Fixed Speed or Modulating (CH/HW Circulation pump)

[marshian]

Recently changed the house boiler

 

From Flexicom 24HX (24kW with a 2.4 Modulation - ie 10 kW min) (Set up as Y plan)

 

To Viessmann 100W 16 kW with a 5.0 Modulation - ie 3.2 kW min) (Currently set up as S Plan)

 

Both are "heat only" boilers but the Viessmann (once the rest of the parts arrive) will be set up to run DHWP

 

During the commisioning process the old Grundfos 15/50 pump speed lever sheared/failed and the pump was jammed on max speed. I was going to replace it with another one identical but it seems only refurbished ones are avaliable so as it's 20 years old and given excellent service I'd rather get something new.

 

I've now replaced it with DAB Evosta3 which has three modes of operation

 

Regulation with Proportional Differential Pressure (3 Speed or 6 Speeds)

 

Regulation with Constant Differential Pressure (3 Speed)

 

Regulation with constant curve (3 Speed)

 

My brain can't cope with this level of options

 

House has 13 rads all with TRV's and there is a ABV in the airing cupboard that will need adjusting once the autumn/winter/spring heating requirement kicks in

 

I currently running it on pump speed 1 as I'm only using the boiler once a day for HW

 

Any help in understanding what would work best

[atlantication]

Variable speed is good for improving efficiency. Lowering the speed when the boiler isn't working at 100% is good to lower the pump electricity consumption and also raise the boiler efficiency due to the lower return temperature increasing condensing. However it also reduces the valve authority of your TRVs and can result in an unbalanced system and some rooms getting insufficent heat input.

 

The problem comes from your bypass valve. In an ideal world there would never be flow through the bypass as this is pointless and reduces efficiency. However with TRVs and an incorrectly set weather compensation curve it's possible for boiler flow to reduce below the minimum without the bypass valve. Thus the bypass valve and constant speed pump. As flow reduces, differential pressure increases until the bypass valve opens assuming the pump speed and valve pressure preset are correct.

 

Having a constant-pressure or proportional-pressure pump in principle breaks the pressure-operated bypass valve as bypass would either be constant, or increase with the flow through the rest of the system (precisely the opposite of what you want!).

 

If I were in your position I would ensure I had weather comp set up correctly and the TRVs adjusted right (so the bypass should never bypass), then set the pump to the lowest constant pressure curve such that all the radiators get warm and the boiler is happy. I wouldn't bother with proportional pressure unless I had a badly designed system with microbore or something. OTOH, keeping it in constant speed mode with the bypass valve set to bypass at low flow will work fine too, and there isn't really that much difference in the running costs/noise levels.

 

This whole thing sort of highlights the benefits of getting a system boiler rather than heat only - the boiler controller can control the pump to ensure optimal flow for efficiency but also that it meets its minimum requirements.

[JohnMo]

3 hours ago, marshian said:

once the rest of the parts arrive) will be set up to run DHWP

You really should state you want Weather Compensation also. Then either dump most the TRVs and run single zone. Then you set the pump flow to match the need of all the radiators. You should get a 25% reduction in gas consumption, because most the time the boiler is in condensing mode. You also no longer need bypass valve.

[marshian]

5 hours ago, JohnMo said:

You really should state you want Weather Compensation also. Then either dump most the TRVs and run single zone. Then you set the pump flow to match the need of all the radiators. You should get a 25% reduction in gas consumption, because most the time the boiler is in condensing mode. You also no longer need bypass valve.

 

Sorry should have stated that DHWP was because I'd had the boiler set up with weather compensation - Long story but the Viessmann specialist didn't quite understand what I was asking for and didn't even know about the HW demand box that tells the boiler it's doing HW

 

Currently S Plan with NC valve for HW and NC valve for CH (this one is being swapped out for NO valve to finish the set up properly)

 

I'm reluctant to dump all the TRV's yet but if I find I can run without and just keep the EB4 Bodies which have internal flow limiters and put a decorators cap over them I will

 

By Pass valve is a legacy of the old boiler which unless I had a little bit of flow thro it the boiler stalled in warm up and short cycled like crazy once the house was coming up to temp due to 4.5 kWh heat loss and 10 kW min boiler output was planning on having it set so it only opens when there is just one rad still calling for heat.......

