Solar Water Heaters linked to Combi boiler

I'm a new joiner to the group so Hi everybody.

I am carrying out some research as to the feasibilty of using a solar water heater to preheat the mains pressure DHW supply to a combi boiler. The reason is obvious but if you getn avergae temperature lift averaged out over the year the savings are worth having provided the simple remain simple. This is the main aim.

The system will have a seperate loop for the solar water heater heating a tank which is mains fed and linked directly to the boiler. Thats the easy bit.

As far as I can make out most combi boilers work on a feed water temperature of around 10 degrees. They achieve delivery temperature by restricting flow through the heat exchanger so increasing the time that the water is in the heat exchanger and so allowing the temperature to be rasied sufficiently. As such, most combi's will not recognise the fact that the DHW feed water is at an elevated temperature ( say 50 degrees) this will mean that the boiler will fire regardless of water temperature so leading to the possibility of dangerously high tap temperatures being delivered.

The idea is to keep the system simple utilising a bog standard solar panel with a glycol mix heating a mains fed tank running directly to the boiler. This does away with the usual gismos that get bolted on to such systems adding expense and making it more complicated than it need be. After all, it is not rocket science. What is difficult is getting the answer as to whether general combi boilers will regulate depending on the temperature of the inlet water. I suspect they do not.

The question is then do combis recognise pre heated water or am I going to need to scope out specific combi boiler that do.

All comments very much appreciated.

Dee

Reply to
morningtonmews
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A quick Google pulls up

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"Solar combi operating procedure

Water is stored at mains pressure and preheated using the sun and solar heat collectors.

When a hot tap is opened sensors, located in the boiler, are used to measure the temperature of the incoming water.

The onboard digital controller automatically adjusts the burner output so as just enough gas is used to raise the water temperature to the set point, typically 55OC.

During the summer months the solar heated water temperature will often be higher than the required set point (>55OC).

A thermostatic blending valve, fixed before the solar Combi water connection, blends cold mains water with water from the solar water store to the required set point.

When the inlet temperature is at set point the Solar Combi Variable Controlled Output burner control feature (VCO) prevents the burner from igniting.

During the winter, or if the store of hot water is used, the water temperature in the store could be lower than the set point (

Reply to
Tony Bryer

On 22 Feb 2006 09:27:04 -0800 someone who may be snipped-for-privacy@btinternet.com wrote this:-

Heat bank, also called a thermal store. Check that the boiler can accept pre-heated water.

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Q: My family is hoping to self-build in the next few months and would be interested in installing a DIY Solartwin panel system. However, I would prefer to have a rising main water system and avoid cold water storage tanks altogether. After some unpleasant past experiences of water tanks - leakage and dead wildlife among them - my prejudice on the matter is strong enough to rule out solar heating on this grounds alone! Is it possible to fit your system without a storage tank?

A: The principle of using solar hot water, of any solar technology, is to usually store water as and when the sun shines and then use it subsequently, so a hot water cylinder, as you know, is always needed but we too would like to do away with the cold water tank (even though it no longer functions as a pigeon/bat/rat/Santa trap when kept covered). High pressure Solartwin solutions now include: using it to heat a low pressure vented heat store through which a high pressure heat exchanger at the top which then pre-feeds, via a thermostatic blender valve to the high pressure cylinder or a solar-ready combi boiler.

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Reply to
David Hansen

Welcome!

Hmm, not really. What you say might be true for an electric shower for example - but not your average combi.

The combi will (these days) typically have a modulating burner that will allow the heat input to be varied in response to the heating requirement. They will also have a limit stat that controls the maximum temperature of the output hot water. So if you set it at 50 then the boiler will add heat as required to give you that output temperature. Note however that most do *not* have any form of flow regulation. Hence if you demand more water flow through it than can be heated to the preset max temperature, all that happens is the temperature of the water will fall. Most users learn to compensate for this by turning down the flow rate at the tap until sufficient temperature is reached. Obviously the more powerful the boiler and the warmer the incoming mains water the less of an issue this is.

See above. What you want to do is (in theory) quite easy and straight forward. However you will need to carefully check the specs on the boiler you choose to ensure it maximum permitted input water temperature is not exceeded by your solar store temperature.

