Solar hot water problems - would love some advice!

[ApprenticeGM]

Hi Everyone,

I have a few problems with a relatively new solar hot water system I'd love your help and advice on. My old twin-element electric heater died 3 days before Christmas 2018. At that time all the plumber supplies had closed for a 2-3 weeks Christmas break. I was able to source a new solar hot water system from eBay from a local reseller that used 30 x evacuated tubes & a 315 litre stainless steel tank which also came with an electrical booster element (connected to off-peak overnight circuit not mains). I picked it up Christmas Eve and installed it a few days later using a professional plumber for the parts that needed a qualified tradesman (to meet building code) - I did the rest myself. Here's a simple diagram of the design logic:

solar_hot_water_design_v2.png

In summer, it works great. The incoming cold water is 25°C, and the entire 315 litre tank reaches 80°C (maximum) so a 55°C temperature increase. We can then have hot showers for >100 minutes. For a family of 4, even allowing for washing up, this is easily 2 days capacity, so if it's cloudy or raining for 2 days, no boosting is needed. Only if 3 days consecutively are cloudy do we need to boost.

In winter, it's woeful, the incoming water is 8°C and the system struggles to raise the temp above 28°C and often less (so a 20°C temperature increase). This means cold-ish showers, and if it's cloudy or raining, very cold showers! So I rigged up a solar reflector to double the amount of sunlight on the evacuated tube heat collector between 10am & 2pm and it was better (around 30°C temperature increase) on good days (but still not quite hot enough) and on cloudy days, terrible performance. So, in winter, we boost all the time. The electric booster element is connected to an off-peak circuit which kicks in between midnight & 6am. It only costs ~15c / kwh for off-peak compared to ~55c / kwh for peak. The off-peak boost though only heats slightly less than half the tank - say 150 litres, and only to 70°C, the limit of the thermostat on the electrical element, a separate thermostat to the solar hot water pump and sensors which can heat the tank to 80°C. This means we end up with 150 litres @ 70°C which is 41.6% of the maximum capacity. However this does not provide 42 minutes of hot showers, because, the cold water is 8°C not 25°C, so you need more hot water in the shower water mix.* We get about 23 minutes of hot water which for 4 people trying to shower and shave and wash up (especially if the wife wants to shampoo her beautiful long hair) just doesn't cut it.

This bad situation (of not enough hot water) is about 7 months (April - October) of the year whilst the other 5 months (November - March) no problem.

So now I'm in a bad situation because in hindsight I should have bought a 500 litre tank with 50 evacuated tubes (but took the advice of the seller, who assured me it would be fine lol), but I have what I have, and I'm trying to make the best of it. That seller has disappeared as well so no advice or help from him or recourse to upgrade or exchange etc.

So, here are my questions based on that information:

1) What kind of performance should I be getting from my system? Is what I've described typical of a 30 evac tube / 315 litre tank system? Or is this possibly some problem I can fix? (I've checked all the tubes, one was broken from a hailstone, replaced it with a spare, all others seem fine)
2) What do you think I should do to improve my hot water production?
a) Add another electric tank say 160 litres connected to off-peak.
b) Add another solar panel / evacuated tube array in series with the existing eg expand to 50x evac tubes
c) Add a second pump to the existing tank utilising the "auxiliary" outlet at the top of the tank that cycles water from the top to the bottom. Run this pump during off-peak (connect it to off-peak) so the entire tank gets boosted to 60°C, provides enough hot water for a day's showers, and allows solar heating if possible during the day.

Thanks all in advance for your thoughts. Stay safe!

* In summer we use 3 litres of 80°C hot water + 7 litres of 25°C cold water per minute to get a 41.5°C shower (total of 10 litres / minute; I'm keeping the maths simple and approximate). In winter we use 5.5 litres of 70°C hot water + 4.5 litres of 8°C cold water per minute to get a 42°C shower.

p.s. The poor performance is not coming from any loss of heat from the system:
a) Hot water pipes are insulated.
b) Climate is mild (never get a frost let alone snow; 20°C / 68°F is a typical winter's day, overnight 5°C is about the minimum)
c) We had a blackout one day when we were out all day (so no hot water use, no pumping possible to allow for solar heating) and the water temps were unchanged from the overnight boost (they did not drop even a tenth of a degree - the tank has multiple sensors and display is in 0.1°C increments). This means there is no measurable heat loss from the system.

