Pump to replace Alpha2 15-60 130, UPS or UPML?

Try try the bypass at a lower setting.
At only 18kw there should be sufficient residual head with a 6m pump...are you certain the boiler heat exchanger/system pipe work etc isn't partially blocked?
Make sure the pump is set as a traditional pump..ie. the fixed speed curve.

The pump is on fixed speed 3. When you say move the ABV to lower setting did you mean a lower opening pressure or lower down the number line. It's at 0.3 at the moment, so go to 0.2 or 0.4?

The system water has always been clear, and while that is no indication of anything there has never been any reason to suspect that the heat exchanger is blocked.

View attachment 130091View attachment 130090

These graphs confirm my thought that the system has been balanced for a differential about 10C, which requires a higher flow rate and, consequently, much greater pressure loss in the heat exchanger and all the system.

Balancing for a higher differential (but no more than 20C) would most likely solve your problem, without needing to buy another pump.

If, at the same time, you increase the flow temp from 60C to 65 and balance each rad for a return temp no lower than 45C, the heat output of each rad will stay approximately the same.

The lower pump speed required for a 20C differential will mean that the ABV will no longer be opening early. You may eve be able to shut off the ABV and run the pump in Auto mode.

Oh no, I was hoping no one would mention balancing! My original installer didn't do anything wrt balancing and since my entire system has TRV based zoning it has always been a struggle to get that balancing done right myself. Anyway, will give that another go.

Previous experience of using the Auto mode on the pump just caused the boiler to go S.53 first thing in the morning.

Dan Robinson said:

Why don't you just hydraulically separate the boiler and use two conventional pumps?

Click to expand...

Hellllooooooooo?

That is not something I can do myself. I will need a pro to put in a low loss header etc. In a few years when I need a new boiler I might not even need any of this. So trying to get past another winter without feeling too frustrated at myself for not paying more attention to the original install done 8 years ago.

You won't need a low loss header.

In which case genuine question, how would I separate the system and run it with two pumps?

Close Couple Tee's

bruce_miranda said:

My original installer didn't do anything wrt balancing and since my entire system has TRV based zoning it has always been a struggle to get that balancing done right myself.

Click to expand...

Woops! Posted too quickly, before I had typed the following:

Your installer was probably under the mistaken belief that it is not necessary to balance a system with TRVs as they are self balancing. Wrong!!!

I building a rig with two temperature sensors at the moment, so I can go around measuring the flow and returns to each radiator. It's slightly made worse by the fact that some TRVs are on the flow but the others on the return. I know they were all dual position TRVs, so that should be fine.

As you say, the TRV and LS valves will be fine. You still adjust the LS valve, even if it is on the return, as you are setting the flow rate through the rad. What goes in is the same as what comes out!

Remember to remove all TRV heads when balancing. Open any wheel valves
Start with pump set to speed 2 and each LS valve open 1/2 turn.
Do not adjust more than one LS valve at a time (1/12 turn), then wait 5 minutes for system to settle down.

The rad differentials should be approximately the same and the boiler differential no more than 20C.

How does one calculate the flow rate through the boiler with my pump? I don't have a flow rate gauge on the pipe work.

bruce_miranda said:

How does one calculate the flow rate through the boiler with my pump? I don't have a flow rate gauge on the pipe work.

Click to expand...

Flow (l/s) x Temp Diff (°C) x 4.187 = Power (kW), (4.187 is Joules Constant). Re-arrange the equation as required.

Why do you need to know the flow rate?

I know this is a slightly old thread, but seemed quite relevant to my question. Hopefully @D_Hailsham can provide some maths genius to answer. Btw - I am going with the advice of this forum and having a LLH fitted. This is more out of curiosity than anything else.

As you know (from my previous threads! - hopefully you remember) my house has multiple zones, but this question relates just to the single upstairs zone. It has an index rad of 1.6m @ 20C (I modified the previous calculation you did and erred on the side of caution, hence the head loss has gone up from your calculated 1.09m). Pump is a Grundfos 25-80. These are the delta Ts I measured at the boiler at varying d0 settings:

24kw
16 delta T
Calculated flow rate:
0.36 l/s
1290 l/h
Boiler head loss at this flow rate (from manual): 2.4m

34kw
21 delta T
Calculated flow rate:
0.39 l/s
1392 l/h
Boiler head loss at this flow rate (from manual): 3.0m

38kw
23 delta T
Calculated flow rate:
0.39 l/s
1421 l/h
Boiler head loss at this flow rate (from manual): 3.0m

What I'm struggling with is the boiler head loss. As flow rate goes up, the boiler head loss goes up. Does the index rad head loss also go up?

Given the 2 conflicting parameters (head loss goes up, flow rate goes down, head loss goes down) and because the pump always operates on it's curve - how do you actually calculate where on the curve it is operating? Hope that made sense.



At 38KW, the minimum flow rate through the boiler is 0.45 l/s and the head loss is 4.05m.

Given the pump head far exceeds the boiler + index rad loss, why is the actual flow rate well below this (i.e. only 0.39 l/s)? Obstruction in pipe? Sludge build up in hex?

Thanks.