Green Building Forum - Solar water heating and condensing combi boilers

Solar water heating and condensing combi boilers

dsearby — Sat, 02 Aug 2008 18:35:58 +0100

Am looking at an evacuated tube solar panel for water heating
I think the best way to do this would be to feed my existing worcester bosch condensing combi boiler - I think this is fully modulating. Anyone got any thoughts on whether this "partnership" is going to be effective?- I am pretty sure my boiler is fully modulating

Thanks

Solar water heating and condensing combi boilers

tony — Sat, 02 Aug 2008 18:40:44 +0100

you are going to need to store solar heated hot water in a cylinder or thermal store, so why not use the heating side of the combi to make this hot too?

ET's way to go.

Solar water heating and condensing combi boilers

matchmade — Fri, 10 Oct 2008 18:59:31 +0100

Hm, I asked Worcester Bosch if you could combine a thermal solar panel with a condensing boiler and they said no, because it made the input water temperature for the boiler too high. Would you clarify why your approach would work in your view?

Solar water heating and condensing combi boilers

JackyR — Fri, 10 Oct 2008 19:09:50 +0100

You CAN combine a thermal solar panel for DHW with a condensing combi: I have one. The combi only condenses when providing space heating, which is a different loop.

Solar water heating and condensing combi boilers

Dantenz — Fri, 10 Oct 2008 19:31:57 +0100

dsearby
The boiler type you refer to is not suitable to accept pre-heated water on the mains inlet side. The fact that the boiler is condensing and fully modulating doesn't make any difference. You can of course use the combi boiler not to function as a combi but as a system boiler and connect it to heat a cylinder together with solar.

Solar water heating and condensing combi boilers

wookey — Fri, 10 Oct 2008 19:45:56 +0100

Also look at the Grant combisol which is a valve that will send water to the combi if <25, direct if >60 or will mix it down to something the combi can accept if in between. Neat solution.

Also check details for your boiler. Combi suppliers will regularly say 'not suitable for water over 25 C', when they are basically just wrong and in fact it'll work fine. Does depend on the model - some really aren't suitable.

Combis not designed with solar in mind are problematic through. See JackyR's informative thread (and that ones that _are_ designed with it in mind). Using the combi as a system boiler may well be a better plan.

Solar water heating and condensing combi boilers

Dantenz — Fri, 10 Oct 2008 20:39:28 +0100

Posted By: wookeyAlso check details for your boiler. Combi suppliers will regularly say 'not suitable for water over 25 C', when they are basically just wrong and in fact it'll work fine.

Agree, more than likely will work but might get overheated hot water from the taps. Ideally, a combi receiving pre-heated water should have a sensor on the mains inlet as well as a sensor on the hot out. This will give far more accurate control thus preventing the boiler from firing at maximum initially and then having to modulate down which can cause high hot water temperatures or even cycling on/off. Perhaps more importantly, if the boiler is not approved to be used with solar then you risk voiding the warranty or worse still the manufacturer refusing to work on it.

Solar water heating and condensing combi boilers

solarguy — Tue, 28 Oct 2008 16:06:02 +0000

I have a Viessmann Combi 6-32C and a small flat plate solar water heater with a 50 gal tank (189 L) that feeds the Combi. I am in the U.S. where Combi boilers and solar are pretty uncommon so I am posting here. When the temperature in the solar tank is hot enough by itself, the boiler will fire briefly (10 seconds maybe) a couple of times in the first 30 seconds or so of water flow. It obviously isn't costing us very much for this to happen, because in the summer our gas bills are running 1-2 dollars (.5-1.5 pounds) per month. On the other hand, it seems like unnecessary wear on the boiler to fire briefly and then shut off for no reason. I wish I could tell the boiler not to run when the solar tank was hot enough, because an electric diverter valve and control costs a few hundred dollars (190 pounds) and will require re working the plumbing. Perhaps I could find the boiler's domestic hot water temperature sensor (assuming it is a thermistor) and have that circuit shorted through a resistor when the solar storage tank were hot enough to "trick" the boiler into thinking the water is hot?

Solar water heating and condensing combi boilers

JackyR — Tue, 28 Oct 2008 17:03:00 +0000

My workaround with a similar system has been to trickle the hot water through to the temp sensor before turning the tap fully on. Tedious but feasible if the pipe run tank-to-temp-sensor is short enough.

