Grundfos Alpha 2L Proportional Pressure

[fezster]

I'm trying to understand how proportional pressure works on the Alpha series pumps. This graph shows PP as blue lines:



You can see the PP2 line maxes out at 4m head at just over 1.5m3/h.

1. Does this mean the pump cannot operate in PP mode at anything over 4m head? What would happen in reality if you attached it to a system with 4.5m head (as an example)? At CP and fixed speed, the pump can still deliver 1.3m3/h, so my guess is it wouldn't just fail to operate at that head in PP?

2. In a fixed speed pump, the pressure differential at the pump INCREASES as the heating demand falls. In PP mode, the pressure differential DECREASES as the heating demand falls. This is according to the documentation. How does the pump achieve this internally? A pump has an impeller which spins at various speeds (fixed speeds traditionally, variable speed in an Alpha). The faster it spins, the more head it can overcome at any given flow rate. How does and Alpha pump REDUCE it's head at those same flow rates in PP mode?

 

[John D v2.0]

Does this mean the pump cannot operate in PP mode at anything over 4m head?

Correct, as increasing the pressure would increase the flow but the pump isn't powerful enough for that.i think you need to look at it the other way round though, as the flow increases ie more rads open, the pump increases the head to allow a decent enough head to get a flow through all the rads.

In a fixed speed pump, the pressure differential at the pump INCREASES as the heating demand falls. In PP mode, the pressure differential DECREASES as the heating demand falls. This is according to the documentation. How does the pump achieve this internally?

Well as the rads close off, the water has nowhere to go, so the pressure will increase until the bypass opens. The pump doesn't really do anything to achieve it. It's like if you're running uphill you'll slow down, but you didn't slow down, you just ran up a hill.

How does and Alpha pump REDUCE it's head at those same flow rates in PP mode?

So the pump has extra function to pump harder as the flow increases ie more rads are open so but the pump has to squeeze more water through the boiler and shared pipework. This would reduce noise and power consumption under low load but still allow sufficient flow in a large system to get the boiler to fire properly and heat all the rads.

 

[John D v2.0]

Fezster I had a quick search and this article provides more detail than I could, have a read. It covers fixed speed, CP and PP so it should be helpful
http://lowara.xylemappliedwater.com/2017/05/12/choose-correct-speed-control-heating-systems/

 

[fezster]

Fezster I had a quick search and this article provides more detail than I could, have a read. It covers fixed speed, CP and PP so it should be helpful
http://lowara.xylemappliedwater.com/2017/05/12/choose-correct-speed-control-heating-systems/

Thank you. Will have a read later of this, as need to pop out.

I was also looking at the MAGNA pumps and the AutoAdapt feature. It seems the AutoAdapt only allows 55% of the max head available of the pump. So (for example), if you require 4.5m head at 1.6m3/hr, Grundfos quick sizer recommends both the MAGNA 3 25-60 and the MAGNA 3 25-80. However, AutoAdapt will only allow 3.5m head on the 25-60 and 4.5m head on the 25-80 (even though both pumps can achieve much higher heads at fixed speed). Given the set points are all software controlled, part of me wonders whether this is just Grundfos trying to push people to buy bigger pumps in order to utilize the energy efficiencies of AutoAdapt.

This video is where I learned this info:

As an aside: Grundfos' quick sizing site is very confusing. Last night, I put in the above figures and was recommended both of the MAGNA 3's I mentioned. But today, the site is only recommending the MAGNA 1 32-XX and MAGNA 3 40-XX!!

 

[fezster]

Fezster I had a quick search and this article provides more detail than I could, have a read. It covers fixed speed, CP and PP so it should be helpful
http://lowara.xylemappliedwater.com/2017/05/12/choose-correct-speed-control-heating-systems/

I realised I had previously read this article. Nonetheless, thanks for pointing it out again, as it is useful. One part I don't understand is:

you open all the thermostats in the house fully (assuming hydraulic balancing has been done), and then increase the power setting slowly until you see that the head pressure is not increasing anymore.

How would you determine when the head pressure is not increasing anymore? I know the more expensive magna's have a small display to give you flow rate and head pressure, but is there a tool to allow you to do this on the simpler Alpha's?

 

[D_Hailsham]

I put your numbers (1.5m³/hr;4.5m) into Grundfos calculator with the Alpha 2 L 15/60 specifically selected. This graph is the result:
.3

As you can see it shows the actual working point as 1.38m³ and 4.32m, on fixed speed 3. Does this mean, if you have selected PP2, the pump will automatically switch to a fixed speed if necessary, or is the graph telling you that you will need to use a fixed speed? The difference in flow rate is equivalent to a 10% increase in differential temperature.

The Grundfos sizing tool always tries to suggest the pump with the lowest Life Cycle cost. This takes into account the "wire to water" efficiency of the pump, which is at its maximum in the middle of the pump's working range. The efficiency of the pump, under the above conditions, is 36.1%; the maximum efficiency is about 50%.

 

[fezster]

Does this mean, if you have selected PP2, the pump will automatically switch to a fixed speed if necessary, or is the graph telling you that you will need to use a fixed speed?

