Runtal Again!

Unknown

While I think TRVs are most excellent and would encourage you to use one, why were you told this? is the Runtal sized to operate at lower water temps than the baseboard needs?

chris_69

Runtal Again!

I recently sent a question about Runtal baseboards and now I have another. It turns out that where we would like to install the Runtals, they would be at the beginning of a loop of traditional fin tube baseboard. We have been informed that we have two options; 1) reverse the loop so that the run of Runtal will be at the end of the loop OR 2) install a TRV on the Runtal and by-pass piping so that when the rooms with Runtal gets warm enough, the heat will by pass those rooms and continue on to the run of fin tube. Any thoughts?

Viv

chris_69

Because Runtal notes in its Installation tips, "Runtal baseboard may be used with fin-tube, but the fin-tube will perform better if the Runtal is at the end of the loop". And because, in response to a previous question of mine, lchmb responded:

"Another thing to keep in mind is the location on the zone itself. If you are running this with baseboard on the same zone, Runtal recommends their baseboard be on the end of the zone so as not to affect standard baseboard" while Brad (White) wrote:

"The concept of using these at the end of a run of fin-tube rather than the beginning is to preserve the hottest water for the fin-tube. With the higher radiant output of the Runtal, it is more forgiving of lower water temperatures."

So there you have it - straight from the mouths of a number of horses, so to speak.

Viv

P.S. I'm learning that this is really a science. Not sure many homeowners appreciate this.

Unknown

well, there are a boat load of assumptions built into that recommendation, but in general yeah, runtals perform better at lower temps, so leaving the hotter water for the BBs is generally best practice. I was just curious as to whether there was a specific design consideration happening here or whether it was just the 'recommended guideline'. There could certainly be a situation where existing BB may be just fine with lower temps and you need higher output at one end, that a Runtal could provide using the hotter water first. But you'd need to crunch the numbers to see if your case is one of those.

Make sure you size the runtal for the inlet temp (or average temp, if you use their correction factors) it's going to see though, and not the supply water temperature at the beginning of the BB loop.

Whatever you do, a TRV is an excellent choice.

lchmb

Viv

My first and foremost suggestion would be to have your installer contact Runtal directly. I have had excellent luck dealing with them in the past and they will suggest the best for your system.

John Ketterman

#2 is certainly the better option, although it means more piping.

If there is more than one Runtal room (daisy-chained) and only one has a TRV, getting every room to heat just right may not be trivial. But if the rooms are connected in parallel (not daisy-chained) and all rooms except the one with the thermostat (including the fin tube rooms) have individual TRVs, then you have an excellent system.

bill clinton_3

makes no sense to me

We install lots of Runtals. Put that radiator on the end of a line with lower water temperatures and you just have to increase its size to compensate. Put it at the front of the line and you have to upsize the baseboards AND figure a way to compensate for the higher pressure drop of the Runtal. Better to pipe the Runtal two pipe: the BB can then be series loop if you want. OR, you can just let the runtal run wild: pipe it so it gets heat when any other part of the system is on, but its return water doesn't become part of the supply for the BB; control it with the TRV.

Bill

Fred Campbell

Piping Runtals w/BB

Any where in a BB loop you add a Runtal radiator, with it's 1/2" tappings and related pressure drop, you're going to slow the flow through the loop. If installed at the end of the loop it may even increase the output of the of the fin tube but the water temp will be lower by the time it hits the Runtal. OK in their book. Installing one at the supply end could create a situation where, by the time the water hits the end of the BB loop, it's cooled enough due to "slower flow" to decrease the output significantly.

It's best to parallel pipe the Runtals.Or, in a retrofit we've had good luck with a bypass pipe and balancing valve without using a TRV.