Pressure drop and flow resistance are critical when installing TRVs.
Thermostatic radiator valves found their way onto the Empire State Building’s radiators in 1929, and they’re a staple on the hot water radiators of Europe these days. Manufacturers build them right into their radiators, which is nice. Back in my rep days, we took on the Danfoss line of TRVs in 1972, which was a year before the big OPEC oil embargo. Fuel was cheap in 1972. My old boss took the line because he believed in comfort.
We were in New York City, where plenty of steam radiators are installed, and TRVs do a nice job of keeping steam-heated rooms from overheating. We were also on Long Island, which has plenty of old diverter-tee systems. TRVs seemed like a lovely way to zone those old radiators, and we set out to do that, as well as other adventurous things. I learned some hard lessons along the way.
One of the first things I learned was that a TRV, even when fully open, still offers a noticeable resistance to flow. When you have radiators on diverter-tee systems, that resistance might be enough to stop the water altogether. And where there is no flow, there is no heat. It looks just like an air problem. But it’s not.
Oh and whether or not this happened to me depended on the level of confidence I was displaying to the contractor at the time. Pressure drop seemed to increase with my level of verbosity. But this is how we learn humility.
The TRV has two parts. The part that attaches to the pipe is a normally open, spring-loaded valve (with a pressure drop). You attach to this the other part of the TRV, which is an operator containing either a fluid or a wax that is very sensitive to changes in air temperature. As the air temperature rises or falls, the fluid or the wax inside the operator will expand and contract, moving the spring-loaded valve open or closed. Control the flow and you’ll control the heat. You can adjust a TRV to whatever temperature you’d like in the room it serves, typically between 50° to 90° F. Sounds great in a sentence, doesn’t it?
But what you need to watch out for in practice, and this is especially true when you’re working with those diverter tees, is that pressure drop. You’ll find this in the valve manufacturers’ literature. They show it as Cv, which is an engineering term that always appears as a number. For example, you might see Cv = 2.5. That 2.5 is the gallons per minute. Any number that appears after the = in the Cv equation will always be gpm. What the equation is saying is that when, in this case, 2.5 gpm flows across this particular valve, there will be a corresponding 1-psig drop in pressure from one side of the valve to the other.
Cv always relates to a delta P of 1 psi. If you look at two valves, say, one where the Cv = 2.5 and the other where the Cv = 3.0, the latter valve will have less of a pressure drop. The higher the number, the lower the pressure drop.
That makes sense, doesn’t it? With the first valve, you get a 1-psi drop in pressure with just 2.5 gpm flowing. The second valve can flow a full 3 gpm before the water suffers that same 1-psi pressure drop.
So if I were choosing between those two TRVs for my diverter-tee system, I’d probably choose the second valve because it has a higher Cv number, which means it offers less resistance to flow. I don’t want the valve, when fully open and just sitting there, to present my flow with so much resistance that the flow just shrugs, gives me a dirty look and stops.
Remember: Where there is no flow, there is no heat. I hope you won’t have to learn that the way I learned it.