Grundfos Alpha 2L Proportional Pressure

[D_Hailsham]

So I measured upstairs in isolation, and this is what I came up with:

34kw (used gas consumption to confirm)
21 delta T (taken from d40 and d41)
Flow rate = 0.39 l/s (1,404 l/h)
Grundfos 25-80 speed 3 - 6.85m head

Meter readings tell you the energy consumed, not the amount of energy which actually heats the water. For the Vaillant 438 this is approximately 90% of the consumption. So your 34kW is actually 30.6kW. which gives a flow rate of 0.349 litres/sec. If we plot this on the pump graph it will produce a higher pressure. What we have to remember is that (using your figures: 0.39 l/s and 6.85m) these are the flow rate and head which the pump is providing, they are not the flow rate and head which the system requires. This can be anywhere along the red line, which is the system curve.

Here is a chart for the UPS2 on my system, with two graphs overlaid . The "original" line is the calculated flow rate and head when all rads are in use. This works out at: flow 0.141 l/s, head 1.503m. As the pump is on fixed speed 1, the actual working point is: flow 0.2051 l/s, head 3.174 m.

I then "turned off" three rads and recalculated flow and head, which was: flow 0.115 l/s, head 1.41 m. This is the "amended" line. Note that the required flow rate and head have both reduced. However as the system curve (red lines) has moved to the left the actual working point has now shifted to: flow 0.1774 l/s, head 3.356. The actual flow has reduced but the head has increased.

 

[fezster]

This works out at: flow 0.141 l/s, head 1.503m. As the pump is on fixed speed 1, the actual working point is: flow 0.2051 l/s, head 3.174 m.

<snip>

I then "turned off" three rads and recalculated flow and head, which was: flow 0.115 l/s, head 1.41 m. This is the "amended" line. Note that the required flow rate and head have both reduced. However as the system curve (red lines) has moved to the left the actual working point has now shifted to: flow 0.1774 l/s, head 3.356. The actual flow has reduced but the head has increased.

So can I take from this that you cannot use the head the pump is operating at to determine the resistance of the system (boiler + radiators)?

If so, is there any reliable way to determine the actual resistance of a system to confirm what has been calculated?

 

[u33446z]

I've just had a Grundfos UPS2 replacement pump fitted which also has both fixed speed and 'Proportional pressure' modes. I tried to quiz Grundfos as to how their PP mode works but couldn't get any sense out of their Customer Service team other than the fact that the pump has no pressure sensor. My assumption is this: a fixed speed pump must achieve the fixed speed by varying the voltage across the pump, so a simple way of achieving a variable speed characteristic would be to operate at a fixed voltage, so that as soon as the outlet resistance increased, the pump would slow down, and vice versa. Goes that sound plausible?

 

[John D v2.0]

resistance increased, the pump would slow down, and vice versa. Goes that sound plausible?

No as that would be inversely proportional.
The fixed speed pump uses a fixed frequency of the mains to spin at the given speed.
The proportional one has to have electrics inside to vary the speed, and it needs some way of knowing the pressure, but given it can just sense the load on the impeller by measuring its speed it can use that as a flow or head sensor.

 

[D_Hailsham]

So can I take from this that you cannot use the head the pump is operating at to determine the resistance of the system (boiler + radiators)?

If so, is there any reliable way to determine the actual resistance of a system to confirm what has been calculated?

Posted in error! I will add a reply soon.

Here it is!

No, you cannot use the pump's working point to determine the system resistance.

However, the working point must be on the system curve, so you should able to calculate the actual resistance if you know the required flow. Which brings me to a query about your test.

You said that the test was done on the first floor, which, according to the floor layout has radiators totalling 21.14kW. So how can the boiler be consuming 34kW? Or is this the number of kilowatt hours consumed in the five minutes the test took?

 

[fezster]

Or is this the number of kilowatt hours consumed in the five minutes the test took?

Correct.

However, the working point must be on the system curve, so you should able to calculate the actual resistance if you know the required flow.

Would love to know if this is possible.

Thanks.

 

[fezster]

This works out at: flow 0.141 l/s, head 1.503m. As the pump is on fixed speed 1, the actual working point is: flow 0.2051 l/s, head 3.174 m.
<snip>
I then "turned off" three rads and recalculated flow and head, which was: flow 0.115 l/s, head 1.41 m. This is the "amended" line. Note that the required flow rate and head have both reduced. However as the system curve (red lines) has moved to the left the actual working point has now shifted to: flow 0.1774 l/s, head 3.356. The actual flow has reduced but the head has increased.

Do the "required head" figures above (from the duty point of the system curve) include the boiler hex resistance? Surely this must be part of the figure as the pump is overcoming it's resistance also.

