Originally posted by HenGus
View Post
DHW needs a really hot flow (70 plus) to recharge the cylinder quickly to 60. My experimental evidence shows that (at least for viessman) the boiler tries to keep flow and return 20 degrees apart - but if the return gets too warm, it shuts the boiler down, there is an inbuilt limiter. what then happens in practice is that your cylinder with a target temp of 60 degrees never gets to target because the boiler keeps shutting off. it interprets return@ close to 60 degrees (which is what it will be , as your cylinder approaches 60) as "gosh the system is too hot, I better shut down". So the viessman hot water kit is to be used AS WELL AS the honeywell one - they do different things. The honeywell evo one controls the zone valve and talks to the controller. The viessman one is there purely to tell the boiler "the demand now is for hot water, so ignore your "return too hot" limiter, and just give it full whack until told otherwise".
I think its almost a generic problem with condensing / low temp target system boilers (certainly for viessman 100) and DHW. You do not get any different behaviour with an evo boiler demand BDR instead of an OT bridge (I know, I have had both on the exact same boiler and system).
I had hoped that the evohome OT bridge / OT protocol would have some additional smarts that could tell the boiler "actually the demand now is HW, please do the right thing", but it appears not.
others on here may have different experience with their hot water cylinder reheat behaviour if they have different system boilers (might have a different behaviour with high return temp), I can only speak for the way my viessman 100 behaves.
viessman cylinder demand terminal box wiring details http://www.viessmanndirect.co.uk/fil...inal%20Box.pdf
It doesn't cost a lot (compared to new boiler and all the work!) think it was order of 100 quid.
oh and there's one more option with viessman if you have the option to change your plumbing - 4 pipe. separate flow/return circuits for CH and HW, can run them at different temps thus achieving the same as I have described but on separate circuits. I'm not the expert, just read about it. This was not an option for me due to the physical layout requirements (can you run more flow/returns in the right place?), talk to your installer..
Comment