Kamstrup Multical 403 Erratic flow readings only during DHW heating okay heating radiators


Mitbushi Zubadan PUD-SHWM100VAA with Level 3 Heat Pump Monitoring, Kamstrup Multical 403.

Flowrate during radiator heating is constant at 22l/m but during DHW heating flowrate is very erratic, dropping to 0.
Here are both radiator heating cycle and DHW heating cycle showing flowrate.

Here is DHW showing okay flow rate 22l/m okay, up to a temperature of approx 43 deg flow temp and 40 deg flow return, but flowrate drops to 0 with few spikes to 14l/m even though DHW heating continues up to 53 deg flow /50 deg return.

Because flowrate is not right then COP is not correct for DHW.

Flowmeter is on return pipe as below.

Pressure is approx 1.4bar, I have just increased this to 1.6bar and will see if it improves.
There is air vent at high point where the flow pipe enters the hot water tank.

Any help would be appreciated.

I had exactly the same with my system which was down to air in the system. I sorted mine by running the radiator heating hot and bleeding the radiators a lot. After a couple of days the DWH cycle starting working correctly


As Dave says, you’ve got air trapped in the hot water circuit, which messes with the flow meter. Kamstrup’s are particularly suceptible. Should be fine once the air is removed.

It sounds like you have some air in the system. Also, the location of your meter on the piecework is not recommended. It looks like your meter is in position F on the inlet to a pump:

The Kamstrup install diagram above states that a meter “must never be placed on the inlet side of a pump”


The meter is located on the return on the outlet side of the pump.

Blue arrows shows return direction,bottom to top, pump followed by meter, followed by filter.
Flow is shown by red arrow down to 3 port valve, yellow arrow right to DHW cylinder, or orange arrow down and back thowards wall and to all radiators.

I did a heating cycle with high temperature followed by bleeding radiators and then a DHW cycle.
Improvement but still flowrate erratic for last half of DHW cycle.

Will try bleading radiators a few more times as people suggested.

Thanks for all suggestions

You will probably get there eventually.
I had the same issue. I installed mine during the summer and it didn’t really fully sort itself until the heating season which helped the air to make its way eventually to rads to be blead off.

Ah right, sorry I didn’t notice it was a split system. Usually for monoblock systems, the return would be heading downwards towards the outdoor unit.

Yup I’d agree with other comments that this looks like air in the system. Had the same issue with my own Kamstrup 403. It took me several days of topping up the system pressure and venting the air out to clear it all. You should hopefully get there after a while.

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On the plus side you’ll be doing the system a lot of good by getting all the air out. There will be less corrosion etc; longer life. :slight_smile:

That’s another win for OEM, for sure. Without this data we’d have [almost] no idea that there was still air trapped in the system. We might think to bleed the radiators once in a while [especially approaching/during heating season] but wouldn’t have an indication like this that the system was not fully vented.