Initial Slab Heating Thermostat Settings

TylerB23 · September 2022

Hey all,

I recently purchased a newly built home with in-floor hydronic radiant heating, heated by propane. My question is, how should I set the thermostats (3 zones) for the initial heating of the slab? I read that it could take a few days for a slab on grade to heat, so should I hold the heat at a specific temperature for a few days or can I start by setting them on a schedule (wake, leave, return, sleep)?

The supply temp is set at 135F, but because it's mid-day and somewhat warm outside the current supply temp is 101F with a return of 98F.

Thanks!

Jamie Hall · September 2022

With a radiant slab, you do NOT want a schedule. The slab can't follow it, and you will just waste energy. Set the thermostat to the temperature you want the space to be and let it get there. It will, given time. And then LEAVE IT ALONE. If your system has outdoor reset for the slab circulating temperature, you may need to adjust the curve on that. If the boiler modulates, then that should be controlled with the outdoor reset. Otherwise, the slab should have a mixing, preferably temperature controlled.

But whatever the setup, don' even think about schedules or set back. Set it and forget it.

TylerB23 · September 2022

Thanks, Jamie.

It's a Navien NCB 240/130 boiler. The outdoor temperature sensor isn't installed at the moment. Is this necessary or is there another way to control temperature, such as the temp-controlled mixing you mentioned?

Also, there's rigid insulation under the slab, but nothing on the exterior edges - I'm planning to add this soon. Temperatures can sometimes drop to -20 celsius, with an average of about -10C through January & February.

hot_rod · September 2022

What type of floor covering do you have? 130F would be a pretty high swt for a concrete slab.

You should add the outdoor sensor and adjust the parameters, that will help provide the best temperature to the slab and prevent over-heating or the flywheel effect. The owners manual should guide you on how to set up the outdoor reset function.

Starting this time of year it should not take days to warm the slab.

EdTheHeaterMan · September 2022

All of the above and... You won't get it right the first time. This will be a learning process, and you may get too hot at times. Just remember that it will take some time to lower the temperature, so don't turn it way down 55° or OFF if it gets too hot. Just set it a few degrees cooler, or adjust the reset curve a little at a time then wait 24 hours before you make the nest adjustment. Record your adjustments or take pictures of your settings with a time stamp. This way you can get it right in a week instead of a month.

Ask me how I know!

pecmsg · September 2022

135 sounds high.

I’d start at 100* and raise 5* per day until your comfortable.

TylerB23 · September 2022

Thanks, everyone for the feedback!

We have vinyl plank laminate flooring. There's also a mini-split in the main living area. It's an 1800 ft2 bungalow (2400 with garage). We've been here for about a week and had the thermostats on a setback schedule until yesterday (72 at wake, 66 during the day, 72 at return and 64 overnight) and the mini-split was set at 24C.

Heating the house hasn't been an issue as it's been very comfortable (it currently drops to 36-39F overnight), but I just want to make sure I'm using the in-floor heating at its highest efficiency. I set the thermostats to 71 last yesterday and this morning some areas were at 66 - I'm assuming zones 2 and 3 because I also noticed that 'SH Zone 1' is the only one showing up on the Navien screen. Do you know if there's a way to turn on/off zones within the Navien boiler or are they all automatically 'ON'?

TylerB23 · September 2022

UPDATE:
Still no heat. I've had the thermostats turned up to 80F for the past few nights but it doesn't seem like the system is circulating anything. I took the attached picture of the set-up. The pex for the DHW is hot to the touch, but the SH pex is cold.

EdTheHeaterMan · September 2022

"80° for the past few nights". means that you have had the thermostat turned off during the day?

Since radiant floor heat can take up to 48 hours to get to the wanted temperature, is it possible that you are not leaving the thermostat on long enough?

With the thermostat set at 80° does the water temperature in any of these pipes shown in red get even a little warm? like maybe 100° or 110°

If not then there are many controls that can be to blame. Each one of those blue toppers on the upper manifold is an electric motorized valve actuator. There may be more than one thermostat in your home. Have you tried leaving all of them on 80° for at least 48 hours?

neilc · September 2022

Does this boiler run with a crossover valve between domestic and heating?
maybe that crossover is stuck to domestic ?

all circs running ?

Jamie Hall · September 2022

As has sort of been implied, setbacks -- even small ones, never mind ones as large as yours -- simply won't work at all with radiant floors. You have to set whatever thermostat is controlling the floors to a set temperature and leave it alone.

There may be another thermostat other than the ones you are playing with which controls the floor temperature. If the floor radiant circuits aren't heating at all, that may be set too low (if it's there). As @EdTheHeaterMan said, the pipes which he outlined in red should be warm if the floors are circulating.

hot_rod · September 2022

You have a supply and return manifold, one with the blue actuators is supply. Looks like the blue actuators pop white when open.

The other manifold has those clear plastic flow-setters. Those may be shut off also, try turning the plastic hex counter clockwise to make sure they are open. Pop the black cap up, remove it, flip it over it is a wrench to turn the meters to adjust.

When the actuators are showing white, the flowmeter should be reading some number, should be 1/2 to full open, maybe .75 gpm reading. If not the loop is air locked or the pump is not running.

Assuming that the supply and returns on the loops are connected properly?

If tubes are crossed at the manifold, you will not get flow.

Dave Carpentier · September 2022

Lotsa zones !
Those uponor valves do show white when zone is open. When testing room thermostats, keep in mind the valve actuates (opens or closes) very (very..) slowly. I've never timed mine, but I'd guess 2 or 3 minutes to open or close.

Initial Slab Heating Thermostat Settings

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