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Assuming:
1) A simple boiler without integrated pump or bypass circuits.
2) A multi-zone system with a separate hot water circuit.
I have a typical S+Plan heating system with a basic boiler and separate circulator.
I have replaced the knackered constant speed pump with a modern variable speed pump (because i couldn't get a fixed speed pump).
I am wondering how to use the variable speed option of this pump whilst maintaining a functional boiler bypass, which in my system, is currently via a ABV on the HW circuit.
My understanding is:
Modern codes of practice recommend the use of variable speed pumps to reduce energy usage.
However, variable speed pumps do not ramp up the pressure as the flow decreases, rendering the standard boiler bypass method of using an ABV pretty pointless. Bypasses are not an issue where the boiler has internal compensation (secondary HE/EX's, etc) but there are a lot of boilers out there which do not do this and as i found out, fixed speed pumps are on the wane.
I've read many posts on this forum, nothing seems conclusive. I've read the paper from the Grundfos products manager stating that variable speed is not possible without a fixed rate bypass. So, it seems i have a few options:
1) Ignore the variable speed modes on the pump and use the fixed speed modes, keep the system (with ABV) as is and take the hit on the electric usage.
2) Follow the Grundfos advice and use the variable speed with a fixed bypass in place of the ABV.
3) New boiler with integrated bypass (!)
4) Something else.....
Option 1 doesn't let me utilize the energy saving of the pump.
Option 2 (i think) effectively de-rates the boiler output and introduces its own losses (heat) on the constant bypass.
Option 3 $$$$$
Option 4 ?
I think its a toss up between option 1 or 2. I have no idea which is best, i suspect option 1 as i can't think that bypassing the boiler constantly is good for efficiency and reliability?
Option 3 will have to wait a few years.....
Option 4, i was toying with using a control valve in place of the ABV. Perhaps a 2 port valve wired in series to the normally-on wiring of the existing 3-off 2 port valves such that should, any of them be open, a 2-port bypass valve would change state from normally open, to closed (thus ensuring a fail safe condition). A bit of a faff but not something too difficult?
Or, is option 2 not such a big deal? The boiler needs 9l/min minimum flow @bypass and that seems quite a good %age of the rated flow of the pump on its DP setting.
As per the system noted, leaving a radiator open isn't possible. I don't want the unnecessary heating and this wouldn't provide a bypass should the HW circuit only be calling for heat.
So, (sorry for the long post), What is the current best practice for the utilization of variable speed pumps in a typical CH system?
Or does everyone just wing it?
Thanks for any answers.
1) A simple boiler without integrated pump or bypass circuits.
2) A multi-zone system with a separate hot water circuit.
I have a typical S+Plan heating system with a basic boiler and separate circulator.
I have replaced the knackered constant speed pump with a modern variable speed pump (because i couldn't get a fixed speed pump).
I am wondering how to use the variable speed option of this pump whilst maintaining a functional boiler bypass, which in my system, is currently via a ABV on the HW circuit.
My understanding is:
Modern codes of practice recommend the use of variable speed pumps to reduce energy usage.
However, variable speed pumps do not ramp up the pressure as the flow decreases, rendering the standard boiler bypass method of using an ABV pretty pointless. Bypasses are not an issue where the boiler has internal compensation (secondary HE/EX's, etc) but there are a lot of boilers out there which do not do this and as i found out, fixed speed pumps are on the wane.
I've read many posts on this forum, nothing seems conclusive. I've read the paper from the Grundfos products manager stating that variable speed is not possible without a fixed rate bypass. So, it seems i have a few options:
1) Ignore the variable speed modes on the pump and use the fixed speed modes, keep the system (with ABV) as is and take the hit on the electric usage.
2) Follow the Grundfos advice and use the variable speed with a fixed bypass in place of the ABV.
3) New boiler with integrated bypass (!)
4) Something else.....
Option 1 doesn't let me utilize the energy saving of the pump.
Option 2 (i think) effectively de-rates the boiler output and introduces its own losses (heat) on the constant bypass.
Option 3 $$$$$
Option 4 ?
I think its a toss up between option 1 or 2. I have no idea which is best, i suspect option 1 as i can't think that bypassing the boiler constantly is good for efficiency and reliability?
Option 3 will have to wait a few years.....
Option 4, i was toying with using a control valve in place of the ABV. Perhaps a 2 port valve wired in series to the normally-on wiring of the existing 3-off 2 port valves such that should, any of them be open, a 2-port bypass valve would change state from normally open, to closed (thus ensuring a fail safe condition). A bit of a faff but not something too difficult?
Or, is option 2 not such a big deal? The boiler needs 9l/min minimum flow @bypass and that seems quite a good %age of the rated flow of the pump on its DP setting.
As per the system noted, leaving a radiator open isn't possible. I don't want the unnecessary heating and this wouldn't provide a bypass should the HW circuit only be calling for heat.
So, (sorry for the long post), What is the current best practice for the utilization of variable speed pumps in a typical CH system?
Or does everyone just wing it?
Thanks for any answers.