Solar water heating and combi boilers

I was wondering this as well, since the install manual made no reference to combimate type devices. (although to give them credit they also neglected dribble style pet elctro/magnetic rocks as well).

Reply to
John Rumm
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Not on the system Mary was talking about - it takes cool water from the cold DHW *feed* at the base the cylinder, runs it through the solar panel (using a small PV powered solar pump connected via microbore pipe), and returns it to the top of the cylinder via the DHW/vent outlet.

Hence the panel and pump are going to be being a reasonably frequent supply of fresh water.

This seems to be one of the more useful documents:

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only thing I can't see is how they prevent the solar pump running when the cylinder temperature already exceeds the panel output temp...)

Reply to
John Rumm

To save potential complications from feeding the combi with prewarmed water (which could in some cases be *hot*), you could do:

--------- mains cold | combi |

--------------------->--| DHW |--->---- --------- | ----- | H | | TMV |---->--- | C | ----- preheated water from store |

---------->--------------------------------

(with optional additional blending valve at the output as you suggest)

Or if you want to take advantage of the prewarmed water for the combis DHW input you might need:

--------- mains cold | combi |

-------------- ->--| DHW |--->----- | | --------- | ----- | ----- | C | | | H | | TMV |- | TMV |---->--- | H | | C | ----- ----- | | | | preheated | | water | | | |

---------->---------------------------------

Set the output temp of the first TMV so that it does not exceed the maximum inlet temp on the combi.

Obviously you will use energy from the store faster this way and will reach the point where the combi is supplying the bulk of the heat sooner (and hence you may need to reduce flowrate to maintain temperature)

Reply to
John Rumm

On Tue, 16 May 2006 17:18:10 +0100 someone who may be Peter Parry wrote this:-

Some boilers have pumps, some of these also have controllers which will activate the pump for short periods to prevent it sticking.

Valves tend to be outside the boiler. If they are sticking often then water treatment may need looking at.

That corrosion is largely caused when the boiler is warming up and acid is being deposited on the surfaces. Warm it up less often and there is less of it, prolonging boiler life.

I pointed out the specific circumstances in which it would save more gas than people think in my original posting. Obviously if the boiler has electric ignition then the saving will not even be that of turning off a pilot light for some months.

The claim people make in this group is that at current prices solar water heating is a long term financial investment. Discussing your distortions of the claims people make does not help your argument one bit.

You have already been pointed to the government report which states that 17-23% of the carbon dioxide benefits can be lost in pumping and running the control system 24 hours a day from mains electricity. BTW, unless variable speed pumping is in use it takes more than a few watts to run the pump.

Against this one has to put the environmental cost of producing the solar panel. However, one also has to allow for the fact that solar powered pumping has stimulated people to think of low water volume systems, with relatively slow speed pumping.

A quick reality check on your assertion is to note what the manufacturer of the system most mentioned has to say on this

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"Need I control water hardness?

"Control for ?direct? Solartwin's is uncomplicated. First phone the water company for ?parts per million calcium carbonate? (also called ?ppm CaCO3?). If they quote ?typical? or ?average? (not maximum) figures, allow 20% tolerance i.e. add 20% to their figure i.e. treat a single figure of 200 as 240.

"* If hardness ever exceeds 200 ppm CaCO3 (very hard water) use either an ion-exchange (salt-regenerated) water softener or an ?indirect? Solartwin. We supply and fit softeners. Please call.

"* At 100 - 200 (maximum) ppm CaCO3, you can use Fernox Superconcentrate Limescale Preventer. Your first little bag of crystals on a string is free. Simply hang it in the cold tank. Replace it twice a year. A free 6-monthly postcard reminds you. Fernox SLP is food grade. It costs about £10 from DIY shops. Or you can use an ion-exchange water softener or an ?indirect? system.

"* Under 100 ppm, and with ?indirect? Solartwins, use no hardness control.

