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hi
I have a DPS pandora heatbank 2005 vintage with DHW plate heat exchanger driven by pump from the heatbank water, primary coil fed from boiler, secondary coil fed from solar thermal, sealed pressurised boiler circuit (expansion built into the top of the pandora) , S-plan plus.With my previous 1990's vintage non-condensing gas boiler driving it, I never had any problems with recovery times or large availability of DHW, running the store at 70deg with this system. The old boiler would run at 80deg or so output all the time, and it got the job done.
I've recently upgraded the system thus:
viessman vt100 wb1b 26kw boiler, honeywell evohome HR92 on every rad (18 of them) evohome DHW stat and controller (single zone valve for HW, the two CH zone valves removed at honeywell recommendation), feeding honeywell opentherm bridge that controls the boiler. The CH system works extremely well, the boiler runs at full blast when the house is cold and all the rads come on, and modulates down nicely when the house is warm, the evohome gives me nice control in every room. all very nice.
however. The DHW is now rubbish. the reheat time from cold (e.g. been off for a week) is a couple of hours at least. fortunately I can use the evohome to switch it on when on the motorway well in advance of getting home!
One key fact is I cannot set an achievable heatbank internal temp of higher than 64 . what happens if DHW only is on, CH is off, the boiler temp very slowly goes up (burner is usually on power level 1, less than a degree a minute) , seems to be tracking the return temp coming up (I believe the viessman tries to maintain a 20 deg delta), hits approx 75 for a minute at most, then it switches off the burner and drops down to 65-67 ish water going round in circles . with the burner OFF (never comes back on). Thus with a setpoint of 64 I can at least achieve a shutdown, this 65 degree water , after about 20 minutes more, gets the store to 64 and it switches off.
If I set the bank temp to the 70 recommended by DPS, it never gets there. and the effect of only having 64 in the heatbank is that I have less DHW, and can run out in the middle of running a bath...
worse, when I have the CH on, the cool water coming back from the rads seems to make the boiler run at low flow temps, and low power which actually cool the heatbank down if the zone valve is open. The system control never seems to be able to (in human terms) say "we need hot water, we need it now, so run at full power please". I have never seen the boiler run full blast for more than a few seconds based on the DHW demand. And because of all the above, recovery time is crap - if it the heatbank needs to recover whilst the CH is in cool low power mode, it frequently never achieves it (it in fact often cools down), it will only do so once all CH has gone off.
I have had ongoing discussions with my installers since it went in as I have not been happy with the change (from DHW point of view). however not really got anywhere yet.
now had a recommendation from honeywell to put master zone valves back on , and implement interlock using the evohome controls to effectively do Y-plan, i.e. shut all CH down when DHW demands. This seems like a workaround at best, and doesn't deal with the core issue of why my boiler temps won't go up fast and stay up.
Is there something about condensing boiler design that isn't suited to recovering a DHW load to a high temp quickly? I understand that the return temp has to be low for condensing, but why should a low return temp mean a low flow temp ? why can't the fact that we have a large DHW demand tell the boiler "yes your return may be at 30, but please, give us full power on the burner and flow at 75+ and just get on with it"? or is there something not setup right? At the moment I feel like my nice heatbank has been ruined by the introduction of the viessman condenser.
can anyone provide learned advice please?
thanks
I have a DPS pandora heatbank 2005 vintage with DHW plate heat exchanger driven by pump from the heatbank water, primary coil fed from boiler, secondary coil fed from solar thermal, sealed pressurised boiler circuit (expansion built into the top of the pandora) , S-plan plus.With my previous 1990's vintage non-condensing gas boiler driving it, I never had any problems with recovery times or large availability of DHW, running the store at 70deg with this system. The old boiler would run at 80deg or so output all the time, and it got the job done.
I've recently upgraded the system thus:
viessman vt100 wb1b 26kw boiler, honeywell evohome HR92 on every rad (18 of them) evohome DHW stat and controller (single zone valve for HW, the two CH zone valves removed at honeywell recommendation), feeding honeywell opentherm bridge that controls the boiler. The CH system works extremely well, the boiler runs at full blast when the house is cold and all the rads come on, and modulates down nicely when the house is warm, the evohome gives me nice control in every room. all very nice.
however. The DHW is now rubbish. the reheat time from cold (e.g. been off for a week) is a couple of hours at least. fortunately I can use the evohome to switch it on when on the motorway well in advance of getting home!
One key fact is I cannot set an achievable heatbank internal temp of higher than 64 . what happens if DHW only is on, CH is off, the boiler temp very slowly goes up (burner is usually on power level 1, less than a degree a minute) , seems to be tracking the return temp coming up (I believe the viessman tries to maintain a 20 deg delta), hits approx 75 for a minute at most, then it switches off the burner and drops down to 65-67 ish water going round in circles . with the burner OFF (never comes back on). Thus with a setpoint of 64 I can at least achieve a shutdown, this 65 degree water , after about 20 minutes more, gets the store to 64 and it switches off.
If I set the bank temp to the 70 recommended by DPS, it never gets there. and the effect of only having 64 in the heatbank is that I have less DHW, and can run out in the middle of running a bath...
worse, when I have the CH on, the cool water coming back from the rads seems to make the boiler run at low flow temps, and low power which actually cool the heatbank down if the zone valve is open. The system control never seems to be able to (in human terms) say "we need hot water, we need it now, so run at full power please". I have never seen the boiler run full blast for more than a few seconds based on the DHW demand. And because of all the above, recovery time is crap - if it the heatbank needs to recover whilst the CH is in cool low power mode, it frequently never achieves it (it in fact often cools down), it will only do so once all CH has gone off.
I have had ongoing discussions with my installers since it went in as I have not been happy with the change (from DHW point of view). however not really got anywhere yet.
now had a recommendation from honeywell to put master zone valves back on , and implement interlock using the evohome controls to effectively do Y-plan, i.e. shut all CH down when DHW demands. This seems like a workaround at best, and doesn't deal with the core issue of why my boiler temps won't go up fast and stay up.
Is there something about condensing boiler design that isn't suited to recovering a DHW load to a high temp quickly? I understand that the return temp has to be low for condensing, but why should a low return temp mean a low flow temp ? why can't the fact that we have a large DHW demand tell the boiler "yes your return may be at 30, but please, give us full power on the burner and flow at 75+ and just get on with it"? or is there something not setup right? At the moment I feel like my nice heatbank has been ruined by the introduction of the viessman condenser.
can anyone provide learned advice please?
thanks
