Flow rate oscillation

[rhymecheat]

Hi there, have a question about my ashp (Altherma 3 H HT). 
 

when the compressor is operating the flow rate oscillates between 29L/m and 31L/m. This oscillation is very quick (I’d say half a second in 31, and down for another half sec in 29 then back up again). I only noticed this as you can hear the pump on the indoor unit sounding almost like it’s getting throttled and it’s quite an annoying sound.
 

I’ve checked the settings and the pump speed is set to “no limit”. If I set it to say “80%” then I get a flow of about 28L/min constant when compressor is on.
 

here’s my questions:

* is this limit okay to set? Or am I compromising efficiency?

* why could the “no limit” be behaving like this? As in, where would I look for what’s potentially throttling the pump. 

 

btw, it feeds UFH and the delta is set at 7

[sharpener]

17 minutes ago, rhymecheat said:

I only noticed this as you can hear the pump on the indoor unit sounding almost like it’s getting throttled

 

Sounds like a classic case of a control system "hunting". If there is a pump in the indoor unit and one in the outdoor unit as well then it may be there are two separate flow control systems fighting each other.

 

If you are reducing the flow rate by <10% then theoretically the delta T will have to rise by 10% to restore the heat transfer rate. I don't think 0.5 deg will make much of a dent in the efficiency. If it shuts the thing up I think I would go with it as a solution.

[JohnMo]

Definitely sounds like you have two pumps not hydraulically separated and both set to modulate. Both end up chasing their tails. Would suggest the one controlled by the heat pump is left in modulation mode the other (UFH) set to a fix duty point at the lowest speed you can.

 

The UFH dT is set via flow meter on each loop, not by pump speed variation. If you want a set dT install Salus self balancing actuators, but leave the UFH pump set to a fixed speed.

[rhymecheat]

1 hour ago, JohnMo said:

Definitely sounds like you have two pumps not hydraulically separated and both set to modulate. Both end up chasing their tails. Would suggest the one controlled by the heat pump is left in modulation mode the other (UFH) set to a fix duty point at the lowest speed you can.

 

The UFH dT is set via flow meter on each loop, not by pump speed variation. If you want a set dT install Salus self balancing actuators, but leave the UFH pump set to a fixed speed.

So I’ve already got Salus actuators on the UFH manifold. That manifold is set to constant pressure speed. Other than that I can set it to 1,2 or 3. If I’m understand this correctly you’re suggesting I change it to a constant speed instead? How long do you think it would take for me to tell if it’s not oscillating anymore?
 

other than the ashp pump there’s also a pump after the buffer tank. That one is set to 2 currently. 

[JohnMo]

Lot of pumps. Change to constant speed, lowest speed you can get away with and not have flow drop of the flow meters. If you only have UFH the pump after the buffer is possibly not needed as the UFH will draw from the buffer itself.

 

2 hours ago, rhymecheat said:

How long do you think it would take for me to tell if it’s not oscillating anymore

One oscillation time period, it will either oscillate or or won't.

[rhymecheat]

29 minutes ago, JohnMo said:

Lot of pumps. Change to constant speed, lowest speed you can get away with and not have flow drop of the flow meters. If you only have UFH the pump after the buffer is possibly not needed as the UFH will draw from the buffer itself.

 

One oscillation time period, it will either oscillate or or won't.

Okay so that still oscillates. I’m now tempted to reduce all the pump to min (even ashp) and start taking flow measures to see. thanks for the insights btw. 
 

also, the pump after loft is there because it serves both downstairs UFH and upstairs rads. 

[JohnMo]

@rhymecheat

 

You have two sides of the buffer, each side has hydraulic seperation. If you have oscillations of the house side of the buffer it's not related to the ASHP circulation pump, only the house side pumps.

 

Switch you UFH off, does the oscillating stop?

Edited January 25 by JohnMo

[rhymecheat]

25 minutes ago, JohnMo said:

@rhymecheat

 

You have two sides of the buffer, each side has hydraulic seperation. If you have oscillations of the house side of the buffer it's not related to the ASHP circulation pump, only the house side pumps.

 

Switch you UFH off, does the oscillating stop?

No, it keeps going.

 

edit: also, I’ve been keeping an eye on energy usage since turning the pump speed down on the ashp, and it seems to be short cycling now 🤔

Edited January 25 by rhymecheat

[JohnMo]

59 minutes ago, rhymecheat said:

since turning the pump speed down on the ashp,

As I said if the oscillating is on the house side the ASHP circulation pump is not related to the issue, the buffer hydraulically separates the two sides of the system.

 

Why not switch off all the pumps on the house side of the buffer and revert the ASHP back to it's original settings. That should leave one pump running (?) and nothing else. If the issue goes away or remains, you know which side of the buffer is the issue is coming from.

[rhymecheat]

20 minutes ago, JohnMo said:

As I said if the oscillating is on the house side the ASHP circulation pump is not related to the issue, the buffer hydraulically separates the two sides of the system.

 

Why not switch off all the pumps on the house side of the buffer and revert the ASHP back to it's original settings. That should leave one pump running (?) and nothing else. If the issue goes away or remains, you know which side of the buffer is the issue is coming from.

Hum, good shout. I'll try this and report back. Thanks!