Topic: Help designing a solar/wetback hot water system...correctly

Help designing a solar/wetback hot water system...correctly 

Hi there,

In the not too distant future I'll be installing a solar hot water system on my house in Palmerston North. Currently there is an old Rinnai inifinty (not sure on size) that supplies just the hot water, so wanting to get rid of that really.

The proposed roof surface is on a 2nd storey roof, facing approximately due north, and at a pitch of 45 degrees, with no shading. At the moment the house is 3 adults, but in future may be 3 adults and 2 children or 2 adults 2 children.

At the moment I was thinking about having a bank of 30 58mm evacuated tubes feeding into a 300 litre twin coil stainless steel indirect hot water cylinder. The idea being that solar for spring/summer/autumn feeding into one coil and then having wetback feeding into the second coil on the cylinder for winter. With a back up electric element for low periods. Wood is free/earned with sweat equity.

A couple of questions really:

Is the 30 evac. tubes feeding into a 300l cylinder a good match in terms of generation/storage capacity? would a 250 litre tank be better?

I've read conflicting information regarding the wetback and feeding into an indirect cylinder. Some things say it's doable provided the wetback loop is vented, others say not doable. Which to believe - anyone have a similar system that does work?

Any other comments about the system?

Look forward to (constructive!!) comments.

Cheers,

Re: Help designing a solar/wetback hot water system...correctly 

Ideally you want your tank to hold 2 days worth of hot water, for 5-4 persons the min size would be 500L, which will require 50 tubes.
30 tubes on 300L may not be enough, its the surface area you want, you may be disappointed especially away from summer conditions.
Wetback on secondary loop - this is the loop higher up in the tank, requires a vented system to prevent possible steam explosions.

Re: Help designing a solar/wetback hot water system...correctly 

manawatuheatingandsolar.co.nz

Re: Help designing a solar/wetback hot water system...correctly 

You are doing it the wrong way round. You use the solar and wetback to heat the low pressure cylinder (with an open vent/header tank)
You then run the tap water, at mains pressure, through the coil.
Low pressure heating.
High pressure taps.

Re: Help designing a solar/wetback hot water system...correctly 

I've got a system as described by Samiam.

I am curious as to the configuration for venting a wetback loop, as well as make-up water for that loop - presume some kind of header tank.

Seeker

Re: Help designing a solar/wetback hot water system...correctly 

Solar and wetback work well together - but setting it up is a different matter.

Mains pressure cylinder with wetback coil. Solar is direct, ie this part is also mains pressure.

Wetback coil has to be open vented with a small tank in the roof - with the risk of boiling everything has to be copper.

Trying to use a coil for "instant" water heating is a complete waste of time.

JK

Re: Help designing a solar/wetback hot water system...correctly 

Why is the use of a coil for instant water a waste of time? It is a state of the art option with many adavatages.
Solar direct is a health risk and deposits from the water can build up in the solar panel pipe system and make it less efficient. This is especially a problem in areas with high iron or minerals in the water.

Re: Help designing a solar/wetback hot water system...correctly 

Of course it requires skills and know how to plan and install a solar water system which is efficient, no health risk and properly functioning.
There are very few people in New Zealand with the necessary training and expertise and the rest work on guess work and luck and they get away with it.

Re: Help designing a solar/wetback hot water system...correctly 

I think the system Samian is describing is called "drain-back" SHW, said to be the simplest in terms of hardware and most efficient type of SHW system. It's also fool-proof in regard to freezing conditions and has good longevity and should combine with wetback fine.

The only problem seems to be because it's a "pre-heater" system I think it will only be very efficient when hot water is being drawn on a regular basis through-out a day. Normally people draw a lot of hot water in a short period and then the pre-heater tank lies unused for long periods.

http://www.solarhotwater-systems.com/advantages-drainback-solar-hot-water-systems/

Re: Help designing a solar/wetback hot water system...correctly 

I suppose a corollary to that is that in a glycol system or even direct system, if a lot of hot water is drawn off most of the re-heating will be done by the electric element as the thermostat will cut in, but this can be offset somewhat by placing the thermostat and coil high-ish up in the cylinder.

