TRV questions

R. Kalia

(a) In a room with two similar radiators, is it adequate to put a TRV on one of them? My hope was that setting it slightly low, and thus letting the other radiator carry the ball for the last few degrees, will work to hold a comfortable temperature?

(b) In one room, for some strange reason we have 2" pipes on both sides of the radiator valve, and the plumber says he can't get an appropriate TRV (he asked the local supply house). Is this something I can get by mail order, or am I out of luck? I couldn't find it on the web.

(c) How does one put TRVs on cast-iron baseboard? Where do they go, physically? There would appear to be insufficient space, unless they make tiny ones. I guess one could also put them in the basement, with remote sensors, but I am told the remote sensor lines are fragile.

Mike T., Swampeast MO

Converted gravity system I presume...

a) If the system is set up for constant circulation (HIGHLY preferable) I think you're asking for trouble if you leave a rad without a TRV. It will have extremely little restriction and flow will be extremely high--even with a good reset curve, the space is likely to overheat.

It's possible to put multiple rads in series served by a single TRV, but beyond the problem of piping it that way in an existing system you might run out of heat in extremely cold weather. Multiple TRVd rads in a single space seem to coexist nicely. You'll likely find that they seem to automatically "stage" themselves based on their location in the room and the outside temperature.

b) 1½" is the largest TRV I've seen listed--not to say that larger don't exist, but they're likely extremely specialized and horridly expensive. Unless that rad is approaching 1,000 square feet EDR, a 1½" valve will be adequate. You'll probably have to remove the rad runouts to the first fitting, install a reducing bushing and install small runouts. A ¾" valve is generally adequate for most any radiator of normal size. That rad with the 2" tappings is likely low and close to the boiler--the most difficult place to achieve gravity circulation.

c) Have never done that so really can't say. TRV valves are available in a number of configurations and you can [probably] "modify" things to get them into most situations. If utterly impossible you can install a straight valve in the basement. While the valves are highly reliable, MAKE CERTAIN THAT SUCH A VALVE HAS SERVICE ACCESS!!!!! The capillary tubes that connect remote temp sensing/adjustment devices do appear to be tiny and fragile, but seem to be surpisingly robust. Small children though are a definite wild card...

R. Kalia

good guess

Thanks for the answers...

>even with a good reset curve, the space is likely

> to overheat.


I assume that if it overheats, it is not a good reset curve. But I plan to have TRVs in every room except the one with the thermostat.

> That rad with

> the 2" tappings is likely low and close to the

> boiler--the most difficult place to achieve

> gravity circulation.


Yup, that's it exactly. I didn't know that was the reason for the large pipe.

Mike T., Swampeast MO

You can get a good reset curve but not a perfect reset curve. While you may be lucky, your system likely has some imbalances in the radiation. Your supply temp has to be adequate for the least radiated space.

Even if the radiation is perfectly balanced you really want a bit of "headroom" in the reset curve. One of the nice things about TRVs is that you can keep a space significantly warmer than the "average" temperature if desired. The headroom also allows you to raise space temperature more quickly.

I still think it best to use a TRV on every radiator and run the circulator constantly. This allows the TRVs to do their best at the job they do so well. If you utterly insist on having a wall thermostat, just install it as a system-wide high-limit that stops the circulator (and boiler if needed). Normally it will be set quite high so that it is NEVER satisfied and the TRVs will maintain the space temperatures. The t-stat will be useful for whole-house setback as when you set it down, it will be satisfied and shut down the system even though the TRVs are not satisfied.

R. Kalia

what if all TRVs shut off?

> still think it best to use a TRV on every

> radiator and run the circulator constantly.


What happens if the reset curve is not perfect, and the rooms are warm enough to shut off all TRVs while the pump tries to keep pumping?

Mike T., Swampeast MO

That's where you use a Differential Pressure Bypass valve. As the TRVs close and head pressure through the system climbs, the differential pressure bypass will open. Depending on how you adjust the valve it can have two different functions.

1) If adjusted above the normal differential pressure in the system, it acts as a high-limit on differential pressure--you only get bypass when pressure climbs above your setting.

2) If adjusted below the normal differential pressure in the system, there will be some bypass constantly and the differential pressure in the system will stay constant regardless of TRV settings. (This seems to work well on gravity conversions with simple iron boilers, but you have to adjust "by ear".)

Location of the valve will depend on the boiler and system piping. If primary/secondary is used, it will be on the secondary side bypassing the radiation. If primary/secondary is not used, it is installed near the boiler between the supply and return main. Regardless the circulator must be "inside" the short-cut loop that the differential pressure bypass valve creates.

With a conventional boiler on a gravity conversion, you can just use a fixed bypass line instead of the differential pressure valve. As all of the TRVs close the velocity through the fixed bypass line (it's significantly smaller than the mains) will increase and noise may become noticeable.

If you're using a condensing or condensing/modulating boiler make certain you follow the boiler's instructions for piping/bypass/etc. when TRVs & constant circulation are used. Viessmann (and I presume others) require a "low-loss" header to be used when the piping system is oversized with extremely low differential pressure.

A warm weather shutdown control (WWSD) is a very good idea on most constantly circulating systems. 55°-60° is generally a good shutdown point for most climates.

R. Kalia

sounds good

Sounds good, to the extent that I understand it! Of course, now I'll have to explain all this to the contractor, who will look suspicious.

Mike T., Swampeast MO

You seem to understand well--you ask just the right questions!

Mike T., Swampeast MO

One last thing...

...make certain the contractor knows that TRVs are directional when it comes to flow. All I have seen are installed on the supply side.

Supply/return didn't matter for hand valves as they are non-directional. They are usually on the supply side, but not always. You have to trace the lines to each and every radiator carefully. Unfortunately that's often easier said than done--particularly when you're in the basement trying to figure out exactly how that rad on the 2nd or 3rd floor is oriented compared to the pipes disappearing into the wall cavity...

If installed on the wrong side--you'll know. You will hear a "bang--bang--bang" as the TRV tries to throttle down...

tp tunstall

trv

as long as the flow direction is correct, trv's can be installed on either side.or even on the middle of a long fin -tube run.

R. Kalia

that''s what I had read...

...so who is right?

Mike T., Swampeast MO

The TRVs that connect to standing iron rads typically have a union connection that goes on the radiator side. Since they're usually a right-angle valve as well, you'd have to do some really goofy piping to install the valve "backwards" in order to have flow in the proper direction.

It's the physical connection that's the problem--not the location of the TRV. As a two-way device, position in the loop (or side of radiator) doesn't matter--as long as you can connect it the way you want that is...

The valves with sweat or NPT connections at BOTH ends are a different matter as they can be installed in either orientation, depending on flow direction.

tp tunstall

trv

most valves, if not all trv manufacturers, have an arrow which shows the direction of flow. the main reason the manufacturers do this is to protect their interest. so follow the arrow! tp