Filter box and UV filter

[mybrotherjohn]

Hi

I have a filter box which is rated at 6000l/h. I have a pump which is unrated (second hand - label lost) and a second hand UV filter. The pipework is pretty convoluted so that it fits into a box I've made to house everything. (See pictures)

• With the UV filter, I get an exit flow rate of c 850l/h.
• Without the UV filter, I get an exit flow rate of 1800l/h

All the pipework is 40mm however the outlet of the filterbox is 50mm so there is a reducer on the filter box exit from 50mm-40mm which, I calculate , will reduce the potential flow by 36% (ratio of cross sectional areas) so limiting the max through flow to 3840l/h. Why is the filter box so limited in practice?

Here is my problem. At 850l/h throughput (that is with the UV filter in line) , the filter box floods. Also, I'm not convinced that the flow rate is high enough for the pond (about 1 cubic meter)

• Does anyone have any ideas why the UV filter should throttle the throughput so much? I've taken the bulb out and the housing looks clear.
• Can I do without the UV filter?
• The pump is only pumping about 1800l/h so whilst the filter box floods if the UV filter is in line, the box flow-rate just about holds its own if there is no UV filter. Once the filters get dirty, I imagine that the flow rate with fall and the box is likely to flood. What is the best way to regulate the pump output (and why don't pumps come with variable outputs?)

Thanks in advance for any information or ideas anyone has!