"All other water hardness treatments or conditioners, including electromagnetic, magnetic, electronic, physical or ultrasonic methods must not be used. The above 239/240 ppm maximum thresholds are reduced to 199/200ppm respectively for all ?fortic? type hot water cylinders as well as irregularly used hot water systems, such as in holiday homes."

Reply to
David Hansen

The message from Steve Firth contains these words:

Cor - got any more?

Reply to
Guy King

"Best practice in water hardness control is more important for direct than for indirect solar water heating systems. Direct solar water heating systems may need robust hardness control: and water softeners may be difficult to install in a few properties. Unless they are plumbed ?indirectly?, water harness control for direct solar is usually achieved by polyphosphates where the water hardness is

100-200 ppm CaCO3 and by the use of an ion exchange softener above this figure. Ion exchange water softeners are thus regarded as essential best practice...

Barry Johnston, MSc Managing Director, Solartwin.com"

Above 200 mg/l as calcium carbonate equivalent includes most of Eastern England in an area bounded on the west by Southampton, Bristol, Birmingham and Lincoln.

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Reply to
Peter Parry

---8 ---------

Reply to
John Stumbles

Solar heating of water and combi-boilers do not mix.The combi is designed to operate on at least 1.2 bar mains water pressure, and water heated by solar means would not be able to deliver this as it would have to be stored before delivery to the boiler. What's the point? A combi boiler is specifically designed for efficiency anyway. Les.

Reply to
Les

If a boiler does _not_ have electric ignition but has a constant pilot light then it is a dinosaur with seasonal efficiency in the region of 65%. The controls are likely to be primitive, too. It will make far more sense to replace it with a high-efficiency boiler with updated controls than put in solar water heating.

(I'm not talking about the OP's installation which, we've already ascertained, is a condensing boiler with proper controls.)

Reply to
John Stumbles

The boiler casing (the outside box) doesn't have acid deposited on it as it is only exposed to incoming air. It does get condensation and damp from the outside atmosphere (to which it is open in a room sealed boiler) when unused. As it is a spare part which is almost impossible to get for older boilers corrosion on it often ends their life.

The "government report" also signally failed to mention that on a cold sunny day with little solar gain the device also happily pumps your gas heated hot water into a nice big radiator on the outside of the roof to cool it down. What is the energy cost of this?

The tiny little Solartwin pump I have at the moment appears to be about a 5W max unit, I'm not sure if this is their common pump (I think this panel came from the trial you refer to).

east of that line have water with more than 200ppm hardness.

Reply to
Peter Parry

I see David Hansen has quoted solartwin's recommendation of phosphor-dosing for all but the hardest water.

Reply to
John Stumbles

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I've now found a place that will supply me factory direct and I'm thinking of importing them myself. There are several different models available either with a 300 litre storage tank, or with a 300 litre thermal store. The thermal store seems like the better option to me, since the unit is sealed, doesn't use a pump and is simpler to install.

Reply to
Steve Firth

That's err a bloody stupid system.

Reply to
Steve Firth

These TMVs, are they something special or the same as shower mixer thermostatic valves?

The above circuit looks suitable to use even with an indirect cylinder, where the Combi is, in the case where the thermal store hasn't got hot enough for some uses or the cylinder is still hot from the night before, the gas boiler having topped it up. So rather than draw hot from the cylinder which may subsequently need heating up by gas this circuit could bypass the indirect cylinder and make better use of the solar heat? It does require both a solar thermal store and a hot water cylinder though.

AJH

Reply to
AJH

The message from "Les" contains these words:

The point is that however efficient the boiler is, the cost of heating water can be reduced further by warming the feed.

Reply to
Guy King

On Wed, 17 May 2006 10:14:10 +0100 someone who may be Steve Firth wrote this:-

No, it's a system that has advantages and disadvantages just like anything else. Proponents believe the former outweigh the latter.

Reply to
David Hansen

On Wed, 17 May 2006 08:59:20 GMT someone who may be John Stumbles wrote this:-

Who said anything about this being an either/or question?