So from my example in my earlier post, the calculated flow rate was 0.36 l/s at 6.9M head on the pump (EDIT - I know I've ignored the 90% combustion you mentioned!).

The REQUIRED flow rate is:
21,140 / 4185 / 21 (21 being my D-T) = 0.24 l/s

View media item 100925
Plotting this on the system curve, this shows a total resistance of 2.98m.

The boiler head loss at 0.24 l/s (865 l/h) is 1.2m.

View media item 100912
Leaving a system resistance of 1.78m. Which is a lot closer to the value I calculated with my more conservative estimates (1.6m) and to the value you calculated originally years ago (1.09m from memory). But I do wonder how much of this is correct, and how much of it is me fudging the figures to try and make them match what I think they should be.

 

[picasso]

I don't mean to be funny but your way over thinking this.

 

[fezster]

I don't mean to be funny but your way over thinking this.

I find it quite interesting.

On a more practical level, though, how do you size a pump to a system, without just using trial and error (a 15-60 "should work" - if not we'll try bigger)

 

[picasso]

work out the heat load, that gives you your flow rate, figure out the resistance of the index circuit and then use a pump curve to select a suitable pump.

 

[fezster]

work out the heat load, that gives you your flow rate, figure out the resistance of the index circuit and then use a pump curve to select a suitable pump.

Yes, indeed . Not always possible to do accurately, though, in an existing system.

This was an attempt to use data gathered from said existing system to prove the calculations were correct (or at least close enough).

 

[D_Hailsham]

How exactly did you calculate the 34kWh? It doesn't make sense as 21kW of rads can only consume 21kWh of gas (ignoring efficiency loss) in one hour

I have assumed that the rad sizes on your plans are the nominal sizes. If not you need to take into account the reduction in output if the actual working temperatures are not 75C, 65C and 20C. For example 75C, 55C, 20C would mean the output was only 85% of nominal.

To answer you later question: yes, the head does include the resistance of the boiler.

Calculating head from experimental data

Basis of calculation

If you know the required flow, actual flow and actual head (from pump graph) you can calculate the head at the required flow using the following formula:

H = kF², where H=head, k is a constant, and F is the flow rate.

What is the value of k? This varies from system to system. It also changes if the system changes, e.g a rad closes or a zone is shut. k can be calculated from the actual flow and head. Using my graph as an example:

Pump flow and head: Fp=0.2051 l/s, Hp= 3.174m

k = 3.174/0.2051² = 75.45

Required flow = 0.141 l/s; so actual head H = kF² = 75.45 x 0.141² = 1.5 m.

Compare this to my calculated head of 1.503m

If you had an accurate figure for the actual flow for the first floor zone, you would be able to plot this on the pump graph and obtain the actual head. This will give k. Obtaining the head at the calculated flow is then simple maths.

 

[fezster]

H = kF², where H=head, k is a constant, and F is the flow rate.

You're a wealth of information @D_Hailsham. Thanks very much for taking the time to explain this. And now the use of Proportional Pressure makes complete sense too, as the required head vs the actual head of a pump on fixed speed are always somewhat different when flow is reduced.

With regards to boiler output, I will need to redo the gas consumption calculations over a longer period. The other problem with the 438 is that it always fires at max output for the first minute or so, so that would skew the figures.

One thing I am more positive about now is that once I have hydraulic separation of the boiler to the radiator circuit, *hopefully* the Alpha 2L 15-60 will be sufficient to drive the entire radiator side.

 

[Gasguru]

Tee into the pipework either side of the pump.
Stick a valve on each tee branch.
Using a SINGLE gauge connect it to either branch and take the pressure reading whilst the system load remains constant.
You can then calc. the differential pressure across the pump and hence find its operating point on the curve.

You might be tempted to use 2 gauges but you'll introduce inaccuracies.
Of course if you've got huge resources you could by a purpose made differential gauge....they're stoopid money.
On my test rig I use an electronic differential pressure sensor which is sensible money.

 

[D_Hailsham]

With regards to boiler output, I will need to redo the gas consumption calculations over a longer period. The other problem with the 438 is that it always fires at max output for the first minute or so, so that would skew the figures.

You really need to take readings as close together as possible, otherwise you are just averaging. Temperatures are easy, d.40 and d.41 provide the info. Unfortunately there is no equivalent for boiler output, so you have to take gas meter readings. 2 minutes apart should be sufficient, but wait until the boiler has stabilized, i.e temperatures are constant. (d.34 gives the fan speed, which controls the output, but Vaillant don't publish a conversion chart.)

I believe later versions of the 4XX boilers have a revised PCB which doesn't restart at max output. My boiler (not a Vaillant) always restarts at min output, then ramps up if necessary.