[marshian]

6 hours ago, atlantication said:

Variable speed is good for improving efficiency. Lowering the speed when the boiler isn't working at 100% is good to lower the pump electricity consumption and also raise the boiler efficiency due to the lower return temperature increasing condensing. However it also reduces the valve authority of your TRVs and can result in an unbalanced system and some rooms getting insufficent heat input.

 

The problem comes from your bypass valve. In an ideal world there would never be flow through the bypass as this is pointless and reduces efficiency. However with TRVs and an incorrectly set weather compensation curve it's possible for boiler flow to reduce below the minimum without the bypass valve. Thus the bypass valve and constant speed pump. As flow reduces, differential pressure increases until the bypass valve opens assuming the pump speed and valve pressure preset are correct.

 

Having a constant-pressure or proportional-pressure pump in principle breaks the pressure-operated bypass valve as bypass would either be constant, or increase with the flow through the rest of the system (precisely the opposite of what you want!).

 

If I were in your position I would ensure I had weather comp set up correctly and the TRVs adjusted right (so the bypass should never bypass), then set the pump to the lowest constant pressure curve such that all the radiators get warm and the boiler is happy. I wouldn't bother with proportional pressure unless I had a badly designed system with microbore or something. OTOH, keeping it in constant speed mode with the bypass valve set to bypass at low flow will work fine too, and there isn't really that much difference in the running costs/noise levels.

 

This whole thing sort of highlights the benefits of getting a system boiler rather than heat only - the boiler controller can control the pump to ensure optimal flow for efficiency but also that it meets its minimum requirements.

 

Useful comments I'll try to get my head round that - thank you

 

Rad circuit is very well balanced (EB4 TRV bodies with flow restrictors so every rad pretty much gets the flow it needs at a sensible flow temp)

 

Main flow and return is all 22mm copper with just 15mm copper tails from the flow and return to the rads (it's all lagged below the ground floor with decent foam lagging) no microbore or plastic anywhere.

 

I'm pretty sure I know what to set the weather compensation curve up from a slope perspective the last few years I've managed to bring the previous boiler flow temp down from 80's to low 50's but any lower was pretty much a bust as min flow temp was 39 (38 was off) and even range rated to 10kW it's initial fire up was 75% of it's max (24kW) so it very quickly shot past the set point if less than 50 was tried

 

The Viessmann system boilers are four pipe (I'm pretty sure) as well as not fitting inside a 300mm Kitchen wall cupboard - I didn't want the disruption of ripping up floors to run a second flow and return for the HW element of the boiler when a heat only with DHWP can do the same with a two pipe set up.

[John Carroll]

19 hours ago, marshian said:

Recently changed the house boiler

 

From Flexicom 24HX (24kW with a 2.4 Modulation - ie 10 kW min) (Set up as Y plan)

 

To Viessmann 100W 16 kW with a 5.0 Modulation - ie 3.2 kW min) (Currently set up as S Plan)

 

Both are "heat only" boilers but the Viessmann (once the rest of the parts arrive) will be set up to run DHWP

 

During the commisioning process the old Grundfos 15/50 pump speed lever sheared/failed and the pump was jammed on max speed. I was going to replace it with another one identical but it seems only refurbished ones are avaliable so as it's 20 years old and given excellent service I'd rather get something new.