Reply to
David Hansen

On Wed, 17 May 2006 10:01:07 +0100 someone who may be Peter Parry wrote this:-

Perhaps you have experience of boilers in corrosive atmospheres, the seaside perhaps, but I have yet to see a boiler in a house suffering from this alleged problem.

Already covered by

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"Q: During the winter months when most of the hot water will be generated by the boiler, what happens if there is enough sun to set the pump in action. At which time there is not enough thermal energy to heat up the water sufficiently so that when it is returned to the hot water tank it reduces the temperature there?

"A: The pump runs only on solar energy in the form of electricity. It has no temperature sensor, only a high pressure bypass in case the panel or its pipes are frozen. In response to your question: First - best not to have the hot water system on all day since this is wasteful anyway and does not allow for optimum solar performance. Most boilers have separate timers for this, but not all. Ideally time the boiler to add heat to the domestic hot water after 4pm. Second, even in winter some hot water is made by Solartwin, not all by the boiler as you say. Third - the panel is well insulated and so will still raise the temperature of water going into it since it collects heat from the sun and not the air. Fourth - at 100% sun and a water input temperature of 50C and air temperature of freezing our mathematical model (based on extensive tests at Napier University) suggests that the water will still leave the panel at least 10C hotter than when it went in. Fifth - if they really want to put cold water in under these circumstances they can connect a second cylinder behind the first and draw water off it! This will also allow for more summer hot storage and is a neat solution for people with AGAs and Rayburns. "

The panel is limited to 4W output and the pump under-run. However, the fact that this system uses low power pumping does not mean that they all do. Even with variable speed pumping I suspect that 50-100W is a fairly typical electricity consumption for a mains powered system.

I have left in what you said, "little or no mention of the fact that for most people living east of a line drawn roughly from Lincoln to Bristol you also need to install an ion exchange water softener".

In response I pointed to the most well known supplier of solar pumped panels, where they explicitly talk about water hardness. Therefore your statement that they make, "little or no mention", is incorrect.

Reply to
David Hansen

On 17 May 2006 01:45:30 -0700 someone who may be "Les" wrote this:-

Water heated by solar would not be able to be at this pressure? Why?

It could be stored in a thermal store, with the supply to the boiler being separate from the stored water.

1) A combination boiler burns gas, the price of which is going up and is likely to continue going up. 2) The price of sunshine is not going up and is unlikely to go up. 3) Therefore it seems sensible to use sunshine as the fuel as much as possible.
Reply to
David Hansen

However not covered,measured or even mentioned by the report you quoted.

Most households want hot water in the morning which means the tank will start hot (unless someone has a remarkably effective water consumption predictor or the last person likes a cold wash).

What about the more realistic situation of less than 100% sun where the pump is running slowly but the heat gain is minimal? Today with an air temperature of about 15deg and solid overcast the solartwin pump is managing to fitfully pump a trickle of water but the inlet temperature and outlet temperature are the same (26 deg +- 0.1deg). If it was cold that would mean hot water would be circulated through the panel and heat loss (the "well insulated" front is simply twinwall polycarbonate).

Whenever an advantage can be gained from a figure it appears - whenever it's a disadvantage we get subjective descriptions. Suppliers of gas boilers have to give comprehensive performance data to buyers and their efficiency is independently checked. Why are "alternative" suppliers not obliged to do the same and give measurements of performance? More to the point why are they so unwilling to supply this data if their products are as good as they say?

Which rather defeats the advertising about a "simple to install" system.

If 4W is adequate to transmit all the power collected by the solar panel on a bright day in midsummer why do others need (or chose) to use much larger pumps?(My 30 year old system used a mains powered 15W pump made specifically for solar hot water pumping).

That seems rather high - a controller should be no more than 3W and the pumps I've seen recently are generally about 30W.

Ah, I had misunderstoop your meaning, I had assumed you were talking about the relative numbers who could use phosphate dosing rather than an ion exchange softener.

Reply to
Peter Parry

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