Re: Help designing a solar/wetback hot water system...correctly 

....and having a large tank as Mike points out above.

Re: Help designing a solar/wetback hot water system...correctly 

Not a drain back.

Seeker

Re: Help designing a solar/wetback hot water system...correctly 

The placement of thermostats, coils, elements, etc affects the performance of SHW systems HUGELY. The distance between the collectors and the storage system is also very important.

Suggest you make contact with someone like Adrian Kerr at the Solar Association and ask your second question - he knows an absolute mountain of info about this as he models performance of systems (quite a few of the systems in NZ that got EECA funding were modeled by Adrian.

Your first question is a bit like how long is a piece of string. There are wide variations between the performance of different makes of 58mm evac tubes.

Seeker

Re: Help designing a solar/wetback hot water system...correctly 

Also SNZ: the larger the tank the better where SHW is concerned - so 300 litres out of your two choices. Which means a second element placed towards the top third of the tank is probably beneficial (for boost).

I've been informed on more than one occasion that our NZ systems are too small - German systems start at 1000 litres - maybe other forum posters can comment on this.

Seeker

Re: Help designing a solar/wetback hot water system...correctly 

Thanks for all the super quick response from everyone. Will spend a little bit of time digesting all this info, and trying to do a bit more research.

Will be back!

Re: Help designing a solar/wetback hot water system...correctly 

Seeker you are quite correct, an efficient solar system requires a much larger storage cylinder than what is typically used in NZ, 500 - 1000 L would be the min required for a 4 person house hold, unfortunately with many retro-fitted systems this isnt possible, so one goes for the biggest standard tank that will fit.
The electric backup element ideally should be fitted in the top 1/3rd of the tank, leaving the bottom water heated entirely by solar for efficiency, or some sort of timer fitted to the main element to prevent its use during periods of sun.
Another option is install a separate solar only heated tank with no electric element, feed the output of this into the bottom of the existing systems tank, providing the pipe run between is well insulated, it works pretty well.

Cheers
Mike

Re: Help designing a solar/wetback hot water system...correctly 

I have a 300L drainback/wetback system in Palmerston North. Drainback avoids any risk of overheating (so can can have larger panels) or freezing, also means the pump isn't running on frosty nights (and what about power cuts?) I have 5.6m^2 of panels which is at the upper limit recommended by Solar Industries NZ (50-100litres per square metre). Even this is not quite enough as the system almost never reaches its cutoff temperature of 75 degrees. In other words, almost no heat is being wasted, but on many cloudy days we don't get enough hot water. (Perhaps because the angle is a
bit east of North).

Correct sizing of the wetback is also important. During a 4 day power cut last winter, once the cylinder had reached boiling we had to stop using the fire because there was insufficent pressure to drive the water through the tempering valve. If the fire is your main source of home heat then boiling could be a problem. It would be good to have a couple of radiators attached to the system that could be turned on or off depending on the availability of heat. However, this winter we are finding that the 6kW wetback unit is just about right as this is our second fire and it runs about 6 hours a day.

The solar is returning about 14% on its cost.

Re: Help designing a solar/wetback hot water system...correctly 

I'm planing a new house. I did already 3 SHW systems open/direct thermosyphone but all with collector close to HWC. Within 5m. Now this one is far away (collector - HWC) Where is the limit? And why? 15m still working? Bigger pipes for lower friction - but more water to loose energy when it stops circulating and cools down? Will the circulation still start? What is the lowest angle/fall between Collector and HWC? Unfortunately I cant set the pipes out to have a steady rise / fall. Thanks for inputs.

Re: Help designing a solar/wetback hot water system...correctly 

I'm also looking at using a solar and wetback system I saw that Rinnai does a gas boosted solar system. This works with the water going into a storage cylinder then passing a diversion valve which sends it to the Rinnai for extra heat if it is not hot enough. I already have what looks to be a drainback system in that I have a header tank in my roof. Half my pipes are coppper and the other half are that black pipe that splits. Also my current low pressure cyllinder has sprung a leak so its definitly time to upgrade but I'm finding it all confusing. I found a cyllinder by Greenglow that that is dual solar and wetback does anybody know if this is a good way forward.