[J.P.M.]

If you don't have them already start by getting low flow shower heads. FWIW, I've got something called (no joke), "The Incredible Head" (I like the name for several reasons - not the least of which is that I get to say the name in mixed and polite company and get away with it). They're commonly available, in the U.S. anyway. I've measured the flowrate at 4.73 l/min at 3 bar line pressure. Works great.

What's your location ?

Consider increasing the tilt angle to something like your latitude plus 15 or 20 deg. That'll help winter production.

Busy just now. more later. Welcome to the neighborhood.

[solarix]

I quit doing solar hot water system years ago. They are just nowhere near the reliability of solar electric. Callbacks all the time for little problems. And the liability risk of running pressurized water through attics. Now with solar PV having come down on price so much - better to just use a regular, low cost electric water heater and run it off your grid-tied solar array. Put a timer on it to only heat during the off-peak time of day if you want to really save.

[ApprenticeGM]

Hi @ J.P.M. & good to be here

I do have low-flow shower heads, I was trying to make the maths simple. We have 3 showers & my favourite is a needle-jet that is very low flow. Despite low flow heads we only get about 25 minutes of shower hot water time, less if washing up / shaving etc happens. Winter is only a quarter of summer's hot water production; OK not strictly true, but a quarter of the hot water capacity, as more hot water is required for the same shower temp given the cold water in winter is much colder than in summer.

My location is Central Coast NSW, just north of Sydney (33°S) and the problem (now) with increasing the evac tube angle to (say) 50 degrees to get them square on to the Winter sun, is that would lift the manifold up about 1.2m+ off the 22° roof, and require plumbing adjustments (currently plumbed in using insulated copper pipe) and significant raising and re-enforcement of the solar reflector. I think it would be cheaper and easier to add another 20 evac tubes than lifting existing.

I'm leaning more towards 2c) the recirculating / mixing off-peak powered pump.

[Mike90250]

My thoughts

1) There is heat loss in the system. Always is, you just are not measuring it.
2) Your storage tank is too small, and runs too hot (increases loss from super hot water)
3) Evacuated tubes are great collectors, till they start to fail. Do you have spares and /or instructions on DIY refurbishing them ?
4) seriously look into a solar powered backup pump for power outages. Evacuated can overheat if the pump is not running and then they fail.

See following for more ideas:

https://www.solarpaneltalk.com/forum/solar-thermal/solar-water-heating-forum/3948-bad-experience-with-evacuated-tubes

https://www.phcppros.com/articles/1455-solar-collector-overheat-prevention

[ApprenticeGM]

I quit doing solar hot water system years ago. They are just nowhere near the reliability of solar electric. Callbacks all the time for little problems. And the liability risk of running pressurized water through attics. Now with solar PV having come down on price so much - better to just use a regular, low cost electric water heater and run it off your grid-tied solar array. Put a timer on it to only heat during the off-peak time of day if you want to really save.

Hi @solarix,

Where I live (Down Under) every home has pressurised water running through the roofspace (we don't really call them attics because they aren't livable, they are large crawl spaces) for normal hot and cold water provisioning regardless of whether the hot water is solar or not. Here, houses are built on concrete slabs with drain / waste plumbing, timber frames are erected, then electricals and plumbing run in the roofspace. Walls are generally brick (sometimes cladding) with interiors and ceilings plasterboard, rooves are trussed with roofing material corrugated iron or concrete tiles. Insulation rests on the ceiling plasterboard and is fibreglass with ratings of 3.5. We don't have the amazing expanded foam delivering insulation numbers like R20 for 75mm etc.

So, having pressurised water runs for rooftop solar isn't an additional risk / liability.

I'm not sure of current numbers, but 10 years ago solar hot water was hugely more efficient than solar PV to make hot water. In summer my system makes more hot water than we can use, but in winter it's awful.

I'm leaning towards adding a pump (option 2c).

