - yes, £380 - for a small 12v 5w brushless pump. I suspect that he's in breach of trade descriptions because the website clearly shows a load of solar panels for sale and he doesn't sell them. There's another company, Thermatwin, who are doing the installations. They charge £266.18 + VAT and carriage for the same pump.

OTOH, a quick look at Amazon and Ebay indicates that fish-tank and pond pumps, which are of the same voltage & wattage, can be picked up for about a fiver. They are all described as "brushless" pumps with a life cycle of about 30,000 hours. Even if the sun shines all day and every day, that should be good for about 5 years.

Experiment time.

PS I have learned from a 3rd party that Solartwin used this pv panel method out of sheer purism. They could have used a much cheaper mains-powered 12v pump and the electricity cost would have been about £3 a year, with a thermal controller deciding when to turn the pump on and off. We could, if necessary, retrofit this.

[Kim]

The solar water system in barakta's mum's house uses a pair^[1] of bog-standard mains powered central heating pumps, with a controller turning them on and off according to the temperature difference between the water in the tank and the water in the panels (it has an assortment of sensors providing that data).  The duty cycle on the pumps isn't particularly high, unless conditions are particularly favourable (summer sun and freshly-depleted tank).  ETA: I think the controller does some sort of crude speed control, as it reports the pump status as either '0%', '33%' '66%' or '100%' - they seem to spend most of their time at 0% or 33%.

It's a closed-loop system, so the circulating water is full of corrosion inhibitor.  The only thing that's ever caused problems is the mixer valve that dilutes the hot water from the thermal store down to the maximum temperature that the gas combi boiler will accept on its inlet and still modulate correctly.  (Not an ideal solution, but it made some sense as a retrofit.)

It works well enough, but represents a triumph of salesmanship over engineering, being one of a series of dubious financial decisions made by barakta's dad over the years.  I've no idea if it's actually managed to pay for itself (not least because the electric shower that's the main consumer of hot water is fed directly from the cold supply because running a pipe from the thermal store would have involved effort).


_{[1] Due to the angle of the house, there's an east-facing panel and a west-facing panel.}

[Cudzoziemiec]

I suspect that he's in preach of trade descriptions

Blessed are the trade descriptors for they shall inherit the juicy fat legal fee.


I don't know about pumps, sorry, but for a fiver it's surely worth it.

[rafletcher]

I'd maybe go for a mains powered CH pump, built for the duty. Our last one lasted 10 years and no doubt had a significantly higher run rime than Wows will have.

[mrcharly-YHT]

I'd maybe go for a mains powered CH pump, built for the duty. Our last one lasted 10 years and no doubt had a significantly higher run rime than Wows will have.

I suspect they will pump the water far too fast.

[Kim]

I'd maybe go for a mains powered CH pump, built for the duty. Our last one lasted 10 years and no doubt had a significantly higher run rime than Wows will have.

I suspect they will pump the water far too fast.

Yes, I think a CH pump would need some proper control logic to get the best performance from the panel, and not waste power.  That's the elegance of using a PV-powered DC pump - you get sunlight-related speed control for free.

I'd certainly try a pond pump in the first instance.

[JohnR]

You have to watch the "head of water" you are trying to pump, I would have thought a pond pump may not have enough head, depending on how much the hot water tank is below the solar panels. all I'm saying is read the details of the pond pump before you buy. although a ch pump may look too big, they usually have 3 speeds.

[Kim]

You have to watch the "head of water" you are trying to pump

Do you, in a closed loop?  IANAP (I just work it out using electricity metaphors), but I'd have thought the weight of the water on the return side would cancel out that on the flow side, so you're left with the head of the pump vs friction loss.  Since a fairly slow flow is desirable for this application, it's not going to be too demanding about head.

[Simon Galgut]

Speak to solarproject.co.uk

I got my panels from him and he also sells the parts for cheapo 12v systems - there are pumps listed on the site.

My system uses standard Grundfos 15/60 pumps (its a SW/SE dual panel system) and they are fine if you reckon on replacing them regularly. The impellers are not really up to the job of glycol laden water at 100C +  Grundfos do make specific solar pumps with brass impellers, but they cost a fortune.

[Wowbagger]

I spoke to him yesterday. He was very helpful but, as in the hindsight bit in my first post, his comment was "Should Solartwin really have sold you such a system in a very hard water area?" A good question, but I think the softener has other benefits. Charlotte really appreciated being under my shower when she stayed here, and loved the softness. I'm sure the washing machine has lasted longer as a result, as have other appliances.

In fact, I think it has been reading the Solarproject website which has helped me cure the problem: the system needed bleeding! Dez disconnected the pump this morning and we took it apart, couldn't see anything wrong, tried pumping some water using a 12v PC supply and it worked fine. Put it back in the loft and it makes noise, but there's no water flow.

I disconnected the hot pipe from just above the copper cylinder and bled it into a bowl. Lots of air came out. Unfortunately, quite a bit of water escaped form the main plumbing as I was trying to put the silicone pipe back on its nozzle, to the extent that Mrs. Wow complained that some water had dripped through the kitchen ceiling. I got quite wet from the back-spray.

However, the tank is now warming up and the pump is making a much healthier noise.

[Kim]

Soft water (by which I mean actually soft water, not what the average resident of South-East England venturing north of Watford may think of as soft) is all well and good for appliances and professional hand-washers, but it's a work of Stan when it comes to showering in.  You can't get the soap off.   

