Converts between heat load and water flow using Q = ṁ × cp × ΔT.
Useful for LTHW checks (80/60, 70/50, 60/40, 55/45). Tender-stage only. Verify final design.
Hydraulic/energy conversion only. Does not select emitters. For design guidance only. Always verify with a qualified engineer.
About this heating output & flow rate calculator
This free heating output & flow rate calculator converts between heat load and water flow rate for hydronic systems, working in either direction: enter a heat load in kW to size the required flow, or enter a flow rate to find the heat capacity it carries. It is built for UK building-services engineers, heating contractors and estimators doing quick LTHW and DHW checks, with built-in flow/return presets for common regimes (80/60, 70/50, 60/40 and 55/45). It is intended for tender-stage indication only. Final design should always be verified by a qualified engineer.
How it works
The calculator applies the standard hydronic energy balance from CIBSE Guide C: Q(kW) = ṁ(kg/s) · cp(kJ/kg·K) · ΔT(K), with volume flow derived as ṁ ÷ ρ. You set the temperature difference ΔT (from a preset or manually), and the specific heat capacity cp and density ρ auto-track the mean fluid temperature of the chosen flow/return preset, editable to override. Selecting an ethylene-glycol mix (25, 35 or 50%) adjusts cp and ρ using factors from ASHRAE Fundamentals Ch.31. Results are shown as mass flow plus volume flow in L/s, L/min and m³/h, alongside a reference table of flow per 10 kW at common ΔTs.
Frequently asked questions
Which standards does it use?
The core conversion follows CIBSE Guide C hydronic first principles (Q = ṁ · cp · ΔT), and the glycol property factors are taken from ASHRAE Fundamentals Chapter 31.
Can I model a glycol mixture?
Yes. Choose 25, 35 or 50% ethylene glycol and the calculator scales specific heat and density accordingly; you can also type cp and ρ directly to override the defaults.
Does it select emitters or pipe sizes?
No. It is a hydraulic and energy conversion only. It does not select emitters, pipes or pumps. Use it for tender-stage flow and output checks, then verify the design with a qualified engineer.