Replacing vented HW/CH cylinder, move to unvented a good idea?

[Rob]

Getting my boiler replaced with a (probably) 18KW WorcesterBosch condensing boiler. Existing HW/CH tank is about 25 years old and is the traditional vented airing cupboard fitted type with a header tank in the roof. The cylinder might need replacing so I might just as well get a solar thermal system installed at the same time.

The options are replace existing vented tank with a bigger (taller, thus loosing some storage in the airing cupboard) version with a solar coil as noticed on the Navitron web site. Or follow the suggestions of my boiler installer & solar installer and go for an unvented tank, probably installed in the roof.

So, what is the benefit of an unvented system over a traditional system apart from mains pressure hot water and no longer needing a shower pump?

I live in a very hardwater area - north wiltshire.

[Jonathan]

No free solar heating.

Jonathan

[Rob]

Care to explain further? Vented tank will have a solar coil and unvented tank will also have a solar coil. They are both compatible with a solar thermal system. Probably have a 20 tube 58mm diameter panel installed. Once installed the water heating will be free, unless I will be getting a bill from Mr Sun for absorbed radiation?

[charles]

interesting thing I noticed last winter. While all the other houses had thick snow on the roof, those with solar water heating panels had virtually none, since the system appeared to work backwards. (taking heat from the house and warming the outside)

[charles]

Yes, it happened. Unfortunately, I was on my local Christmas card delivery and didn't have a camera with me. The problem affected 5 new-build houses which were intended to be very energy saving.

[The Natural Philosopher]

Thst it really. Its also insulated and you don't need a header tank. So you can put it anywhere it will fit.

You DO need a water softener though.

Then you MUST fit a water softener.

[Adrian Brentnall]

On a solar DHW install we did at the last house, the controller had an 'anti-freeze' option which would run the circulating pump when the panel temperature dropped below a certain preset level (think it was -3c by default). The logic was that it was better to 'waste' some heat from the DHW cylinder than to replace a frozen/burst collector... Maybe that's what you were seeing...?

Adrian

[charles]

very likely.

[John Rumm]

Chances are there will be significant scale deposits in it by now.

On the plus side, mains pressure, potable hot water, no header tank, no airlocks. Better quality tank, decent showers and bath filling (if you cold main is up to it). Easily available in larger capacities.

On the down side, more expensive tank (although cheaper overall if fitting from scratch), need routine safety checks.

Fit a water softener, or consider a thermal store with an external heat exchanger. Makes it simpler to descale when required and the tank will not scale at all. Also very easy to aggregate different heat sources.

[cynic]

rry

r

My solar hot water collector circuit is filled with anti-freeze mix. Absolutley no point in throwing heat away in the manner you describe

[dennis]

Or more likely the snow slid of the panels, decreased the albedo, so absorbed the sun and cleared more of the snow.

[dennis]

Why? You either add antifreeze or you make them self draining so there is nothing to freeze.

[Adrian Brentnall]

I couldn't say - that's how the controller (Thermomax) was installed, and I recall the engineer who commissioned it making mention of it.

A quick Google shows that this certainly was the case - but it was a few years back - maybe things have changed ?

"In climates where freezing occurs infrequently, a recirculation-type differential control will turn the circulating pump on when the collector inlet temperature falls to 40°F (4.4°C). The philosophy behind this design is that the cost of heating your collectors with hot water from your tank is low cost freeze protection if only required occasionally."

Adrian

[charles]

what sun? It stayed behind the clouds.

[charles]

[Snip]

There was sufficient snow to block any air passage under the panels.

[Peter Parry]

No, what happens is that even when not warm enough to produce worthwhile heat for the house the solar panels still absorb enough IR to keep their ambient temperature above freezing. Combined with a smooth surface snow either melts as it lands or slides off.

Most if not all modern solar controllers only allow water circulation if there is going to be a transfer of heat from outside to inside.

[Peter Parry]

Even on a heavily overcast day there is enough IR to warm a panel by quite few degrees if no heat is being transferred away. If you have an IR non-contact thermometer it easy to show - put out a matt black panel and a white shiny one on an overcast day and measure their respective surface temperatures after half an hour.

[Mark]

+1 vote for a thermal store.

[Jonathan]

Sorry I misread your original post and mistakenly assumed you were talking about a combi boiler.

Jonathan

[Jonathan]

y

ar

Then they must have been badly fitted. most have a thermostat so they only work when the panels are hotter than the water being heated.

Jonathan