Grundfos Home Booster - how to set properly

[panicboy]

Hello,

We've just had one of these babies installed along with a new Vaillant combi boiler: previously we had two tanks and a shower pump with a standard non-condensing boiler. The new system looks very impressive.

With the old system when you turned on the hot tap the pump would kick in and you'd get a steady stream of hot water. With this one however the hot water comes out at medium pressure, then the Grundfos kicks in to give high pressure, and then it stops and the pressure goes back to what it was, then it kicks in again, repeat. The temperature fluctuates with the pressure change, it going cold for a bit as the pump kicks in.

Anyone know what the problem is here? Do I need to change the settings on the boiler or Grundfos unit? Or is this not right at all and should I call the plumbers back in?

Andrew

 

[doitall]

It sounds like the cut-in, cut-out pressures need to be adjusted.



which compares the actual pressure to the user adjustable pressure values for cut-in and cut-out.
The pump is started if a demand causes the system pressure to fall to the cut-in pressure. The pump will continue to run until the cut-out pressure is reached, and then stops after a small run on delay to prevent pump cycling. The 8 litre pressure vessel makes up for small water demands thereby preventing unnecessary pump starts.

 

[panicboy]

Aha, so perhaps the pressures for cut in and out are too low? I think they're 2.8 for cut out and 1 below this for cut in. I might bump it up to 3.5 and see what that does. It's not the most user-friendly control panel I've seen: what other device asks for a password each time??

I suppose it's quite possible the values mean it's set to turn on at a value that's too low for a shower, hence the pressure keeps dropping. Anyway I now feel confident enough to try adjusting the beast...

 

[doitall]

I wouldn't bump it up, you have to think of the boilers maximum pressure.

I think you may need to increase the expansion vessel or better still fit a small accumulator.

 

[panicboy]

Hmm, good point. I've put it back down to 2.8 which is what the plumber put it at (I assume: I wasn't here when he left) and just decreased the cut-in differential to 0.5, which means the pump should kick in before the pressure drops too much.

You know I'm not convinced we needed this enormous thing however the landlady's architect friend seemed very sure.

 

[doitall]

You can always switch it off and open the by-pass to try it without.

 

[panicboy]

Haha very true, but the last thing I want to find out is that she spent £1600 on something she didn't really need. Naturally I wouldn't have to tell her that but the knowledge would haunt me. In any case I think we now have the best hot water system in this part of London: it's amazing.

Plus I wouldn't know where to find this by-pass: I'm rubbish. I wish I knew more about plumbing. I'd happily watch the whole installation and have everything explained to me but I think the plumbers would find that rather annoying.

 

[simond]

If she'd fitted an accumulator/TCWS system it would give you the same good performance without the noise or the pulsating pressure differences.

I can point you at a fair number of installs in london with a better performance

 

[Agile]

I am confused! First its "we have had" and then later "she has paid....".

Who is we and who is she?

As far as I recall, there are THREE settings which are relevant and for optimum performance they should all be set appropriately. Cut-in, cut-out and the air charge pressure on the EXV.

Simon is the expert on these kind of thing but I would have thought a pressure of 2.5 and a differential on about 0.25 and an air charge of about 2.5 would have been about right.

Tony