Fishing for specialty t-stat

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Open Neutral

Senior Member
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Engineer
Have a hydronic heating system with multiple zones. Most are controlled by wall thermostats to an uponor Zone Control and thermal valve actuators.

But in the master bath, there are 2 special zones; one is the heated towel rack. The other is the granite pedestal the tub sits within; it's lined with hydronic tubing on its own loop.

Both have the issue of "where do you put a thermostat?" The floor has one for its zone. There's no place on the rack for one, nor the tub. So my approach is to put a t-stat in the hydronic return line to the manifold. I've got thermowells with 1_Wire sensor probes in therm, but nothing like an adjustable on-off t-stat. Where should I look for such?

Suggestions?
 
First time I ever heard of a hydronically heated towel rack. Usually, they are electric with an integral thermostat. As to your dilemma, there are plenty of industrial controllers with sensing probes that can go in your wells. Problem is the controllers probably won't lend themselves to your decor. Maybe there are some that are blue tooth, WiFi or ethernet enabled so that you can control them from your phone?

-Hal
 
Sure, Honeywell makes those. Dunno though if that's what he is looking for. I kind of think he wants to be able to control them from the wall in the bathroom like the floor thermo. Yes, no?

-Hal
 
I would think that a desired setting for each could be established, and remote on-off control is easy.

Each could still be adjusted, perhaps with seasonal changes, with a trip down to the basement.
 
Probably Tekmar 519 air thermostat with a slab sensor for radiant. Slab sensor would be attached to the tub mass, to read it and respond.

519 - tekmar

Radiant Thermostat, One Stage Heat (Includes Slab Sensor 079)
www.watts.com www.watts.com

You don't say, but the radiant loop would normally have to be on an "outside air temp reset" controller, to mix down the boiler water temp, down below 130F typically and adjusting automatically with OAT.

Hot water reset - Wikipedia

en.wikipedia.org en.wikipedia.org

Straight boiler water temp at 180F into the radiant loop would be a problem if that's what you're doing. Tekmar also has the reset controllers for either mixing vale or proportional injection pump reset loops, plus good docs on how / why they work or are needed. Should be OAT, outside air temp reset, to the tub radiant loop. Then you can call the tub zone with the Tekmar 519.

Tekmar 519.png
 
I would put a mixing valve in the Hydronic’s piping to each zone, and a zone valve just controlled by a wall switch or timer, the mixing valve will control the temperature of the water going through the towel warmer and tub heat.
 
bgard said:
I would put a mixing valve in the Hydronic’s piping to each zone, and a zone valve just controlled by a wall switch or timer, the mixing valve will control the temperature of the water going through the towel warmer and tub heat.
Click to expand...
Not a fixed mixing valve. Seen that done. They put one in on a second try to fix it, still not right. Needs to vary loop supply water temp, with outside air temp, is the right way to do it, "outdoor air reset". With radiant, you are always in contact with the radiator. Fixed temp would be too hot on mild days, not enough when it's 20 F outside.

Depending on the OP's boiler, modern condensing gas boiler would have OAT reset built in. Just need to implement it. If the boiler does not have it, it would be in the near boiler piping for all of the radiant zone loops, and OAT reset injection pump or OAT reset mixing valve is the right way to do it. Tekmar docs are good to explain this.

Proper implementation of radiant with outdoor air reset can be "always on" with no indoor stat. That does work. There could still be an indoor stat but it's cycle time and necessary work would be much less. Indoor stat cannot compensate for wrong loop water temp. Especially if it's straight boiler water. Even with a fixed temp mixing valve and indoor stats, they could not get it working right.
 
The op is not talking about radiators for room heat, they are special zone for heating a towel rack and a bathtub, I don’t think it matters what the outdoor temperature is for these zones.
 
The tub is the radiator in this instance. The slab sensor of the Tekmar 519, attached to read the mass of the tub, could maintain setpoint temp for the tub.

OAT, outside air temp, is a proxy for heat loss or demand, and it's a lot more efficient and especially, comfortable, to match heat input to demand, which is what the OAT hot water reset controller for the radiant loop loads, hot water supply temp, does.

That's a different Tekmar controller along with near boiler piping, known as primary secondary piping. Unless the boiler is modern gas condensing then it has OAT reset built in already.

It's a lot easier to do it right the first time (it's a premium adder system). You will know every day if the tub is 10 F too hot or too cold. And for the customer, it's a legit complaint. It would be wrong temp and incorrect non standard implementation if the loop water temps are not OAT reset.
 
If it's 20 F outside the tub could be 90 - 95 F (wag guessing) on a reset schedule.

