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Apologies LittleInch.It must when there is DHW demand (presumably)?. All I've measured are with temperature clamps a fairly rapid decay in the inlet temperature of ~ 2.5 °F/min to the air handler coil and the temperature in the outlet increasing (which presumably means it's not moving through...

@ LittleInch: Yes the pump (Grundfos non-submersible circulation pump) only active when heat called. I believe there is intermittent operation to prevent Legionella. Schematic here (not entire property)- not showing the DHW loads (eg. shower, faucets, etc)

@HVAC_Novice: yes it's the detail from the manufacturer. As noted earlier, it's a new build (4 years roughly). The plans approved by the local municipality showed a 199 kBTU/h unit whereas they installed 160 kBTU/h. They don't have the same capacity obviously. Hydronic coil draws 2 GPM steady...

@PEDARRIN2: I'm stuck with 1 x single unit of tankless (condensing type). Not boiler. The heat exchange life diminishes to 3 years from 15 years if used in combo DHW + space heating (vs standalone DHW).

I believe the building code here has showers fixtures rated a minimum 1.50 w.s.f.u. (or 1.5 gpm) minimum. Two 2x showers would be 3 gpm simultaneously. Space is the issue. Can't fit a storage tank. Mech designer used a method to size conventional storage type tanks (using FHR-first hour...

The HX is a hydronic air handler. Coil sits above the blower fan. Water comes in at 120 F exists 94 F (delta T = 26 F), which leads to 25,000 btu/h of supply at 2 GPM. Heat loss ~ 19,000 btu/h There is prioritization for DHW.

1600 sq.ft. Outdoor design temperature = -18 C (-0.4 F) in winter.

It is separate.

Question du jour about sizing tankless water heaters (instantaneous type): Tankless water heater installed in conjunction with a hydronic heating coil in an air handler. -Tankless input: 160,000 BTU/h -Tankless output: 157,670 BTU/h -Space heating capacity (Hydronic coil with air handler)...

What did you use to get 15W/m^2?

Air temperature of the room is controlled via thermostat. It's maintained at 21 C. The void under the slab corresponds to a space that is obviously heated by the radiator that sits approx 8 inches below the slab, but above the drop ceiling as shown above with insulation. The drop ceiling...

I suppose was looking for guidance on the heat flux, temperature distribution on the slab at steady state as it's fairly non-uniform.

Fair point I did not ask the correct question. I suppose what I meant to say is that the heater at the source is 2 kW (approx) and the amount coming through is much less obviously. But you're right there is a long transient to start feeling it above....and the floor temperature on the other side...

The cavity is air and slab is irradiated...so not quite direct. I could be wrong here...the slab has a lot of mass that continuously radiates the space above

Cavity dimensions are hard to get as slab geometry is complex... basically it's a ceiling of a parking structure with drop ceiling drywall suspended and batts of insulation.

I only have rough thermal images ...based on that it's around 30-41C ...locally. Of course the heat I'm assuming conducts radially in the slab somewhat (elliptically given line source) and decays with distance. Top surface inside is ambient. May have to instrument this with a heat flux sensor in...

Looking for some guidance on how to best calculate the heat generated from an electric heater that is installed beneath a concrete slab. It's 275 W/ft heater (7 ft long) 347 V, 5.76A electric baseboard heater that was installed beneath the slab (sits about 8 inches beneath it on some mounting...