I recently replaced the Hot Water Recirc Pump (used to give Instant Hot Water to all sink faucets). Purchased the exact model number from the manufacturers plumbing supplier in my area. Also bought the end flanges with all new gaskets & square "O" rings. After I placed the system back in service & vented all piping of air, from all faucets especially the upper floor & now when we flush a toilet the water at the faucet is very low flow. This house has had larger diameter pipes feeding the faucets since the rebuild, that are the longest away from the hot water heater just for that reason so if someone flushes a toilet & someone is taking a shower, you do not notice a flow change & the water stays at the same temp. System was installed during rebuild/upgrade of the house after a fire & this system was installed over 15 years ago & this was the first failure of this system. We get hot water within 3 to 5 seconds in the upstairs master bath. It's just very low flow, but as soon the the low flush toilet tank is almost full, the flow is back. All piping is copper except for the new flanges which are bronse with 1" pipe threads which the older ones were 3/4" threads & half the gaskets were gone but there were no leaks. Problem was noticed as it was taking 3 to 5 min to get hot water. Pump was not running due to rust/corrosion & had the thermal protection cut it off. These Grundfos (Model UP-15-42F) pumps are rated for water temp to 230 deg F. We also installed whole house filter system with two types of filters, one for odor & one for sediment & these were just changed while I was doing this pump change. Also replaced a float switch in the water softner & all of this is working fine except for the flow to the sinks for hot water.

ABoccia, February 2006

If we think about the problem and trace it back from when the system was working properly with a decent water flow.
The pump has been changed to the exact new model due to rust. New filters have been put in, the old pipes have now gone and the bore size has increased.
This is where the problem lies, water enclosed in a pipe pushed by a pump through 15mm standard copper will have a good pressure to go round the house and feed each faucet. As soon as you increase the width of the pipe the pressure drops as less force is applied. The answer is buy a high rated pump.

Mat, March 2006