Very good question, and one I asked myself also. This is even more prevalent between the MAGNA 3 25-60 and 25-80 - both capable of head well in excess of 4m, but the smaller one only provides up to 3.5m in AutoAdapt.

I also discovered why the site seemingly made different suggestions day by day - the country selection somehow reverted to USA. Setting to UK brought back consistent results.

 

[fezster]

The Alpha 2 with AutoAdapt is probably more appropriate for this scenario, as it adjusts within the band shown, although at 1m3/hr and 5m head, I'm not sure if it would automatically revert to fixed speed (probably not).

View media item 100909

 

[D_Hailsham]

In a fixed speed pump, the pressure differential at the pump INCREASES as the heating demand falls. In PP mode, the pressure differential DECREASES as the heating demand falls. This is according to the documentation. How does the pump achieve this internally? A pump has an impeller which spins at various speeds (fixed speeds traditionally, variable speed in an Alpha). The faster it spins, the more head it can overcome at any given flow rate. How does and Alpha pump REDUCE it's head at those same flow rates in PP mode?

You are looking at it the wrong way round.

The pressure differential of a fixed speed pump increases with decreasing flow rate because it is rotating at a fixed speed. That does not mean that the resistance of the circuit increases as the flow increases; it is the opposite, the resistance of the circuit varies proportionally to flow squared. So if the flow halves, the resistance is a quarter. So, if the flow rate halves the pump only needs to produce a head which is a quarter the size.

Variable speed pumps have synchronous motors so the speed is locked to the supply frequency. The speed is changed by by varying the frequency of the supply.

Re your latest post

The Alpha2 15-60 is no longer listed; it has been superseded by the Alpha3 15-50/60. When I put in your parameters, 1m³/5m, it selected a point on speed 3, outside the Autoadapt range, 1.01m³/5.118m.

 

[fezster]

You are looking at it the wrong way round.

The pressure differential of a fixed speed pump increases with decreasing flow rate because it is rotating at a fixed speed. That does not mean that the resistance of the circuit increases as the flow increases; it is the opposite, the resistance of the circuit varies proportionally to flow squared. So if the flow halves, the resistance is a quarter. So, if the flow rate halves the pump only needs to produce a head which is a quarter the size.

So translating this to real world scenario:

Fixed speed pump

1. Head required in a balanced system is equal to the index circuit.
2. As trv's close down, the resistance increases (?), flow rate reduces and pump head increases following the pump curve.

Variable speed pump

1. Head required in a balanced system is equal to the index circuit.
2. As trv's close down, the resistance increases (?) , pump senses back pressure increasing so reduces its speed. Flow rate reduces and pump head decreases following the PP line/curve.

I'm sure most of that is wrong (hence the question marks above), because if resistance increases, surely more head is required? On the other hand, the reduction in flow rate makes sense.

 

[John D v2.0]

You were spot on actually

because if resistance increases, surely more head is required?

Resistance increased because the rads closed, so you only need enough water for one radiator

only more head is needed if you want the same quantity of water that was running through all the reads running round the remaining open rads. That's certainly not the case as that radiator would not need all that water.

Usually you want to keep the same head across the rads, not across the system, so reducing the pump head is necessary if there's shared pipework which is also serving closed radiators.
If you have very little resistance in the shared pipe work then a constant pressure would be ok, although it too would reducing its pumping effort as radiators close down.

 

[picasso]

Unless your running your pump off a low loss header or some sort of buffer pp is pointless as the minimum flow rate for the boiler cant be guaranteed and if a bypass is fitted it wont work with pp, the way most boilers/controls are set up in the uk a fixed speed pump is needed.

 

[fezster]

Resistance increased because the rads closed, so you only need enough water for one radiator

Thank you. And so as flow rate reduces, as David said "the resistance of the circuit varies proportionally to flow squared", the head required is proportionally less - therefore the reduction in head on a variable pump is still sufficient to overcome the head of the remaining open rads.

 

[John D v2.0]

yes although as per picasso's point, they still have a minimum head (so not directly proportional) which you need to ensure provides a minimum flow through the boiler via the bypass

 

[fezster]

Unless your running your pump off a low loss header or some sort of buffer pp is pointless as the minimum flow rate for the boiler cant be guaranteed and if a bypass is fitted it wont work with pp, the way most boilers/controls are set up in the uk a fixed speed pump is needed.

This is actually the reason I started looking at understanding this (though curiosity took over). I am having a LLH installed, and have a pump which is designed to overcome the boiler resistance. I want to use an appropriately sized pump on the secondary circuit as it consists of multiple zones and varying loads (because of TRVs).

David was kind enough to do an index calculation for me a few years ago, for each of my zones, but the reality is somewhat different as I've discovered by using flow and return temps to determine the actual flow rates in my system (and therefore the actual resistance of the system too). An Alpha 2L might be just within spec when the boiler is firing at full capacity (34KW and -1 outside I believe), although perhaps not in PP mode. A MAGNA 3 25-80 would probably be more appropriate, but given the significant extra cost, I may be better off trying the Alpha 2L first.