[ApprenticeGM]

My thoughts

1) There is heat loss in the system. Always is, you just are not measuring it.
2) Your storage tank is too small, and runs too hot (increases loss from super hot water)
3) Evacuated tubes are great collectors, till they start to fail. Do you have spares and /or instructions on DIY refurbishing them ?
4) seriously look into a solar powered backup pump for power outages. Evacuated can overheat if the pump is not running and then they fail.

See following for more ideas:

https://www.solarpaneltalk.com/forum/solar-thermal/solar-water-heating-forum/3948-bad-experience-with-evacuated-tubes

https://www.phcppros.com/articles/1455-solar-collector-overheat-prevention

1) Heat loss is < 0.1°C during the day, so it's just so negligible it doesn't count. We know as overnight the off-peak heated top half of tank to 70°C per normal, we were out all day (no hot water use), there was a blackout (no pumping possible), it was cold and raining (no solar heating possible even if the pump worked which it didn't) and when we returned the top half of the tank was still 70°C. The problem is the system is not producing enough hot water, not that it is producing enough, or nearly enough, and loss is impacting at any discernible level. As I said in the maths (math) section in my original post, we're down to 23 minutes of hot water in winter from off-peak generation. We can see how quickly the tank temp sensor drops to reflect the layering between the top, hot half of the storage tank, and the bottom, cold water part, as the cold water moves through the tank. These layers stay separated, they don't mix, this is standard design for all hot water tank systems for use perspective (different to heating cycles).
Heat loss is not a factor here, I get that you and others think it often is for other cases, but not this one. I have the actual data, measured, with multiple sensors.
2) Yeah, I should have gone with the 50x tubes / 500 litre tank, then I wouldn't be having this problem.
3) The tubes are only 16 months old, installed late December 2018. 1 had died thanks to hail breaking the glass, it's been replaced.
4) I will consider a solar powered pump, but in my case - not sure about other evac tube systems - the evac tubes have heat pipes running into a manifold where heat is exchanged into the water, the pump moves the hot water back into the tank and replaces with colder water from the tank, repeat. If the pump dies or isn't running, or if the storage tank reaches max temp (80°C in my case, which it does in summer quite often) then the manifold has a pressure release valve it purges boiling water / steam safely.

[LucMan]

It doesn't sound like there is a problem with your system to me. It is the nature of the beast, low solar insolation = reduced performance. Yes you can add more tubes for a slight increase of performance but It will not give you the water temps that you require, and you would have to add a dump zone for the your highest performing month's.
Download the free "Solar Pathfinder " software to calculate your best projection.

[J.P.M.]

I quit doing solar hot water system years ago. They are just nowhere near the reliability of solar electric. Callbacks all the time for little problems. And the liability risk of running pressurized water through attics. Now with solar PV having come down on price so much - better to just use a regular, low cost electric water heater and run it off your grid-tied solar array. Put a timer on it to only heat during the off-peak time of day if you want to really save.

Pretty much agree. If I was doing it today, and being in a moderate climate as I am, and I couldn't get or didn't have nat. gas available, I'd consider PV with a HPWH, although I feel I'd be taking a chance on the HPWH as I'm of the opinion that, at this time HPWHs aren't yet ready for prime time.

I will say also however, that a well designed solar thermal flat plate system, or better yet, a batch or "bread box" heater (in moderate climates) can be trouble free and pretty close to, but not completely maint. free. That they are not completely maint. free, coupled with the observed but still anecdotal reality that most any solar thermal system in residential service almost always gets no maint., or at least no more than the hot water tank sitting in the basement or garage gets, and that is the source of most solar thermal complaints. That's just the reality as I see it.