Obviously what's needed is a balance of hardness such that you can rinse yourself off in a timely manner, while not driving innocent heating appliances to an untimely demise through scale build-up, and without leaving crunchy bits floating in your tea.  If we cross-reference this with the additional requirement that the water shouldn't taste of ming (Manchester need not apply) or be held to ransom by Welsh people, then the answer is clear and clarion-compliant.   

[JohnR]

You have to watch the "head of water" you are trying to pump

Do you, in a closed loop?  IANAP (I just work it out using electricity metaphors), but I'd have thought the weight of the water on the return side would cancel out that on the flow side, so you're left with the head of the pump vs friction loss.  Since a fairly slow flow is desirable for this application, it's not going to be too demanding about head.

Thinking about it further, you are right, the pump will only be overcoming friction round the system. Note to self - check before posting 

[Wowbagger]

Soft water (by which I mean actually soft water, not what the average resident of South-East England venturing north of Watford may think of as soft) is all well and good for appliances and professional hand-washers, but it's a work of Stan when it comes to showering in.  You can't get the soap off.   

Obviously what's needed is a balance of hardness such that you can rinse yourself off in a timely manner, while not driving innocent heating appliances to an untimely demise through scale build-up, and without leaving crunchy bits floating in your tea.  If we cross-reference this with the additional requirement that the water shouldn't taste of ming (Manchester need not apply) or be held to ransom by Welsh people, then the answer is clear and clarion-compliant.   

We don't soften the drinking water. This is a Bad Thing from the point of view of the ingestion of sodium salts and their effect on the cardiovascular system. As it happens, although the water is hard, we don't have a problem with the kettle furring up. It is known technically as "egg-shell hardness" and I understand from my brother (retired water co. chemist) that it exists where the water is supplied from large, shallow reservoirs (Abberton, Hanningfield, Ardleigh being the three largest in Essex) are affected by algal bloom. For some reason that I don't know, this prevents the hardness from building up in great chunks. Washing powder clearly negates the effect and in the past, I have removed lumps from under the washing machine drum the size of house-bricks. The kettle, on the other hand, merely accumulates little bits very similar to egg shell.

I agree about the shower and removing the slime from soap. Hand-washing is best carried out at our kitchen sink where the hot water has been softened but the cold has not. Rinsing can be done with the cold tap. Charlotte's enthusiasm whilst in the shower was as a result of the soft water being much gentler with her tender skin.

[Quisling]

Interesting to see your system Wow uses a primary circuit and directly pumps water from the cylinder into the panels and back.  This of course avoids the need to spend £1500 on a 250 litre indirect twin coil cylinder.  However, these tend to have a dedicated solar coil at the bottom (with large surface area for good heat transfer) and avoid the issue of hard water affecting the pumps and panels since they are on a closed indirect circuit containing glycol and the suchlike to prevent freezing in the panels.  How does your system resolve the freezing risk - does it drain back into the cylinder when the pump isn't running in order to leave the panels empty?  Most indirect solar circuits run on a command from the pump controller and temperature sensors which detects a minimum temperature differential between the panels and the cylinder, and stops pumping if the differential is below say 5 to 8 degrees C in order to prevent wasted electricity for pumping.

[Kim]

Sounds like Wow's is using flexible silicone hoses rather than rigid pipe, which presumably mitigates the damage from freezing, and the panel is left to fend for itself?

[pcolbeck]

My thermal store is a triple coil one. Oil boiler, back boiler and solar. You can also insert an immersion heater if you want a fourth option !
I haven’t got round to fitting solar yet though.

[Wowbagger]

Sounds like Wow's is using flexible silicone hoses rather than rigid pipe, which presumably mitigates the damage from freezing, and the panel is left to fend for itself?

This woman knows what she is talking about - with knobs on!

[Cudzoziemiec]

Charlotte really appreciated being under my shower when she stayed here, and loved the softness.

We should certainly be reading less into this than we are.

[Wowbagger]

Charlotte really appreciated being under my shower when she stayed here, and loved the softness.

We should certainly be reading less into this than we are.

I don't know what you mean. 

[Simon Galgut]

My thermal store is a triple coil one. Oil boiler, back boiler and solar. You can also insert an immersion heater if you want a fourth option !
I haven’t got round to fitting solar yet though.

Mine too. I haven't got around to connecting the back boiler from the woodburner, but the solar, immersion, gas boiler all work. Custom made by Newark for £550

[Poly Hive]

it really pays to go round now and again and bleed your rads. We had a room that used to really warm then failed to warm though the rad felt warm enough, then after getting a suprisingly small amount of air out the room get really cosy again.

PH

[Wowbagger]

Update time!

I bought a pump from an Ebay droid for under £6 and fitted it. It didn't work and I had to get Dez to check my working. He concluded that I had fitted the pump correctly but that the sunlight was not sufficiently strong to generate 12v from the panel - the old, very expensive, pump wasn't so fussy. Then we had a series of cloudy days and I went away for the weekend and forgot about the whole thing for a while.

I noticed on Wednesday morning that the cheap pump appears to be doing its job. Because of having a lot of other stuff to do I didn't spend as long playing with it as I would have liked. However, when I washed my hands this morning I noticed that the water was warm! Checking the tank revealed 33 deg C in the top, 28 deg C half-way down!

I think we will regard that as a success.