Low rate effect on running cost

[Jordan]

My installer has identified that my flow rate is 10/L p/m and the engineer told me it’s very low and should be at least 15-20 on my 8 kw ecodan , what effect will this have on running the system with low rate will it have effects on cost and efficiency for example 

[MikeSharp01]

Do you know why it is so low? Is it the pump not running hard enough or the pipework being so tortuous (perhaps small bores) that the flow rate cannot be achieved.

[Jordan]

I’m not sure but the engineer said I will require an additional pump on the return, we have 15mm pipe throughout . I’m just interested to know if I will have over paid to heat my home over last few months due to this? If so I am going to take it up with the installer I assume the ashp will be having to work twice as hard to get the required temp in the rads if the water isn’t getting around the system quick enough?

[PeterW]

2 minutes ago, Jordan said:

we have 15mm pipe throughout

There is your reason. Mitsubishi recommend 22mm or even 28mm primary pipe runs. 
 

Is this a retrofit for a boiler or a new build..?

[Jordan]

Sorry my mistake I believe we have 22mm reduced to 15mm at the rads, it is a retrofit.

[ReedRichards]

The slower the pump speed the greater will be the temperature differential across your radiators.

The greater the temperature differential the lower will be the average temperature of the radiator.

The lower the average temperature of the radiator the smaller will be its heat output.

So the heat pump will have to make the leaving water temperature higher to get the average up and achieve a sufficient output.

So either you will be too cold or the heat pump will be working less efficiently than it could.  

[ProDave]

Most heat pumps demand a high flow rate.  Be thankful it works.  Had you installed an LG like mine it would have refused to run if the inbuilt flow switch was not satisfied, forcing me to add the second pump.

 

I doubt it will affect efficiency.  More likely with a lower flow rate, it will limit the amount of power through the heat exchanger so the heat pump might never run at it's maximum power, which if anything might make it more efficient.

[ReedRichards]

I have an LG ASHP and a second pump for the central heating.  The thing I like least about the entire system is that the flow of water through some of the pipes and radiators is quite audible whereas what I had before was completely silent.  But in my previous house the gas boiler had quite a loud pump so maybe it's just pot luck. 

[ProDave]

1 minute ago, ReedRichards said:

I have an LG ASHP and a second pump for the central heating.  The thing I like least about the entire system is that the flow of water through some of the pipes and radiators is quite audible whereas what I had before was completely silent.  But in my previous house the gas boiler had quite a loud pump so maybe it's just pot luck. 

I changed my pump from a cheap one to a grundfoss, way quieter.  Wilo are also pretty quiet pumps, I put those on the UFH manifolds to quieten those down as well.

[ReedRichards]

Fortunately my installer gave me a Grundfos pump from the outset.  And the pump itself really is quiet; it seems to be the flow that makes most of the noise.