A couple of comments:
- Reducing hot water demand is more efficient and cost effective than additional/increased sizing for any system and so is always a better first step before any methods of meetings a load.
- If solar is still a choice for meeting some/all remaining and reduced load, and if available space, on a roof or elsewhere is a constraint, in a moderate climate, a solar thermal system will produce the same amount of thermal energy, even considering piping and tank standby losses, as a PV system by itself but require something like ~ 1/2 the footprint size. Reason: PV operates at something like ~ 16 - 18 % efficient as measured by (delivered energy to the load/incident energy). A well designed solar thermal flat plate will operate at something like 30-35% efficiency as (delivered energy to the load/incident energy).
- I've lived in very cold and also cloudy climates (Buffalo) and goldilocks climates (Albuquerque and San Diego). Solar thermal in cold climates using most any solar thermal flat plate system is mostly not a practical idea unless it's drained and not used if during times of the year when freezing is even a remote possibility. Then, doing that usually makes such a flat plate systems about as cost effective as evac. tube systems, which is to say, and depending on specific climate, probably cost ineffective.
- For the duty of residential domestic water heating, evacuated tube systems are overkill and not the best way to do solar water heating. Kind of analogous to using a Mercedes 12 cylinder roadster as a grocery hauler. Evacuated tubes also seem to have poor reliability as compared to simpler systems such as flat plates. Such evac. tube systems are better utilized for process heat applications that require temps well over 100 C. That's where they are more efficient than flat plate systems. At temps. that DHW systems operate, maybe 50 C to 75 C or so, the flat plate's greater optical efficiency works to make that system's overall efficiency as measured by useful energy delivered to a load greater than that of the evac. tube system, usually for a lot less $$/m^2 of system size to boot.

[Mike90250]

I'm not sure why your pump is allowing your tank to stratify, with the hot water at the top, and not being equally mixed. Do you have an intermediate heat exchanger that allows thermal stratification in the storage tank ? Simply adding a small 5watt circulation pump will allow you to store more hot water than just the top portion of the tank.

[Ampster]

Pretty much agree. If I was doing it today, and being in a moderate climate as I am, and I couldn't get or didn't have nat. gas available, I'd consider PV with a HPWH, although I feel I'd be taking a chance on the HPWH as I'm of the opinion that, at this time HPWHs aren't yet ready for prime time.
.....................

The only thing I would add from installing HPWHs in four instances for myself or a relative is the following:
The first one I installed was in 2011 and it is still going strong. It was a retrofit compressor that sat on top of an existing electric water heater. That manufacturer is no longer in business.
The second one was a GE Geospring installed in 2014 and the refrigeration portion has worked flawlessly, but the quality of one of the pipe assemblies caused enough corrosion that I was concerned about a leak. GE has sold that part of the business to AO Smith and they had no interest in pursuing a warranty claim, so I repaired that assembly with a copper replacement that required some time to access. I would not recommend AO Smith and Geosprings are no longer manufactured. They may be the reason HPWHs have a bad reputation.
The last two HPWH were Rheems and they have been in service for over 3 years now. I had done research on Rheem and their HPWH warranty is 10 years. In my situation the payback of the additional cost was three years compared to a regular electric water heater. I would recommend them.

[ApprenticeGM]

I'm not sure why your pump is allowing your tank to stratify, with the hot water at the top, and not being equally mixed. Do you have an intermediate heat exchanger that allows thermal stratification in the storage tank ? Simply adding a small 5watt circulation pump will allow you to store more hot water than just the top portion of the tank.

Hi Mike,

1) If by stratify you mean the layering of hot water / cold water in a hot water system, this is by design; it keeps the top half of the tank (that could have been boosted) separate to the bottom half during the solar heating cycle. Once the bottom half equalises in temp to the top half, then future solar heating effectively heats the whole tank (as hot water rises). Until the bottom half rises sufficiently in temperature from solar heating, the top half stays hot for use in the home.
2) In my original post I gave 2c) as a potential solution, a small pump that would circulate water through the whole tank whilst off-peak heating, and this is the solution I am leaning to.A 5W sounds ideal, electrically connected to the off-peak heating circuit, so it only comes on and works when off-peak heating is functioning.

[nwdiver]

Pretty much agree. If I was doing it today, and being in a moderate climate as I am, and I couldn't get or didn't have nat. gas available, I'd consider PV with a HPWH, although I feel I'd be taking a chance on the HPWH as I'm of the opinion that, at this time HPWHs aren't yet ready for prime time.

To back up what @Ampster posted I think the current crop of HPWH are great. The Rheem HPWH is currently on sale at Home Depot. The Geo Springs had some issues but take a look at the reviews for the Rheem. My sister has had a Rheem for ~3 years now with no issues. The energy savings is crazy. ~70% reduction in energy use.