Grundfos Alpha 2 does not self prime

[Richard Chambers]

I have also re-checked the numbers, and CP1 = 20W; CP2= 35W. Some of the thermostatic valves are now shut as the weather is warm (Devon is 25C)

 

[John.g]

You could go for the smaller burner, only downside is a longer heat up time on a cold morning if the boiler has been shutdown overnight, cycling doesn't seem to have a big effect on boiler efficiency if one can believe the numbers I've seen. I have a 20kw oil fired (Firebird) and the only time it cycles rapidly is on HW only when the water temperature is approaching 60C when the coil is probably only absorbing 2 or 3 kw, but the coil only absorbs 7/8 kw from cold.

 

[John.g]

I have also re-checked the numbers, and CP1 = 20W; CP2= 35W. Some of the thermostatic valves are now shut as the weather is warm (Devon is 25C)

It's a air conditioning unit you need just now.

What is the ABV setting now?.

 

[Richard Chambers]

ABV 5+

 

[doitmyself]

Something else that caught my eye there in the numbers, see attachment, if you look at the head/flowrates for CP2 and CP1, CP2 is showing a flow rate of 0.95 m3/[email protected] and CP1, 1.1 M3/hr@3M, maybe I'm reading the curves incorrectly but for the same system resistance these two readings are incompattible?, a flow of 1.1 M3/h@ 3m = 1.35 M3/[email protected] or 0.95 M3/[email protected] = 0.78 M3/hr@3M.

Where do you get your numbers from:

1. Top table?
2. Middle table?
3. The graphs (estimated)?

 

[John.g]

The (very) top table (composite file) are my results from the below:

I used the power (watts) values from Richard's readings ( table), I then went to the bottom chart and where the power (vert.axis) met the pump curve(S) I read off the flow rate in M3/hr (horiz axis), I then used these flow rates in the top chart (horiz axis) and where they met the pump curves I read off the head in M (vertical axis).
The table in that composite is more correct than the ones in my post, above.

 

[Riley]

Wow this is a proper pump love in isn’t it

 

[Harvest Fields]

Wow this is a proper pump love in isn’t it

I got bored and switched off!

 

[doitmyself]

The (very) top table (composite file) are my results from the below:

etc etc

I thought that's what you had done; just wanted to be sure.

Don't forget that when the pump is in a constant pressure mode it will vary the output to maintain the required pressure. The fact that the pressure may be too high or too low is irrelevant.

 

[Richard Chambers]

As this is proving so interesting, maybe I could ask a supplimentary question. I have always had a problem in the winter with cool rads when the both the CH and HW are calling for heat. I have now fitted a gate valve to act as a restrictor on the HW tank.
Should I restrict the flow into the tank so that the power W reading is the same for CH only and HW only of say PP2, given they both have approx the same power rating? Or just screw it in half way. The HW circuit is very short and all 22mm whereas the CH is 22mm into 8mm to the rads.
Thanks

 

[doitmyself]

The HW circuit needs to be balanced in the same way as the CH. If it's not balanced the HW will grab the lions share of the flow.

Of course, if you timed the HW to only come on when CH is not required, e.g. during the night, balancing would not be so important.

Balancing is best done using one of the fixed speeds, not a CP or PP setting.

 

[John.g]

I thought that's what you had done; just wanted to be sure.

Don't forget that when the pump is in a constant pressure mode it will vary the output to maintain the required pressure. The fact that the pressure may be too high or too low is irrelevant.

That's correct and as flow is proportional to the sq.root of pressure then if the flow is 1/1.1 M3/hr@3M then it should be 1.34 M3/[email protected] so I would have expected the pump power when c/o to CP2 (4.5M) to be 45W and not 37W but no big deal really, its probably down to my interpretation of the pump curves.