Two q's: radiant floor temp sensor and NEST, and DHW IWH and anti-legionnairre water temp

modconwannabe

Hi all
I have newly installed under-floor radiant (Uponor plates between joists) and a Nest thermostat. Nest has a built-in algorithm called True Radiant that theoretically learns your home and then accounts for time to heat and automatically adjust setback times to be optimal. However it is weird to me that one would use an ambient air temperature sensor rather than one touching the floor itself—not least as I don't want to risk popping my 3/4 4" oak floors by increasing temps too high. What is the best option here: can I add a sensor that touches the floor to be the input to the NEST?

My second question is what is recommended for a 60 gallon TT Smart 60 IWH and safe temperatures to prevent legionella. Currently the system is piped p/s with ODR and the DHW is connected to that rather than my TT Solo 110's built-in DHW supply (I don't know why) and so quite often the water temp is below 120 deg, which I understand becomes a breeding ground for legionella. There is no cold water mix valve attached to DHW so whatever it gets heated to is what goes to my faucets. Is there a setting in the 110 that's smart enough to heat the P/S system to 125 whenever there's a DHW call and otherwise modulate to whatever is needed for heat?
I love having $35 gas bills but am afraid I'm taking a risk here.

Ironman

You don't want to use setback with radiant floors due to the long response time unless the setback is for an extended time. Just doing it for a few hours will yield no savings. What's controlling the water temp to the floor and what's it set at?
Unless the indirect is serving as a buffer for the space heating, it should be connected to the domestic provisions on the boiler. It takes 140*+ to kill legionella.

Jamie Hall

It's not just legionella -- there are other charming critters that live in warm water whom you do not want to make the acquaintance of. Your water heater should be set to hold as close to 140 as you can (note that some ultra green city codes may not allow this, on the theory that it wastes energy. Please don't get me going). However. You should also use a mixing valve on the hot water supply to the rest of the house so that the actual water at the tap doesn't exceed 110 to 120, to avoid scalding the unwary.

modconwannabe

Thanks for the replies: the "setback" on the floors I suspect is minor, just easing off a couple degrees here and there as it does indeed take time to raise back. But for me I'd prefer a more sensitive control that takes into account floor temperature vs ambient, which is kind of the whole thing with radiant no?

As for legionella, yeah that's what I don't understand. I was curious as to why he was piping the DHW as a zone in the secondary loop which makes no sense unless there's more sophisticated things going on--maybe the DHW makes a call that automatically jacks temps up to 140 or whatever. I had read that 125 was sufficient to prevent the breeding of most nasties, but that 140 was the kill temperature, but wasn't sure if my small system for a two family 3 storey house is at issue.

Zman

Your in floor wants to run at the lowest possible temp to maximize efficiency and comfort. The boiler is designed to have an outdoor sensor installed and outdoor reset enabled.
The water heater needs a constant temp and will perform better with dhw priority.
If the boiler is set up per the manufactures instructions, this is all possible. You can even have the tank temp spike occasionally to kill off the ugly stuff.
Post pictures and we can tell you what it will take to get it there.

Rich_49

While floor surface temps cannot be ignored , the king is MEAN RADIANT TEMP , ambient is an illusion with radiant .

modconwannabe

My point exactly Rich. It may be comfortably warm regardless of a low thermostat or high thermostat setting, so my larger concern is that I might turn up the thermostat and go above 80 or so degrees floor surface which might pop my floors. The underfloor has a cold water mixing valve at the manifold, which has 4 loops. I don't recall last heating season what the circulating temp was but we insulated the joists and have sealed the basement ceiling now so it will need to be lowered. The top floor has wall radiators and basement has cast iron baseboard.

Rich_49

RH% is a much larger issue with the floors than surface temps . <85* should be fine . Don't forget about the relative humidity inside is my point , much bigger problem .

hot_rod

modconwannabe said:

My point exactly Rich. It may be comfortably warm regardless of a low thermostat or high thermostat setting, so my larger concern is that I might turn up the thermostat and go above 80 or so degrees floor surface which might pop my floors. The underfloor has a cold water mixing valve at the manifold, which has 4 loops. I don't recall last heating season what the circulating temp was but we insulated the joists and have sealed the basement ceiling now so it will need to be lowered. The top floor has wall radiators and basement has cast iron baseboard.

The temperature of the radiant panel needs to be sufficient to cover the load of the room. So it all goes back to the heat load and the design of the radiant. You should design to heat the room on a design day with temperatures below 85 surface temperature.

If the floor needs to exceed 85F surface temperature to cover the load, supplemental heat emitters should be considered.

It's fine line, if you run the floor surface much below skin temperatures you may not "feel" warm floors. Radiant floors in low load homes often get complaints of cold floors, even though they may be warming the space and satisfying the control stats.

A nice way to control a radiant floor is a combo stats that watches floor and air temperature. The tekmar controls are one of the best ways to control radiants, I don't know that the Nest has all the logic onboard that a true radiant stat has?

modconwannabe

Just as an update for future readers, I consulted with Triangle Tube, which confirmed that my IWH should not be piped as a secondary, and only as a direct connection to the boiler so that temperature is consistent instead of being subject to modulation guided by the ODR. So plumber is returning to fix that. It should also have a mixing valve after the IWH going to house supply.