You may also need to choose a boiler with a wide power modulation range since in the summer it is quite possible that your solar water may not be that far off usable temperature and hence require only a small heat input from the boiler. With most combination boilers if the output water temperature is going to exceed the maximum setting on the stat, and the burner is already running at minimum power it has no alternative but to shut off the burner. This may lead to fluctuating temperature. Note that the combi boilers that include a very small store of pre heated water for "instant" hot water without the typical 40 secs warm up time are better at coping with this situation. (it is the same problem as wanting a slow trickle of hot water from a top).

They will all cope with some variation in input temperature (even ground water can swing from 5 to 15 with the change in season), but you will need to check if this is enough for your application.

Reply to
John Rumm

First what you already know - solar water heating needs a water store. Traditional gravity fed systems provide stores by default, and so do vented and unvented mains pressure systems. You just need a cylinder with an additional solar coil. However mains unvented systems, which store hot water at mains pressure are not ideal and need annual inspections.

I think what you're suggesting is storing hot water at up to 70 degrees (I'm told it gets this hot in the summer) at mains pressure - not a good idea. It may be better to have the store at low pressure and pass the mains through a heat exchanger within it, a la vented mains pressure systems. However, would you get enough heat out of it? Probably - otherwise these systems wouldn't work.

Vented mains pressure systems are expensive. So I suspect that to put something together that will work and be safe would be expensive. Then there's still the issue that combi boilers are not designed to heat water coming in at 20-40 degrees (much hotter and you wouldn't want to heat it anyway). You're probably better off ditching the combi and converting to either a vented mains pressure system or a conventional gravity-fed system if you have a decent loft and can raise the tank >1m.

Personally I can't see a solar add-on ever working with combi boilers that aren't specifically engineered for high input temperatures, and I'd be surprised if any are. However it would be great if you could solve the problem, with combis being so popular. To me it's a real shame that combis are so popular as they preclude the fitting of add-on solar systems. The cost to go solar for people with combis involves a new boiler, some re-plumbing as well as the new cylinder that is generally needed. Prohibitive for most folk no doubt, never mind having to give up some space for the cylinder.

By the way, the system I designed for my house has a conventional gravity-fed (raised tank) water system with conventional condensing gas boiler and solar (20 evacuated tubes) heating of a 210l dual-coil cylinder. Completed in September, so I'm looking forward to seeing how well the solar side of it performs this summer.

Mark

Reply to
MarkK

Hi Mark,

Your solar system is exactly the one I am considering. I'd be very gratful for any advice about which company to buy from and if you rate the offers that have come up on eBay recently for =A31000 complete evacuated tube systems. Any other advice would be most welcome. Thank you, Neil

Reply to
nafuk

there are a few possible reasons, and the reply depends entirely on which it is

depends on the above

a header fed tank would be much easier

I wonder what gizmos you mean

Simple reflectors that put 2 suns onto the solar panel make a big performance improvement all round. Cant be done with all types of collector tho. I could talk about differeing collector types and combinations, but...

To be honest there are just too many variables. Tell us why youre doing this, and more about your aims. Wehn there are at least 3 posssibilities, it is not obvious which one you have in mind.

NT

Reply to
meow2222

Mark, A post here pointed to a combi that can work with a solar pre-heater. I "think" the Japanese multi points, like Rinnai, can have preheated solar heated water.

One post pointed to a heat bank. Is it possible to bypass the combi when the heat bank has enough hot water available? This can't be that difficult to do. Interesting.

Reply to
timegoesby

This is probably quite an expensive way of acheiving an effective solar assisted HW system.

The cost of this includes a "solar water store" this is almost certainly some sort of mains unvented cylinder, or a thermal store neither of these items is cheap and they take up space.

However this system looks like it combines the best of functionality and integrates solar benefits.

There are doubtless cheaper ways of organising solar water heating but the common factor is they need a HW store of some sort somewhere.

Solar heating is a Catch-22

1) Solar heating is currently too expensive relative to energy prices to be economic. 2) Solar heating would be a lot cheaper if more systems were sold. 3) More systems won't be sold until it's economic to do so.

The only proviso is that if you diy the panels and the install, then provided your time is _really_ cheap then you could make it pay.

