hi all

apologies for the long first post, here goes:

I have the following system
DPS Pandora thermal store / heat bank for DHW, zone valve controlled by evohome hot water kit
19 radiators all with HR92ís (180sq m house)
Viessman 100-W 26kw heat-only boiler
Evohome controlling boiler with Opentherm bridge
Bypass loop (which I have checked is not activating in the below scenarios)

All of the Evohome and the new boiler were installed by evohome certified installers in November 15.
I'm measuring temps with an maplins IR spot thermometer at close range, with black insulating tape on the pipes, moving it to find the hottest point on that pipe.

Prior to the upgrade, I had the pandora with S-Plan plus (we had DHW zone + 2 zone valves for up and downstairs, the latter were removed during the evo install) , standard TRVís on all but 1 rad per zone, and a non-condensing 1990ís potterton boiler. During 10 years in the house with that previous system I never had any problem with hot water availability, the only problems were due to breakdown due to boiler age, and some rooms not heating as well as would have been liked due to distance from the stat.

Since the upgrade to evohome and new boiler, the heating comfort level has gone up massively (I can now run the rooms far away from the pre-upgrade zone-stat locations at the temp I want them) however my hot water performance is rubbish. My installers have been back several times and canít figure it out, I am getting to the point of needing to ask the wider world for help, so here I am.

My system has a state, which I have seen multiple times, where in the evening the house is close to heating setpoint , and so the boiler is backing off, running at 50 degrees or so flow, firing every now and then on power level 1 to keep the system just topped up. Which would be fine Ė except the hot water store is also calling for heat with a target of 60+! So the temp of the flow side is actually cooling the DHW down! It never seems to figure out that it needs to fire up to max , to get the store heated. The store worked fine with the old boiler (boiler flow temp was at 80+ I guess when old boiler set to max), but with the new boiler / evohome / opentherm control, it will never ever get anywhere near target. We canít have baths any more, and can only shower when we are sure the tank has been charged in advance of having the heating on.

The only way I can reliably get hot water is to effectively run a manual hot water priority approach Ė time the hot water to come on without the heating, or if I find we need more hot water in the middle of heating being on, put the heating into eco or away mode using quick action. This is not family friendly and could arguably be devaluing my house. so I am pretty fed up with it.

Iíve done testing, when the system is running hot water only, it drives the return temp up as the tank heats up, which drives the flow temp up (because I am told the viessman tries to maintain a 20degrees delta between return and flow), so it will then heat the store, but only to a certain point.

with the hot water zone by itself, the hottest flow the boiler will run and still keep the burner on is 76. If it goes to 77, with return temp around 64 (store has reached 64), it shuts the burner down, and ends up in a steady state pumping water round in circles at 66 flow, 66 return, with the tank at 66. This can go on forever with intermittent small burns that are never big enough to hit the setpoint of 70 . Being a thermal store the recommended setpoint is 70 not 60.
Because the tank never reaches it target temp of 70, and the boiler never supplies enough heat for long enough to get the tank up to temp so it can shut down, I have to set a lower target temp (64 is the best it can do consistently) for it to be an actually achievable target so it doesn't run for ever, and this target can take a long time - even with the lower target if we need more hot water when the heating is on and in the "topping up" stage with flow at 50-ish, then we usually can't get any more hot water without manually stopping the heat demand completely.

contrast this with the old boiler I could run at a setpoint of 75 or even 80 (if the wife wanted an uber-bath) on the store no problem, and there was no dependency between hot water and heating state , I could have both on at the same time no problem . And this isn't boiler power issue - the old boiler was the same rating give or take, and the new one is running at its lowest modulation level most of the time (which is the issue). I feel like I've stepped back as far on the hot water as far as I've stepped forward on the heating with the new system.

I understand the physics of the condensing targets, however I don't understand why is there not a function in the evohome/openflow to say to the boiler words to the effect of ďwe need the highest flow temp you can do and we need it now , because the demand is the hot water, so stop this low power modulating/condensing/target delta business and just give us max power pleaseĒ ? or does this function in fact exist and my system is not working / setup properly?

I have the boiler output control dial (the only thing available on it ) set at max, but this seems to only ever be an upper limit that is reduced by the system depending on its mood, rather than the actual output temp.

any suggestion please?