A radiator’s output falls much faster than the water temperature: taking output as proportional to the 1.3 power of the difference between mean water and room temperature, a radiator that gives 100% at 50 K above the room gives 87% at a 65 °C mean, 63% at 55 °C, 52% at 50 °C and 21% at 35 °C. Test each salvaged radiator for its real output, size the number of radiators for the loop temperature you will actually run, and use floor heating only where the loop will run at 35 to 45 °C.
Part of District Heating
The emitters in each house: how radiator output depends on water temperature, how to size and test salvaged units, and when floor heating fits.
Output Falls With The Temperature Difference
A radiator emits heat roughly in proportion to (mean water temperature − room temperature) raised to a power near 1.3. The exponent is a recalled value for hot-water convector-radiators and varies by type, so measure rather than rely on it. With room temperature 20 °C and a reference difference of 50 K as 100%:
| Water supply / return | Mean water | Difference | Output against the 50 K reference |
|---|---|---|---|
| 90 / 60 | 75 °C | 55 K | 113% |
| 80 / 50 | 65 °C | 45 K | 87% |
| 70 / 40 | 55 °C | 35 K | 63% |
| 60 / 40 | 50 °C | 30 K | 52% |
| 40 / 30 (floor) | 35 °C | 15 K | 21% |
Working, 70 / 40: mean = 55; difference = 35; (35 ÷ 50)^1.3 = 0.7^1.3 = 0.63, or 63%. This is why a cooler network, which loses less heat in the pipes (see Design Temperature & Flow Rate), demands more radiators in the house.
Test A Salvaged Radiator
A salvaged radiator has no usable rating. Measure it.
- Connect it to a flow of hot water, with a thermometer at the inlet and one at the outlet, in a room at a steady temperature.
- Measure the flow by catching the outlet in a bucket for a timed period.
- Compute the output Q = m × c × (T_in − T_out). Note the mean water temperature and the room.
- Scale to the loop temperatures you plan to run, with the 1.3 exponent: Q₂ = Q₁ × (ΔT₂ ÷ ΔT₁)^1.3.
Worked Example: How Many Radiators For A 4 kW House
Inputs: a tested radiator gives 0.80 kW at a mean water temperature of 55 °C in a 20 °C room (difference 35 K); the house needs 4 kW at design.
- Run 80 / 50 (mean 65, difference 45 K): output = 0.80 × (45 ÷ 35)^1.3 = 0.80 × 1.386 = 1.11 kW each; 4 ÷ 1.11 = 3.6, so 4 radiators.
- Run 60 / 40 (mean 50, difference 30 K): output = 0.80 × (30 ÷ 35)^1.3 = 0.80 × 0.818 = 0.65 kW each; 4 ÷ 0.65 = 6.1, so 7 radiators.
Lowering the loop temperature from 65 to 50 °C mean takes the house from 4 to 7 of the same radiators. Mount them below windows, as the carrier page advises, with an air vent at the top and a drain at the bottom.
Underfloor Heating
The carrier page gives 35 to 45 °C water in a floor slab, installed during construction, as the most comfortable method. Two figures I am recalling and not taking from the corpus: a floor surface kept at or below about 29 °C for comfort, and a combined heat-transfer coefficient from floor to room of about 11 W/m²K. At a floor at 27 °C in a 20 °C room that is 11 × 7 = 77 W/m², so 4 kW needs 4,000 ÷ 77 = 52 m² of heated floor. A small house may not have that much floor, and the floor heat output falls as insulation goes.
Practical points:
- Heat stored in the slab continues after the water stops; the corpus Hypocaust) page gives a 20 m² slab-and-earth floor 15 cm thick as storing 50 to 63 MJ per 10 K of cooling and releasing it over 9 to 12 hours.
- Pipe inside concrete is a permanent joint-free job: use continuous pipe (copper, or plastic rated for the temperature; see Pipe Materials (Clay, Steel, Plastic)) with no joints under the slab.
- Pressure-test at 1.5 times working pressure before covering (Pressure Testing).
- Mix the supply down to the floor temperature; an 80 °C supply into a slab damages it and the floor.
Common Mistakes
- Counting radiators at nameplate. Test, and rate them at your loop temperature.
- Hot water straight into a floor. Mix down.
- Burying a joint. A leak under a slab means breaking the floor.
Safety
Hot surfaces and hidden leaks
A radiator surface at 65 °C or more burns a child’s skin; guard or place them out of reach in children’s rooms. A leaking floor loop saturates the slab and its insulation silently. Pressure-test before covering, and if the hypocaust-style flue floor of the corpus ondol page is used, check the floor for cracks, since a cracked flue floor leaks carbon monoxide.
What’s Next
- Learn next: Domestic Hot Water Takeoff — the other use of the district heat
- Combine with: Supply & Return Piping Layout — what the emitter temperatures do to a series loop