The skills required to install a solar HW system include a medium level of plumbing competence (E.g. exchange the HW cylinder level). A moderate ability to follow electronics and solder (to make the comparator unit).

Reply to
Ed Sirett

Hi,

If a normal combi I'd have a blending valve on the input to mix mains water and store water. This would keep the inlet temp to the combi below the max permitted, and keep the outlet temp from getting too hot.

Then I'd have a 3 port valve connected to the combi output and store output. This would be controlled by a pipe thermostat right by the outlet from the store. This would switch from the combi to the store when the store is hot enough.

cheers, Pete.

Reply to
Pete C

So far you've had a mix of good and misinformation. For example solar DHW doesnt need any new tank installing, nor do you need any electronic knowledge, nor does it need to cost much, but there really are too many options to make any suggestion at this point. Tell us more about your requirement and reasons for it first, not gonna write a book for you.

NT

Reply to
meow2222

On Thu, 23 Feb 2006 18:34:40 +0000 someone who may be Ed Sirett wrote this:-

Assuming one wants to change the cylinder. If it is well insulated and not "solar ready" then the Solartwin approach has great merits, as the most difficult plumbing is inserting a couple of tees.

If one is making one, rather then buying one. An alternative is to use a solar powered pump, in which case the pump runs when sun is falling on the panel. Solartwin and Imagination offer this approach.

Reply to
David Hansen

The point of this is to keep it simple. If the combi boiler is fed directly from a mains pressure tank that is heated by the solar WH then you do away with temperature differential controller and thermostats. I'm not interested in squeezing every last drop of energy out of it. I want a simple system that will provide a temperature lift. A reasonable temp lift will be neought o make it worthwhile. Sizing of the panel and mains tank is the critcal factor. We use the same amount of Hot water summer and winter. So achieving vast amounts of Hot water during the summer is bugger all use at the end of the day if you can't use it. No is there any point getting the temperaure of the water up to

70-80 degress when you only have to cool it down again. If the sizing is done in an intelligent manner the system can be designed to max out at a desired temp (65). Go to achieve this in the summer and forget the winter. It costs too much to try to get all year round use out of it. Never mind maximising output for winter period, this isn't France here. The associated cost incurred by persuing the extra 8-12 increase in seasonal efficiency is not worth it. Go for a reasonable year round temperature lift and save on costly installation and controls.

What I need to know is do most combis regulate the heat input depending on the temperature ofthe inlet water .

If not then I need to think about boiler bypass valves which sort of defeats the goal of simplicity.

Is the combi boiler burner either OFF or ON ? Anybody!

Reply to
morningtonmews

Thank God

Reply to
morningtonmews

Well, I'm not sure what you're thanking Him for, but I'm very happy for you that you've seen the light.

Perhaps you could try a religious group for more information?

Reply to
Chris Bacon

blah blah blah

Reply to
manatbandq

On 24 Feb 2006 05:08:11 -0800 someone who may be snipped-for-privacy@btinternet.com wrote this:-

You need to ask the manufacturers about this and a number of your other questions. Most have web sites from which all sorts of information can be downloaded.

Reply to
David Hansen

Most (if not all) new condensing combis (and non-combis) will modulate the burner in order to acheive the desired DHW/HW temp.

Reply to
Phil

Energy prices are rising like a kite. Try Navitron for cheap solar equipment. It is now economically feasible.

Reply to
Doctor Drivel

If you want simple, use the solar to preheat the header tank, and insulate it.

Youre going to lose the majority of what youve gained if you use no stat, and circulate the water thru the panel all the time. Most hot water use occurs before the sun has heated it up, so you need overnight heat storage to maximise output. That means no pumping at night, or it'll just go cold.

system design is the critical factor. If you dont make the efffort to get that right..

If you only want summer heat, a basic draindown system will give you that at a tiny cost. The cheapest solar panel is a pancake of hose on polystyrene/wood under polythene, add a pump and heat your header tank. This will get steaming hot in summer, and needs to be drained when frost approaches. 2 valves needed for that: turn feed off, and open the panel output to drain valve.

Stagnation temp will vary depending on time of year, there is no set temp.

It costs little extra to get a system that also runs in winter. You need reflectors, antifreeze and a heat exchanger.

NT

Reply to
meow2222

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