Multiple inlets for cold water tank in loft

[salixsorbus]

I am putting in a rainwater harvesting system, but want to avoid (needlessly) pumping mains water from ground level. The system I am going with is a cold water tank in the loft, fed from the rainwater tank in the ground, with a mains water backup. Normally, the mains water backup is built into the ground tank.

I am aware of the need for an air gap (type AB in this case) and may do this via a small tank higher up to provide an indirect mains water feed.

The best way I can think of to provide the mains water backup is to have two inlets into the cold water tank in the loft, with the mains water inlet operated through a float that is lower down than the rainwater inlet. This would mean that the lower float would be permanently submerged until the rainwater tank is empty and can no longer fill the loft tank, which could put stress onto the float valve.

So the question : Is the stress on the float valve going to be a problem. If it is, is there a better way of providing a secondary inlet which only gets used when the first inlet is not providing water?

 

[muggles]

Not hugely familiar with these systems so I can only offer general guidance from my knowledge and experience, but here goes...

You can't have one tank with two inlets in the way that you have suggested - immersing the mains water float valve presents major health risk problems with regard to potential backflow. If the immersed float valve were to stick open and mains water was then switched off and the cold water system drained, rainwater would be drawn into the cold water system through the float valve. It is possible that this could be subsequently consumed. You can't simply fit a double check valve to the supply to the float valve either - rainwater is fluid risk category 4 at best, and DCV's are only approved to category 3 (water is divided into five categories, where 1 is what normally comes out of your kitchen cold tap, and 5 is, amongst other things, what's in your toilet after you've used it).

The indirect feed idea would work quite well, and be within regs, and you could fit something like a Fluidmaster side entry float valve which would put much less strain on the side of the storage tank than a traditional ball valve when immersed.

Another way to do it is to have two unconnected storage tanks, one for mains and one for rainwater, with a manual changeover valve situated somewhere convenient like the airing cupboard (if you have one) for those times when it is required. In this instance, both tanks in the loft would be installed at the same level.

Hope this helps.

 

[salixsorbus]

The indirect feed idea would work quite well, and be within regs, and you could fit something like a Fluidmaster side entry float valve which would put much less strain on the side of the storage tank than a traditional ball valve when immersed.

Another way to do it is to have two unconnected storage tanks, one for mains and one for rainwater, with a manual changeover valve situated somewhere convenient like the airing cupboard (if you have one) for those times when it is required. In this instance, both tanks in the loft would be installed at the same level.

Hope this helps.

Muggles, thank you for the reply. Contamination of the mains water is not going to happen - any solution that doesn't inherently provide a type AB air gap will have an indirect feed instead, and I had already discounted the check valve! The changeover valve is a very simple option, but we would be always forgetting to change it back to the rainwater, which is why I need something that doesn't require our intervention.

Your suggestion of the reduced stress from using a Fluidmaster side entry float valve sounds like this is the method we can go for.