We get a lot of tech support calls asking about proper piping of a point-of-distribution master mixing valve when a domestic hot water system has a recirculation pump. Unfortunately, there are a LOT of incorrectly piped systems out there, resulting in either temperature creep or temperature droop in the recirculation piping at night when there is no draw from fixtures. The first hot water users in the morning can have overheated (creep) or underheated (droop) hot water when the system is not piped like we show in Caleffi idronics™ journal (Issue #11), Figure 7-5, or in the Installation Instructions for our 521, 5231 Series MixCal™ or 6000 Series LEGIOMIX® valves.
Here are some short answers (see those literature references for details) to common temperature creep and temperature droop questions:
1) Make sure there is always enough flow to meet the valve minimum flow GPM requirement. All control valves need enough flow to work properly. A correctly sized, constantly running recirculation pump will usually assure enough flow.
2) A master point-of-distribution mixing valve that does not have 100% close-off on it's inlet ports (most thermostatic mixing valves) can cause creep or droop if not properly piped. Most digital mixing valves, such as the 6000 Series LEGIOMIX, have 100% close-off, so creep and droop are a non-issue.
3) Figure 7-5 in idronics #12 shows proper piping with a return valve, which throttles the amount of recirculation water going back to the cold inlet of the mixing valve, and a bypass valve which throttles the amount of recirculation water going back to the storage tank. These valves can be simple valves, like the Caleffi 142 Series Flo-Set™. The bypass valve needs to be open only slightly, allowing only enough recirculation water to flow back to the tank and out to the mixing valve hot inlet in order to make up for the heat losses in the supply piping going out to the fixtures. Page 54 in idronics #11 describes how to set that up. The return valve is typically fully open (sometimes not even needed). Too much water going back to the storage tank causes creep; too little causes droop. These valves balance out the return flows.
Check out those piping schematics, and if you need more help contact us!