Solar water heating and condensing combi boilers

steve61155 — Tue, 28 Oct 2008 19:21:33 +0000

Using a combi for solar application is ok if you use the approved method. Take the hot water out let of the combi to a single point of use. (sink tap). Install a pressurised cylinder to all other outlets.Cap off the cylinder side of the sink tap.
From the heating flow of the combi branch off and install twin zone valves with a 22mm auto by-pass, take one valve to the cylinder (dont forget to wire the aquastat through the zone valve and differential controller to act as the 1st stage safety method) second valve to the heating flow, treat the returns from cylinder/heating as a system boiler. Wire the controls as a standard heat only system with programmer,room stat.
This system will work with out any problems associated with overheating of the combi.

What must be realised is that when the boilers are are used in a manner not recomended by the manufacture you are invalidating any guarantee or warrenty of the boiler, also because the boiler is not instaled as manufactures recomendations the house insurance may be invalid if a major problem arises.

There are combi boilers which accept the pre heat water. Alpha, manufacture, a kit, of boiler,collector and pressurised cylinder in one pack for this application. This works by the drain back method and the performance is very impressive.

RM solar the B& Q kit suppliers, (The older model 1004 Genersys collector) offer a kit for a combi, you need to check with the manufacture if the combi model will accept pre heat.

Hope this helps.

Solar water heating and condensing combi boilers

joken — Wed, 29 Oct 2008 20:59:47 +0000

all combi boilers will accept water up to 25 degrees grant combi sol has wras approval the boiler manufacturers will say no but think about it if cold main is next to a heating pipe and water is not run for a while the water may reach this temp tried explaning this to a few manufacturers but they still say no their lying

Solar water heating and condensing combi boilers

wookey — Fri, 31 Oct 2008 01:52:24 +0000

solarguy, yes you need an electrical interlock to turn the combi off when the water is hot enough. Whther it needs a thermostat to make/break some connection, or a resistor/thermister depends on how your combi works.

Solar water heating and condensing combi boilers

solarguy — Sat, 08 Nov 2008 18:38:48 +0000

What I ended up doing is putting in an automatic control to turn the combi off when the solar heated water is already hot enough, like Wookey suggested in the Oct. 31st post. I located the switch in the boiler that senses water flow and wired an aquastat (Johnson Controls A419) in line with it. I used the Normally Closed (normally switched on) contacts so when the solar tank is not hot enough, the combi operates like normal. When the tank is hot enough (I use 49 degrees C, since that is also what I set the combi thermostat to), it opens the contacts (switches off). When the water flow switch is off because the aqustat contacts are open, the hot water from the solar tank just flows through it, without the boiler firing. This approach works great and the controller was only about U.S. $70 and gives me a second temperature reading in the top of the tank.

Solar water heating and condensing combi boilers

Gervase-Cooke — Fri, 09 Jul 2010 16:02:45 +0100

"Combis not designed with solar in mind are problematic through. See JackyR's informative thread (and that ones that _are_ designed with it in mind). Using the combi as a system boiler may well be a better plan."

Can anyone point me to this thread. How do I find threads in general. I feel I'm just wandering aimlessly through this forum occasionally bumping into what I want. It's all very interesting but time is of the essence.

Thanks

Gervase

Solar water heating and condensing combi boilers

tony — Fri, 09 Jul 2010 16:31:04 +0100

try the search function

Solar water heating and condensing combi boilers

JonnySolar — Tue, 03 Aug 2010 16:32:13 +0100

Another solution is the IntaSol valve set. There is a vid on youtube which shows it working. http://www.youtube.com/watch?v=o9wTE8zvh2E

Solar water heating and condensing combi boilers

Davipon — Sat, 07 Aug 2010 14:26:36 +0100

This valve is quite usefull http://www.grantuk.com/product/22

Solar water heating and condensing combi boilers

Dantenz — Sat, 07 Aug 2010 20:19:30 +0100

Fit whatever you want at your peril. Unless a combi boiler (of which there are very few) that have approval to accept pre-heated water then your warranty will be null & void. Non-approval for pre-heated water means not exceeding 25'C on the mains cold inlet, makes solar a complete waste of time in this application!

Solar water heating and condensing combi boilers

DamonHD — Sat, 07 Aug 2010 21:14:53 +0100

Our inlet (mains) water temperature varies between 9C and 21C depending on time of year: being able to boost from winter temperatures to effectively summer temperatures with a mixer would be very valuable (40C bathwater then requires 30% less input from gas and thus 30% smaller carbon footprint) and I was seriously consider doing it.

If we can do (tankless/instant) ASHP with a similar pre-heat/tempering/mixer scheme then I'll consider it again.

Rgds

Damon