- Messages
- 36
I have an open-vent S-Plan CH system (22 radiators, 2 DHW cylinders and long pipe-runs) driven from a new 35KW Viessmann 100W WB1B boiler. When most TRVs were open, the Grundfos UPS2 15-50/60 pump was struggling to keep an adequate flow through the boiler (the latter modulated and cycled excessively) so I changed it for its big brother the UPS2 25-80 which I currently run at speed ‘1’ in constant speed mode.
I also added a Honeywell DU144 ABV (the previous boiler didn’t use pump overrun but the new one does).
To determine the ABV’s setting, I needed to estimate a minimum flow when (e.g.) the room thermostat stops demanding heat or the programmer switches ‘Off’ (i.e. when all the flow would be directed through the ABV) . The boiler’s recommended minimum flow is 0.4 cu.m/hr. Using this and the pump’s speed ‘1’ head of 6m (which is constant for 0 - 2.3 cu.m/hr flows), the ABV’s setting chart gives a setting of 0.5, which is what I am now using.
If this isn't total rubbish, it leaves me wondering how to set the ABV if I want to use a faster pump speed (say speed ‘3’, which gives an 8m head). The ABV setting chart says that for an 8m head and with the largest available setting on the ABV (0.6), the flow rate is 0.8 cu.m/hr - nearly 60% of the recommended maximum for the boiler!
I just think I am misunderstanding something very basic!
I also added a Honeywell DU144 ABV (the previous boiler didn’t use pump overrun but the new one does).
To determine the ABV’s setting, I needed to estimate a minimum flow when (e.g.) the room thermostat stops demanding heat or the programmer switches ‘Off’ (i.e. when all the flow would be directed through the ABV) . The boiler’s recommended minimum flow is 0.4 cu.m/hr. Using this and the pump’s speed ‘1’ head of 6m (which is constant for 0 - 2.3 cu.m/hr flows), the ABV’s setting chart gives a setting of 0.5, which is what I am now using.
If this isn't total rubbish, it leaves me wondering how to set the ABV if I want to use a faster pump speed (say speed ‘3’, which gives an 8m head). The ABV setting chart says that for an 8m head and with the largest available setting on the ABV (0.6), the flow rate is 0.8 cu.m/hr - nearly 60% of the recommended maximum for the boiler!
I just think I am misunderstanding something very basic!