If it's 45 F outside the tub might be 75 F. OAT reset is the only way to do that. And you're saving money on your heat input. Not too much when it's warm out, more, comfy warm when it's freezing out.
 
hbiss said:
Not sure how that "sacrificial tub" works. I can't see the granite base heating the water. I think all it does is make it so you don't chill your butt when you sit on it to go in the tub.

-Hal
Click to expand...
I'm not thinking it's one big rock with a tub cut in it, but could be.

Magnus Home Products - 71" Kensington Stone Double-Slipper Tub with Pedestal

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Pricing pretty reasonable too but might need some I beams under it

"The other is the granite pedestal the tub sits within; it's lined with hydronic tubing on its own loop."

That reads like a more conventional surround with a drop in tub, usually acrylic or cast iron.

For the zone to work as intended, the PEX loops would have to be in direct contact with the tub for heat transfer. If the tubing is embedded in the surround and there's air space between the surround and the tub, the tub zone would be a room heater and not a tub heater.

Tubing would have to have direct contact plus an embedding method to the tub shell to be a tub heater. Tubing loops would have to wrap the tub shell. Or for a freestanding, embedded in the base.
 
Tekmar 361 OAT reset injection pump controller.

361 - tekmar

Variable Speed Mixing Control
www.watts.com www.watts.com

Typical OAT reset schedule and also shows the primary secondary near boiler piping using closely spaced tees for hydronic flow balancing. This is the right way to do it.

Second picture I've posted before is the implementation for my house. With two 361's visible on the wall, Pri Sec piping, Froling cordwood gasification boiler. Out to all in slab radiant heat. This system is four cord / year all in.

Tekmar OAT reset controller.pngP1000710.JPG
 
The "tub" heat is really the granite surrounding it; so you sit on it in the winter.....

The time factor & appearance issue means we'll just put a t-stat in the water return leg, and see how that works.
 
Open Neutral said:
The "tub" heat is really the granite surrounding it; so you sit on it in the winter.....

The time factor & appearance issue means we'll just put a t-stat in the water return leg, and see how that works.
Click to expand...
Very predictable what will happen. An aquastat, t stat on the water line, will only short cycle.

Load loop delta T, heat transfer to the load and water temp drop while flowing, is not enough to keep the return water temp from coming immediately up pretty close to supply water temp. Aquastat sees this and turns off. Then the stationary water cools and the aquastat calls again. It will have no idea what the surround temp is.

The surround is a room radiant heater. Regular air stat will know the temp and maintain air temp setpoint, regardless of the surround temp but better than the aquastat.

Tekmar 519 is what you asked for. The slab sensor can be embedded in the surround and the 519 can be programmed to ignore room air temp and only maintain setpoint of the slab sensor. Or it might give you the option to use both mass and air temp sensing. Say set the surround to 75 and the air to 72.

One thing about the 519. The pid algorithm is tuned for radiant, high mass loads. It will do its own PWM, short cycle on and off to maintain setpoint perfectly, but with a sensor to the actual thing you want at temp.

What are you doing to set the radiant loop supply water temp. Cannot be straight 180 deg boiler water. Gas boiler with reset built in or something else. That can give you trouble also, success or failure.
 
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A) It's GSHP fed with a water loop of say 110F.
B) The bath coils are to hear the granite, not the water.
C) It would impossible to have the added t-stats in the room. We have some Honeywell ?T104? with an external RTD sensors. It might be possible to epoxy one to the stone, before the tub is set. And then remote the t-stat to the basement.
 
110 F sounds good. PEX will like it. I'm guessing the surround is wood framed and plywood with granite tiling. Plywood being an insulator and where the PEX likely attaches to, similar to PEX underfloor radiant, they attach the PEX to plywood with aluminum heat spreader plates.

Water to water heat pump (ground source to radiant hydronic) is on my list of things to try. The surround with PEX attached would be just like a (radiant) panel radiator.

Honeywell T104 comes up as a thermostatic radiator valve, which is (afaik) high end European design, low temp boiler loop water with panel radiators and thermostatic valves. The remote sensor looks like an air temp responding and non electronic, capillary tube connection to the actuator. Looks tough to fish down to the basement. Panel rads with thermostatic valve are popular in Europe, where they are decades ahead of us in green design. It's on my list of things to try.

Tekmar 519, the slab sensor could be embedded in the surround mass for mass sensing and CL2 cable the sensor to the basement, where the actual 519 could be located, programmed to ignore the air temp and give you surround, mass sensing only. The 519 will maintain setpoint temp of the target mass.
 
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