Space heating
How big a heat pump — and what puts the heat in?
Most sizing tools stop at the kilowatts. This one goes on to the part that decides whether the building is actually warm: the fan coils or radiant panels that move that heat into the room, and how much of their rated output survives on heat pump water rather than boiler water.
The space
How big it is, and how it is built.
Floor area to heat
120m²
The heated floor, not the plot. One room, one floor or a whole house.
Ceiling height
3.0m
What it is attached to
A flat between two heated flats loses heat through
far less surface than a detached house of the same area.
Glazing
15%
Window and glass door area, as a share of the external wall.
Temperatures
The difference across the wall is what costs money.
Nearest place
A starting point only. The slider below stays editable.
Outside design temperature
2°C
The cold the system still has to hold the room at — not the record minimum, and not the winter average.
Wanted inside
21°C
Every degree costs. Raising the room by
1 °C adds roughly 5 % to the load in a cold climate, and a
radiant system feels warm at a lower air temperature than a fan
system does.
How the heat gets in
Fan coils, or radiant panels with no fan.
A fan coil moves air across a coil. A radiant panel
has no fan — silent, nothing to service, but its output falls
away much faster as the water cools.
Water leaving the heat pump
50°C
Lower is cheaper to make and kinder to the COP; higher makes the emitters smaller. This is the trade-off the whole design turns on.
Temperature drop across the emitters
5°C
Fan coil size, each
1.5TR
Nominal cooling tonnage, which is how fan coils are sold here. The heating output at your water temperature is worked out from it.
The plant room
One machine or several, and whether you want a spare.
Standby machine
A standby is one spare machine of the same model,
piped in parallel and rotated with the duty units.
Most machines you would accept
4
Two smaller machines often fit the load far more closely than one large one, and they stage better through a mild spell.
Sizing margin
15%
Headroom for warm-up after a setback, plan shape, and a colder day than the design one.
What you pay for energy
Change these to your own tariff.
What heats the space today
The saving is measured against this one. The chart
still shows the others.
Electricity
9₹/kWh
Diesel
92₹/litre
LPG (commercial)
95₹/kg
PNG
55₹/SCM
Hours heated per day
12hours
Days of heating per year
120days
Construction and comparison assumptions change only if you know why
Wall U-value
1.4W/m²K
Roof U-value
1.4W/m²K
Floor U-value
0.9W/m²K
Glazing U-value
3.2W/m²K
Air changes per hour
0.9ACH
Part-load factor over the heating hours
65%
The design load is the worst hour of the year. Over a season the average runs well below it.
Fan coil sensible heat ratio
72%
A cooling rating includes latent heat; only its sensible share describes the coil that also heats.
Fan coil air-to-water difference when cooling
17.5°C
The difference behind the nominal tonnage: 27 °C air against a mean 9.5 °C chilled water.
Electric heating efficiency
100%
Diesel boiler efficiency
85%
Diesel energy content
10kWh/litre
LPG boiler efficiency
90%
LPG energy content
12.8kWh/kg
PNG boiler efficiency
90%
PNG energy content
10.5kWh/SCM
Whatever figure you have — ours or anyone
else’s, installed. Nothing is sent anywhere until you fill in the
form below.
Grid electricity carbon
0.71kg CO₂/kWh
The Indian grid average. Lower it if you have rooftop solar or a green tariff.
Diesel carbon
2.68kg CO₂/litre
LPG carbon
2.98kg CO₂/kg
PNG carbon
1.9kg CO₂/SCM
Coal / biomass carbon
1.8kg CO₂/kg
Swings hugely with grade. Set it near zero for biomass, which is usually counted as biogenic.
Required heat pump capacity
—
Heat into the room
—
The machines that cover it
—
Heat loss at design
—kW
Where the heat goes
Cost to run for one day
You would save
—
What it pays back, and what it stops
Pays for itself in
—
Carbon avoided
—
Want this sizing checked by an engineer?
Your name, phone and email go on the saved sheet and let our team confirm the load, the emitter selection and the pipe sizing against your actual building. All three are needed before the sheet can be sent or saved.
How this is worked out
- Heat loss = area × U-value × the inside-to-outside difference, summed over walls, glazing, roof and floor, plus 0.33 × air changes × volume × the same difference for the air. The ground floor is charged half the difference — the earth under a slab sits far nearer room temperature than the outside air does.
- Sizing is on the peak load, not on a run window. A hot water machine is sized on the day’s energy because storage rides the peak. A building loses heat at the design rate for as long as it is that cold, and no tank rides that out.
- Fan coils are forced convection: the fan sets the air-side heat transfer, so output is very nearly proportional to the water-to-room difference. That is why a fan coil copes with heat pump water while a radiator struggles. Sizing here is in tons of nominal cooling, because that is how fan coils are bought in India, with the heating output worked out from the coil’s sensible conductance.
