noise when new pump starts up

[nick83]

At last managed to get central heating working after another powerflush.Fitted new Danfoss pump,and when my heating starts
,especially from cold I get about a 0.25 pressure drop and noise as if air is circulating around the pump for about 10seconds.
pressure gradually returns after a minute or so--any ideas.It is a sealed system ,with ordinary oil fired system boiler and expansion tank.
many thanks

 

[Best]

I assume you meant Grundfos circulating pump?
It can take many days for air to leave pipes after a refill.
If system holds water pressure for months or longer, then you should have little bother.
Is the oil boiler and expansion vessel serviced each year?

 

[nick83]

I assume you meant Grundfos circulating pump?
It can take many days for air to leave pipes after a refill.
If system holds water pressure for months or longer, then you should have little bother.
Is the oil boiler and expansion vessel serviced each year?

It is done annually.Will that be just air in the pipework and the pump 'pulling' it around.

 

[Best]

It is done annually.Will that be just air in the pipework and the pump 'pulling' it around.

Probably. Some systems have terrible pipework which traps air.
If your rads work okay, then must just be air bubbles in system.
I would be wary of auto vents allowing air into pipes

 

[Chuck]

when my heating starts
,especially from cold I get about a 0.25 pressure drop and noise as if air is circulating around the pump for about 10seconds.
pressure gradually returns after a minute or so--any ideas.It is a sealed system

Another possible explanation, if it's a 'smart pump' and depending on its setting, could be the pump starting up at maximum circulation, which will be noisy due to turbulence, and then its 'smart' controller reducing the flow to the correct lower rate.

 

[nick83]

Another possible explanation, if it's a 'smart pump' and depending on its setting, could be the pump starting up at maximum circulation, which will be noisy due to turbulence, and then its 'smart' controller reducing the flow to the correct lower rate.

It could be,it is on speed 2 and hopefully on fixed speed even though i found the light system confusing between pp and cp.Would that account for pressure drop?It is quite a loud 'wooshing' with what sounds like gravel going around for a few seconds

 

[nick83]

Probably. Some systems have terrible pipework which traps air.
If your rads work okay, then must just be air bubbles in system.
I would be wary of auto vents allowing air into pipes

I tend to keep it closed and then open it a few times a day to let build up of air out

 

[Chuck]

Would that account for pressure drop?

It's a plausible explanation. When the pump is running, the pressure on the inlet side will be reduced and the pressure on the outlet side will be increased. What your pressure gauge does will depend where it is tapped into the pipework with respect to the pump.

It is quite a loud 'wooshing' with what sounds like gravel going around for a few seconds

That's a reasonable description of what cavitation/turbulence sounds like. But it's also a reasonable description of air in the system, which Best has discussed. Both sound similar but, to me, air tends to sound more like the trickling of a stream running over stones and rocks and turbulence tends to sound more like the sound of a strong wind.

 

[Ben-gee]

Almost poetic Chuck