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Evacuated Tubes for Hot Water

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Evacuated Tubes for Hot Water

Posted 30 May '18 07:48 PM

Hi All
I've been a lurker on the forum, gathering info from all the really interesting posts, but had a question that I wanted to post as can't quite get an answer.

We're building in Christchurch, and will have a 5 bedroom property of about 330 sqm. We'll build so it's solar ready but are not putting PV on yet as I can't justify the cost/payback yet. Perhaps in a few years when battery costs fall (as in my opinion this is the next area of rapid evolution rather than the panels themselves) but not just yet.

I do however want to install Evacuated Tubes for solar hot water, as I'm of the opinion we'll get about 70% of our annual hot water from this source, and with a growing family of girls, this will be a massive benefit in my opinion.

So my question is, why do I see so few properties with these installed? I would guesstimate that about 30% of the new builds in the area have PV, but probably less than 10% with evacuated tubes? Is this because they are yesterday's technology, or people just don't like the idea of water circulating around their home in case of leaks etc?

Any opinions welcomed, thanks.


RE: Evacuated Tubes for Hot Water

Posted 02 Jun '18 11:53 PM

The technology is still current and a very effective way to cut your annual hot water bill by 70-80% PA. However there are other systems out there that are equally effective, eg Hot water heat pump, PV direct power to hot water cylinder element.

In the past solar hot water panel suppliers here in NZ were mostly cowboys doing shoddy installs of badly designed products; this resulted in many of these systems failing in a few years and the suppliers going insolvent or vanishing, leaving no backup and no one to fix. Thus solar hot water systems are perhaps now seen by many as a liability waiting to happen, so no one wants them. Many plumbers haven't a clue how these systems work and are of little help.

This should not be the case now, long term suppliers and installers whom are still in business would be standing by their product and providing quality installations. However quality does not come cheap, you need to get a full costing and work out how many years it is going to take before the system breaks even and you are gaining ie free hot water. In my opinion the markups on components for these systems here in NZ are perhaps excessive, so making the costs quite high. If the system doesn't break even by the time you have lived in the house and then sold it, then it is a waste of time installing it, also prospective owners may see it as a potential liability not an incentive to purchase.

To gain the most from such a system requires a bigger than normal hot water cylinder, for a family of 4-5, 400-500 liters would be optimum, solar tube array size 50 tubes minimum. Make sure the system uses a magnetic drive brush-less circulating pump (50,000 hr life).

Hot water heat pump may be more cost effective, there are suppliers in CCH, but these are not so effective in cold frosty winters with the COP very low, however a normal 280L cylinder can be used as the heat pump runs as required. After 5-7 years some components may need replacing, talk to the supplier on expected component life spans.

Direct PV power via a switching device to a second element and thermostat in the hot water cylinder is probably the most cost effective now, PV panels are now $1 per watt, there are no moving parts to fail and unaffected by frosts etc, however the roof space required is 4 time bigger as the PV panels are only 16% efficient. depending on your section, they could be pole mounted off the house so no roof penetrations required and no consents, however this tech is new, there may not be any suppliers here in NZ, there are some that combine this tech with an existing gridtie solar setup, I don't know of any that have stand alone systems. I have designed and built my own DIY system and will be installing it on a rental property, but is not for commercial sale.

I would install a wood burner with wetback, mount the HWC directly behind it in a small room (utility shed) side off the house so pipe runs are short. Burning wood is carbon neutral, so you get cheap hot water and heating for the house, Install PV to power the element in the summer when the fire isn't required.

You have lots of decisions.


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