 

I've now replaced it with DAB Evosta3 which has three modes of operation

 

Regulation with Proportional Differential Pressure (3 Speed or 6 Speeds)

 

Regulation with Constant Differential Pressure (3 Speed)

 

Regulation with constant curve (3 Speed)

 

My brain can't cope with this level of options

 

House has 13 rads all with TRV's and there is a ABV in the airing cupboard that will need adjusting once the autumn/winter/spring heating requirement kicks in

 

I currently running it on pump speed 1 as I'm only using the boiler once a day for HW

 

Any help in understanding what would work best

You have wisely choosen a pump that displays the flowrate in m3/hr which you may find very useful when you commission the CH, I have tuned a few relations systems with/without  WC and I set the flowrate to 1.0LPM per KW of rated rad(s) output, your system, assuming total rated rad output of 15kw would require a flowrate of 15.0LPM (0.94m3/hr), would suggest setting the pump on CC2 (constant curve) "speed 2" or CP2 (constant pressure) both 3M pump head,  and see how it goes, the setting can, if required be increased to CP3 (4.5M pump head.)

 

I would also suggest that you install a digital roomstat somewhere, ( I have mine in the combined two rooms where we spend most of our time), Viessmann seem to think that WC alone is only required?  but again IMO, the OT can fall or rise by 6 or 8C over a 1 or two hour period which can result in over/undershoot of the room temps, at least you have a master control with one roomstat.

Edited August 22 by John Carroll

[marshian]

1 hour ago, John Carroll said:

You have wisely choosen a pump that displays the flowrate in m3/hr which you may find very useful when you commission the CH, I have tuned a few relations systems with/without  WC and I set the flowrate to 1.0LPM per KW of rated rad(s) output, your system, assuming total rated rad output of 15kw would require a flowrate of 15.0LPM (0.94m3/hr), would suggest setting the pump on CC2 (constant curve) "speed 2" or CP2 (constant pressure) both 3M pump head,  and see how it goes, the setting can, if required be increased to CP3 (4.5M pump head.)

 

I would also suggest that you install a digital roomstat somewhere, ( I have mine in the combined two rooms where we spend most of our time), Viessmann seem to think that WC alone is only required?  but again IMO, the OT can fall or rise by 6 or 8C over a 1 or two hour period which can result in over/undershoot of the room temps, at least you have a master control with one roomstat.

Yeah the flow rate info and head is quite useful info - it's actual consumption is also handy

 

I don't think it's actually as powerful as the old 15/50 so was expecting to have to run it a little faster speed wise ( I need to do a little experiment to confirm that)

 

House does have two separate roomstats - one in the hall which if that ever gets to target it will turn off the boiler and one in the main living room which takes a fair while to get up to temp due to room dimensions and rad position (Long room - single rad at one end)

[John Carroll]

1 hour ago, marshian said:

Yeah the flow rate info and head is quite useful info - it's actual consumption is also handy

 

I don't think it's actually as powerful as the old 15/50 so was expecting to have to run it a little faster speed wise ( I need to do a little experiment to confirm that)

 

House does have two separate roomstats - one in the hall which if that ever gets to target it will turn off the boiler and one in the main living room which takes a fair while to get up to temp due to room dimensions and rad position (Long room - single rad at one end)

 

The "push button" UPS2 50/60 130 flows ~ 10/15% more than the Evosta3  60 (6M) but the old Grundfos selectric is a good bit less, you do have the Evosta3 60 (6M) and not the Evosta3 40 (4m) model ??.

[marshian]

19 minutes ago, John Carroll said:

 

The "push button" UPS2 50/60 130 flows ~ 10/15% more than the Evosta3  60 (6M) but the old Grundfos selectric is a good bit less, you do have the Evosta3 60 (6M) and not the Evosta3 40 (4m) model ??.

Old Pump - Grundfos 15/50 Selectric 3 Speed (it was 25 years old to be fair - purchased in 1999 for £47.99 as a direct replacement for the previous 15/50 which was fitted when the house was built in 1982)

 

This has been replaced with DAB Evosta3 60/130

 

DAB PN - 60193271

 

Max Flow rate - 3.6m3/h

 

Max Head 6m