- Radiant panels follow the EN 442 exponent of 1.3: natural convection and radiation both strengthen faster than the temperature difference itself, so output falls away sharply as the water cools. A panel rated at the standard 50 K difference gives well under half that on 50 °C heat pump water. This is the single commonest reason a heat pump installation fails to heat a building — panels sized off the catalogue figure.
- The water temperature is the design decision. Cooler water means a better COP and a cheaper bill, but bigger emitters. The slider makes that trade visible: move it and watch both the panel count and the running cost change.
- A buffer tank of 12 to 25 litres per kilowatt gives the defrost cycle somewhere to draw from and stops the compressor short-cycling against a small circuit.
- Machine selection interpolates each model’s own published capacity and COP to your design temperature. SE-AH is published down to 5 °C, SE-EVI down to −20 °C — which is why the recommendation moves to EVI as you drag the temperature down.
- U-values and emitter ratings are indicative, chosen for the construction type you picked. They are not Sunniva product data, and they are all sliders. Replace them with real figures before ordering; the saved sheet records exactly what was used.
- Payback is simple payback — the price you type in, divided by the annual saving. No finance, no maintenance, no discounting, and no price of ours is assumed or disclosed. Carbon is the displaced fuel’s emissions minus the electricity the heat pump draws, at the Indian grid average of 0.71 kg CO₂ per unit. Every factor is a slider, and the saved sheet records what was used.
- Indicative only. Room use, orientation, shading and occupancy all affect the real load — confirm with Sunniva Encon LLP before ordering.
Space heating sizing
Sunniva Encon LLP
Calculated for
—
What you entered
Result
Required capacity
—
—
Running cost & savings
How this was worked out
Talk to Sunniva
SalesWhatsApp our sales desk
LinkedInFollow Sunniva Encon
YouTubeSee the machines running
Review usRate us on GoogleContact
+91 93213 65103sales@sunnivaencon.com
www.sunnivaencon.com
Office
C‑443, Oshiwara Industrial Centre Premises CHS, New Link Road, Goregaon West, Mumbai© 2026 Sunniva Encon LLP. The design and code of this calculator are our copyright. Results are indicative and offered in good faith — the method is set out above. Confirm the final selection with us before ordering.
1
Space heating
How big a heat pump — and what puts the heat in?
Most sizing tools stop at the kilowatts. This one goes on to the part that decides whether the building is actually warm: the fan coils or radiant panels that move that heat into the room, and how much of their rated output survives on heat pump water rather than boiler water.
The space
How big it is, and how it is built.
Floor area to heat
120m²
The heated floor, not the plot. One room, one floor or a whole house.
Ceiling height
3.0m
What it is attached to
A flat between two heated flats loses heat through
far less surface than a detached house of the same area.
Glazing
15%
Window and glass door area, as a share of the external wall.
Temperatures
The difference across the wall is what costs money.
Nearest place
A starting point only. The slider below stays editable.
Outside design temperature
2°C
The cold the system still has to hold the room at — not the record minimum, and not the winter average.
Wanted inside
21°C
Every degree costs. Raising the room by
1 °C adds roughly 5 % to the load in a cold climate, and a
radiant system feels warm at a lower air temperature than a fan
system does.
How the heat gets in
Fan coils, or radiant panels with no fan.
A fan coil moves air across a coil. A radiant panel
has no fan — silent, nothing to service, but its output falls
away much faster as the water cools.
Water leaving the heat pump
50°C
Lower is cheaper to make and kinder to the COP; higher makes the emitters smaller. This is the trade-off the whole design turns on.
Temperature drop across the emitters
5°C
Fan coil size, each
1.5TR
Nominal cooling tonnage, which is how fan coils are sold here. The heating output at your water temperature is worked out from it.
The plant room
One machine or several, and whether you want a spare.
Standby machine
A standby is one spare machine of the same model,
piped in parallel and rotated with the duty units.
Most machines you would accept
4
Two smaller machines often fit the load far more closely than one large one, and they stage better through a mild spell.
Sizing margin
15%
Headroom for warm-up after a setback, plan shape, and a colder day than the design one.
What you pay for energy
Change these to your own tariff.
What heats the space today
The saving is measured against this one. The chart
still shows the others.
Electricity
9₹/kWh
Diesel
92₹/litre
LPG (commercial)
95₹/kg
PNG
55₹/SCM
Hours heated per day
12hours
Days of heating per year
120days
Construction and comparison assumptions change only if you know why
Wall U-value
1.4W/m²K
Roof U-value
1.4W/m²K
Floor U-value
0.9W/m²K
Glazing U-value
3.2W/m²K
Air changes per hour
0.9ACH
Part-load factor over the heating hours
65%
The design load is the worst hour of the year. Over a season the average runs well below it.