Cheer Sue

Re: Help designing a solar/wetback hot water system...correctly 

Solar, wetback, greenglow, plus a Rinnai - an extremely expensive water heating option.

Personally I think wetback systems are vastly overated - and expensive to install with a mains pressure cylinder. (unless you have free wood which alters things somewhat).

Really the choice is between solar and a hot water heat pump - throughout most of New Zealand both systems save similar amounts of energy in a year. A heat pump is cheaper and more versatile, solar should last longer than 15 years. (also note a heat pump can be fitted to ANY cylinder).

JK

Re: Help designing a solar/wetback hot water system...correctly 

Hi Sue

I agree with JK. I also think that the combination of solar, wetback, greenglow, Rinnai may give 4 different sets of headaches.

I was wondering what your objective was, and how long you intend to stay at the place where this set up would be.

Seeker

Re: Help designing a solar/wetback hot water system...correctly 

Hi Tui

I think one of the secrets with passive solar hot water, given the thinking behind the whole thermo-siphon setup, is that the system needs to be very close to the cylinder, allowing for a seperation distance of a metre or two height-wise for the siphoning flows in a steady rise fall - which does not seem to be achievable according to your post, and which would suggest thermo-siphon is not really an appropriate technology to use in this case.

(In terms of distances, even with my pumped system I am losing a significant amount of heat in the lagged piping between the cylinder and my evac tubes less that 3 metres directly above the cylinder - and I have more captured energy to play with in a pumped set-up.)

A thermo-siphon system is a passive, low impact kind of arrangement, and does not collect as much heat energy as more active (pumped systems). You can increase the output by having a large collector area - so it depends on what your primary objective is as well - what is it that you want to achieve with your hot water service?

Seeker

Re: Help designing a solar/wetback hot water system...correctly 

The thing is I do live up in the bush and almost all of my wood I grow and get from pruning. I'm planting a whole lot more trees this weekend.

I've just spoken to a technical person from Rinnai. I've decided to drop the solar idea, but given I already have all the copper in for a wetback I wanted to explore options. I've had it suggested that I could use a coopers cylinder with a coil (apprarently don't even need a thermo-siphon system). The coil which heats the water is low pressure the rest of the water in the tank is mains pressure, a flow diversion valve automatically directs the water to the infinity in summer when it is not hot enough. (A temporing valve is also required if the system gets too hot.) The only extra cost is really the coopers storage cylinder. Of interest Rinnai has just bought coopers. I'm costing this out at the moment. I really want to get gas for cooking. Its too hard to get cylinders up to mine and not economical to have piped gas just for cooking but my calculations show that using gas to top up the water rather than electric is effective. environmentally the suggestion is that this is an equivalent option. Although I've had a whole lot of conflicting advice.
Your thoughts?
Sue

Re: Help designing a solar/wetback hot water system...correctly 

With free wood it makes sense to go wetback.

The decision to make is the backup for this.

Electric, gas, solar, or heatpump.

As you will have a cylinder there is little point in gas, as gas water heating costs more than straight electric - with extra capital cost.

Electric is least capital cost, solar makes most sense as it goes well with a wetback (plus you will have electric as well).

Heat pump costs a lot less than solar and would mean you would never need electric - cost should be about $3500.

With induction hobs I would question gas for cooking.

JK

Re: Help designing a solar/wetback hot water system...correctly 

Hi Seeker

I would like to build the system my self. Except for the HWC I'm going to buy a good one available on the market. Do you know a good brand? The rest I can do my self, incl the collector. That saves me huge on money, I'm talking under 1000$ for everything except HWC. No pump, no sensors, no air release valve, no heat exchangers etc. Just pure simplicity. No power required. More efficient, than closed loop. etc.

I know the perfect world would be as close as possible to the HWC and I know I should head for that, but my question was ... if anybody here has made some experiments or gained otherwise some knowledge on how far you could stretch it?
Cheers