<blockquote><cite>Posted By: alec</cite>The latter I agree with, the rest is well know within the industry, we call it the pump kick to check the presence of water...system cleanliness seems to be an issue as the sensor was positioned in such a way that dirt collected in it...

but why does this issue not appear in my installs...

any way its good to know of a client who appreciates how poorly supported us installers are...

this may entertain you yet more:

http://vaillantcyclingproblem.blogspot.co.uk/</blockquote>

My installer first thought that it was dirt on the sensor, but there is a filter fitted to the sensor to reduce this possibility and he did change the sensor a couple of times (on different call-outs) just to be sure. The system was power flushed for a couple of hours or so when the boiler was fitted, too, and the water still looks very clean, even now, three years later. It's always been run with inhibitor, too. As I mentioned before, I feel sorry for the chap that installed it, as he was as frustrated as I was (plus it must have cost him a bob or two in call outs under warranty). He didn't express an opinion about Vaillant after I told him about the mod kit, or at least not one I could reasonably reproduce here..............

The problem does seem to be fairly well-known now, as this installers web site shows: http://www.vaillantservice.co.uk/ECOTEC_F.75_Fault_Code.html Interestingly this chap seems to have added the bit about the mod kit after I first read it! A quick web search through a few of the DIY sites soon finds a crop of customers complaining about EcoTECs failing to start with an F.75 code, too, some of which are down to the same pressure sensor positioning problem.

The comment in your link about Vaillant not responding mirrors my experience. I even sent a recorded delivery letter, asking them to advise on the reason for the problem, to no avail. It seems they don't talk to their installers either, which surprises me; I'd assumed they were only ignoring me because I was an end user, rather than an installer.

Posted By: alecwhy have a cylinder and a combi... the combi flow rate will have to be limited, so you get no real benefit.

Fair point re flow rate when the combi is in use (unless you go for some of the really high output models), but the mixer valve options will mean that the flow rate can be higher when no or limited preheating is needed.

Otherwise the arguments in favour of a combi are the same as usual - reduced standing losses, and reduced space requirement (you need more cylinder capacity to be able to run a solar+gas cylinder(s) at the same efficiency as a preheat cylinder+combi.

... or perhaps you just already have a combi installed, and want to add solar to it?

The other issue is that a combi has to work to a dhw set point, and its minimum output will make that impossible to acheive..say if the set point is 50c and the solar water preheated to 48c how many watts are required for the 2c difference at 10l/m

Yes, the combi will cycle in this circumstance, but if you have downstream mixer valves (I have), then this doesn't seem to matter very much in practise.

I know mixing valves for combis exist, but even they are a bit if a compromise if you want a consistent supply of hot water...

Ok, the temp (if no downstream mixer valve), and possibly the max flow rate will vary a bit, but I'd guess this normally doesn't matter much? Might bother some people I suppose...

I'm not saying it's the best+only solution by any means, but I think it is an effective option.


Tim.

Wookey, I intend to go with pre-heat plus combi (later HP combi) given the chance.

Rgds

Damon

PS. OT, just pre-ordered a CuBox to see if saving another W or two is viable for my server(s)!

Solar hot water is best for houses with high hot water usage such as families and then with a cylinder and a system boiler or other sort of top up..

The problem is the minimum heat requirement from the boiler as has been discussed... I reckon one of the reasons that their isnt much of an offer with a combi is to do with the fact that a combi probably doesn't use much less gas if the mains is 25c or 15c because of lowering the temperature of the return...

in other words raising the incoming mains in a decent combi that does condense in hot water mode is going to have nil effect on gas consumption as the lower return temp in the main heat exchanger just picks up more latent heat from the burnt gas....