Other discovery was that my floors were heating up despite being turned off, because stat calls from other zones were forcing water past the radiant floor's pump and through the system, and plumber forgot/neglected to put zone valve to prevent it. He's a real peach.

I also have come to realize that IMO the NEST really isn't so functional for radiant systems, or at least hot water ones (maybe steam makes sense still), where you really have little to no setback during heating season. It's nifty to be able to check temperature and humidity remotely, and the interface is wonderful, but it's meant to learn your habits for turning on and off central heating/AC which just isn't the case for my system.

Thanks all for the input.

Jamie Hall

modconwannabe said:

I also have come to realize that IMO the NEST really isn't so functional for radiant systems, or at least hot water ones (maybe steam makes sense still), where you really have little to no setback during heating season. It's nifty to be able to check temperature and humidity remotely, and the interface is wonderful, but it's meant to learn your habits for turning on and off central heating/AC which just isn't the case for my system.

I've been saying that ever since they came out!

Rich_49

How in the world would the boiler know where the indirect is connected in the piping ? As long as the sensor (aquastat) is connected to the DHW and the boiler pump drops out and the IDWH circ takes over why would it be influenced by ODR since the boiler has or should have determined that this is a DHW call ?

modconwannabe

I don't quite know, Rich. My hope was that the aqua stat and zone would be somehow listed as DHW via the Taco relay (a dedicated terminal or setting), and so when there was a DHW call it would heat the primary to whatever setting is required, 140 or so. But TT says this is not the case, or at least that you shouldn't have the DHW as a secondary in P/S piping. Currently the DHW is heated to whatever temp the primary is circulating, which is influenced by ODR. All summer in modulated down to under 120 degrees. Now it's 120-130.

Rich_49

Just looked at the TT manual for the Solo 110 . Did the installer even open the manual ? Although I do not place the DHW on the secondary side I did just have to assist in design for a Utica SSC which always has the boiler circ on and the DHW should be on the secondary side . I cannot understand why someone would connect the sensor wiring and circ to the Taco relay unless they did not read the manual . Nice boiler and the controls are easy enough .

Ironman

Rich said:

How in the world would the boiler know where the indirect is connected in the piping ? As long as the sensor (aquastat) is connected to the DHW and the boiler pump drops out and the IDWH circ takes over why would it be influenced by ODR since the boiler has or should have determined that this is a DHW call ?

Rich,
There's a separate connection for domestic at the boiler. I may be wrong, but I don't believe the control will allow the CH pump to run when there's a domestic call. At least that's how it was on the old MCBA control.

modconwannabe

Rich said:

Just looked at the TT manual for the Solo 110 . Did the installer even open the manual ? Although I do not place the DHW on the secondary side I did just have to assist in design for a Utica SSC which always has the boiler circ on and the DHW should be on the secondary side . I cannot understand why someone would connect the sensor wiring and circ to the Taco relay unless they did not read the manual . Nice boiler and the controls are easy enough .

When it was originally piped and he explained teh system, I asked him why he didn't pipe DHW directly to the boiler DHW output, and he pushed past the question insisting this was the most efficient way to do it and so on. I can see somewhat the logic, but then you realize that means you can't really modulate heating calls below 140 or whatever your hot water setpoint is. Or what the high temp is either, without a mixing valve.

Ironman said:

Rich said:

There's a separate connection for domestic at the boiler. I may be wrong, but I don't believe the control will allow the CH pump to run when there's a domestic call. At least that's how it was on the old MCBA control.

I believe there's a priority setting for DHW in the boiler settings; I know the Taco relay has a switch in it.

ajomiii

...can I add a sensor that touches the floor to be the input to the NEST?

Hi,

I see that this is an old question, but was there ever a consensus on whether a floor sensor could be combined as input to the Nest?

I currently have hydronic radiant floor heat under hard wood; it uses little Watts Radiant (P-3285) thermostats that allow you to set air temperature and also have a floor sensor allowing you to set a maximum and minimum floor temperature. They aren't programmable. I'd like to replace them with Nest thermostats in the rooms that I use the most so I can control those remotely (and leave all the other ones at a low temperature generally). Is the floor sensor important to have? I noticed that most thermostats DO NOT have floor sensors. There are a few Honeywell (that aren't WIFI) and some by TekMar (now owned by Watts).

modconwannabe, do you end up removing the nest?

Thanks
AJ

SuperTech

@ajomiii read the thread again. It's a well established fact that the Nest is a poor choice of thermostat for hydronic applications, especially radiant. If you want remote smart phone access to the thermostat buy a Honeywell T6 or vision pro 8000.

Anyone who wants the best possible control for a radiant or hydronic heating system should research the controls made by Tekmar. Slab/Floor temperature sensing as well as indoor ambient temperature sensors and outdoor temperature sensors for reset. Smart phone access is also an option.