Should I bother with installing a flow meter?

[sfriedri]

Hello all.

In my system I'll be using a 3-speed pump on my heat load loop and I just have a single load loop (water to air exchanger). I've done all the pressure drop calculations and know what pump to use based on my loop length (160') and pipe size (1 1/4"), but I'm also considering installing a flow meter so that I can pick the best pump speed based on real data rather than pump curves. Would it be just as good to set the pump speed based on the temperature drop across the exchanger? If I do decide to go with a flow meter, does anyone have information about sources for inexpensive rotameters?

One related question I have is about the best way to plumb my planned side-arm exchanger. My water heater is located next to the furnace that will have the water-air exchanger. Should I plumb the side-arm in parallel with the water-air exchanger or in series after the water-air exchanger? I'm wondering if the temperature of the circulating fluid will still be high enough after passing through the water-air exchanger to provide enough BTUs to the sidearm exchanger. If it is in series the sidearm would receive flow whenever the house calls for heat, which is not optimal but very simple. If I run it in parallel I could also either use part of the flow whenever the house calls for heat, or I could get much more complicated and install a couple of zone valves and control system and have the circulating pump turn on if either the house or DHW needs heat. Opinions?

Thanks for your help.

 

[Nofossil]

I personally wouldn't worry too much about using a flowmeter. If you've done your calculations and selected a pump that's about right, there's a lot of latitude in terms of actual flow rates that will give acceptable performance. I'd go with the lowest speed that keeps bubbles purged and delivers enought flow to keep the return temperature comfortably high.

I'd put the sidearm in series BEFORE the water / air HX to ensure adequate hot water and a better response time. In any event, you'll want to install an anti-scald valve (I like the Honeywell AM101) on the DHW tank to avoid scalding people.