Pump Sizing/Troubleshooting

OcrRzr

I have a Taco Veridian VT2218 pump circulating to two Modine HCH22 hydronic unit heaters. I'm not getting as much heat out of them as I expected and am trying to figure out where I went wrong and what I can do about it. I'm an idiot, so be gentle and don't expect too much out of me.

Problem 1: Either the VT2218 is too small or I have a constriction in the piping or maybe both. At full speed the pump only moves 2 gal/min (per the pump display), looking at the pump output curves that would indicate I've got about 19ft of head in the system. The HCH22's are plumbed in series with no valve control, each is 4.9ft of head so 9.8 total head for just the units. Rest of the plumbing is approx 120ft of 3/4" PEX with 3/4" copper at the units and pump, and approx (8) 90deg bends. Does 9ft of head pressure loss sound about right for the 120ft and bends or do we think there is a constriction somewhere in the system?

Problem 2: Even at only 2 gal/min I'm only getting about a 10-15deg temp drop with one unit running, and about 20-22deg drop max with both running. That's at about 120-130deg supply and about 60deg entering air temp. At 8.33 BTU/gallon/deg that's only about 20k-22k BTU I'm getting out of them. I'm a little confused on this one - if I increase the circulating flow I'll reduce the temp drop, no? The HCH22 manual lists the 22k BTU at 140 entering water temp and 2.2 gal/min @ 60 deg entering temp and 20 degree drop. I guess the 140 deg supply is enough of a difference to still get 20 deg drop at 2.2 gal/min per unit?

Am I correct that I need a larger pump? What's a good pump for this application? I had hoped to use the features of the Veridian to control the speed of the pump - increasing when both units were running - but since it just runs full out that didn't really work out…

Jamie Hall

A bigger pump would reduce the temperature drop, yes… but you can only get just so much heat out of those unit heaters, and that amount is governed by the water temperature going in to them and the airflow through them. Your problem isn't water flow. If you need more heat, you need higher temperature or greater air flow.

Joe Mattiello

not sure if this is a new system, or an existing system you started having issues with?

Based on you line size at 3/4”, at 120 lineal feet, the 2218 should be fine.

I suggest calling Taco tech support at 401-942-8000 for additional support.

hot_rod

120’ total of 3/4 Pex? 8 ells, any other valves or fittings?

4.4 gpm is about the limit for 3/4pex, just under 4 fps velocity. More like. 11.6’ head in the Pex circuit

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Dave H_2

I'm estimating about 13' of head in the system since the heaters are piped in series and with the elbows and fittings at each heater.

At 4.5 gpm, the VT2218 is sized correctly even though getting close to the max capability of the circulator

Is this circ hydraulically isolated from the rest of the system, what's your boiler? Whats the piping look like around it?