Fan coil sensible heat ratio
72%
A cooling rating includes latent heat; only its sensible share describes the coil that also heats.
Fan coil air-to-water difference when cooling
17.5°C
The difference behind the nominal tonnage: 27 °C air against a mean 9.5 °C chilled water.
Electric heating efficiency
100%
Diesel boiler efficiency
85%
Diesel energy content
10kWh/litre
LPG boiler efficiency
90%
LPG energy content
12.8kWh/kg
PNG boiler efficiency
90%
PNG energy content
10.5kWh/SCM
Whatever figure you have — ours or anyone
else’s, installed. Nothing is sent anywhere until you fill in the
form below.
Grid electricity carbon
0.71kg CO₂/kWh
The Indian grid average. Lower it if you have rooftop solar or a green tariff.
Diesel carbon
2.68kg CO₂/litre
LPG carbon
2.98kg CO₂/kg
PNG carbon
1.9kg CO₂/SCM
Coal / biomass carbon
1.8kg CO₂/kg
Swings hugely with grade. Set it near zero for biomass, which is usually counted as biogenic.
Required heat pump capacity
—
Heat into the room
—
The machines that cover it
—
Heat loss at design
—kW
Where the heat goes
Cost to run for one day
You would save
—
What it pays back, and what it stops
Pays for itself in
—
Carbon avoided
—
Want this sizing checked by an engineer?
Your name, phone and email go on the saved sheet and let our team confirm the load, the emitter selection and the pipe sizing against your actual building. All three are needed before the sheet can be sent or saved.
How this is worked out
- Heat loss = area × U-value × the inside-to-outside difference, summed over walls, glazing, roof and floor, plus 0.33 × air changes × volume × the same difference for the air. The ground floor is charged half the difference — the earth under a slab sits far nearer room temperature than the outside air does.
- Sizing is on the peak load, not on a run window. A hot water machine is sized on the day’s energy because storage rides the peak. A building loses heat at the design rate for as long as it is that cold, and no tank rides that out.
- Fan coils are forced convection: the fan sets the air-side heat transfer, so output is very nearly proportional to the water-to-room difference. That is why a fan coil copes with heat pump water while a radiator struggles. Sizing here is in tons of nominal cooling, because that is how fan coils are bought in India, with the heating output worked out from the coil’s sensible conductance.
- Radiant panels follow the EN 442 exponent of 1.3: natural convection and radiation both strengthen faster than the temperature difference itself, so output falls away sharply as the water cools. A panel rated at the standard 50 K difference gives well under half that on 50 °C heat pump water. This is the single commonest reason a heat pump installation fails to heat a building — panels sized off the catalogue figure.
- The water temperature is the design decision. Cooler water means a better COP and a cheaper bill, but bigger emitters. The slider makes that trade visible: move it and watch both the panel count and the running cost change.
- A buffer tank of 12 to 25 litres per kilowatt gives the defrost cycle somewhere to draw from and stops the compressor short-cycling against a small circuit.
- Machine selection interpolates each model’s own published capacity and COP to your design temperature. SE-AH is published down to 5 °C, SE-EVI down to −20 °C — which is why the recommendation moves to EVI as you drag the temperature down.
- U-values and emitter ratings are indicative, chosen for the construction type you picked. They are not Sunniva product data, and they are all sliders. Replace them with real figures before ordering; the saved sheet records exactly what was used.
- Payback is simple payback — the price you type in, divided by the annual saving. No finance, no maintenance, no discounting, and no price of ours is assumed or disclosed. Carbon is the displaced fuel’s emissions minus the electricity the heat pump draws, at the Indian grid average of 0.71 kg CO₂ per unit. Every factor is a slider, and the saved sheet records what was used.
- Indicative only. Room use, orientation, shading and occupancy all affect the real load — confirm with Sunniva Encon LLP before ordering.
Space heating sizing
Sunniva Encon LLP
Calculated for
—
What you entered
Result
Required capacity
—
—
Running cost & savings
How this was worked out
Talk to Sunniva
SalesWhatsApp our sales deskLinkedInFollow Sunniva EnconYouTubeSee the machines runningReview usRate us on GoogleContact
+91 93213 65103sales@sunnivaencon.com
www.sunnivaencon.com
Office
C‑443, Oshiwara Industrial Centre Premises CHS, New Link Road, Goregaon West, Mumbai© 2026 Sunniva Encon LLP. The design and code of this calculator are our copyright. Results are indicative and offered in good faith — the method is set out above. Confirm the final selection with us before ordering.
1