Answer first
The survey must find an appropriate hot-water solution and confirm the radiators, pipework, electrical supply and outdoor position. A former combi cupboard is not automatically large enough for all indoor equipment, so the whole route matters.
In the 742-home Electrification of Heat project, 81% of installed homes needed a new thermal store because they had a combi boiler or an existing cylinder with an unsuitable coil. That is evidence from one project, not a rule that every home needs the same tank.
Project homes needing a new thermal store such as a hot-water tank
View sourceProject flats using heat batteries where a hot-water tank would not fit
View sourceMCS design requirement for heat-pump cylinder capability at design conditions
View sourceAsk FLO how this would work in your home
Share your current heating, property and the practical question you are trying to answer. FLO can explain the sensible next check without forcing you into a survey.
A combi boiler and heat pump make hot water differently
A combi boiler uses high power to heat mains water as it flows. A domestic air-to-water heat pump is usually designed around lower power delivered for longer periods, so storing hot water is the practical way to meet baths, showers and taps.
The cylinder is sized from household demand, recovery time and available space. Bigger is not automatically better: excessive storage adds space, standing loss and cost, while undersizing produces poor service.
The existing cylinder may or may not be suitable
If the home previously had a system boiler, the cylinder needs its size, condition, insulation and heat-exchanger coil checked. Heat pumps normally need a larger coil surface than a boiler because heat is transferred at a lower water temperature.
MCS design requirements say a heat-pump hot-water cylinder must follow manufacturer sizing and heat-exchanger recommendations. Reusing a cylinder should be an evidenced decision, not an assumption made to lower the first quote.
Find space before choosing equipment
Possible locations include an airing cupboard, utility room, garage, loft designed for the load, or a purpose-made internal alteration. The survey considers structural support, access for replacement, pipe runs, noise from pumps and heat loss from distribution.
Where a conventional cylinder genuinely cannot fit, some projects consider compact stores or heat batteries. The demonstration project used heat batteries in 33 flats. Product suitability, funding rules, capacity and service support still need checking.
- Household peak hot-water use
- Available height, width and service clearance
- Distance from heat pump and outlets
- Floor loading and safe access
- Pipe heat loss and insulation
- Immersion and hygiene-cycle provision
Hot-water behaviour may feel different
Stored hot water is finite, so simultaneous showers and large baths need to be included in the design. Recovery is planned around the cylinder, heat-pump capacity and household schedule rather than an instant combi flow rate.
The controls should explain normal target temperature, reheat windows, boost and the periodic hygiene cycle. Unnecessary permanent high temperatures or frequent immersion use can increase running cost.
The rest of the heating system still needs design
Replacing a combi is not just a hot-water project. Lower-temperature radiator output, pipe flow, heat loss, outdoor-unit location, electrics and controls determine whether the whole home remains comfortable.
Ask the quote to state what happens to the boiler, flue, gas meter or supply, condensate and old controls. If the gas connection is removed, confirm the supplier process and whether its standing charge ends.
What to read next
Related answers and services
Questions people also ask
Can a heat pump provide instant hot water?+
Most domestic air-to-water installations use stored hot water. Some specialist arrangements differ, but their flow, efficiency, funding and suitability need project-specific checks.
Will I run out of hot water?+
A correctly sized and scheduled cylinder should meet the agreed household pattern. Tell the designer about baths, high-flow showers and simultaneous use.
Can the cylinder go where the combi boiler was?+
Sometimes, but dimensions, floor loading, pipe routes and service clearance must be checked. The old boiler space alone does not prove suitability.
Evidence used for this answer
Figures are explained in context and links go to the independent source. Prices, tariffs, scheme rules and standards can change, so check the source again before making a financial or technical decision.
- Energy Systems Catapult: home surveys and installation reportEvidence from 742 installations, including radiators, cylinders, survey constraints and typical installation duration.
- MCS heat-pump design standardTechnical requirements for heat loss, equipment selection, domestic hot water and system design.
- MCS domestic heat-pump guideConsumer guidance covering design, installation, controls, handover and maintenance.
- Home Energy Scotland Grant and LoanCurrent eligibility, grant, optional loan, rural uplift and MCS requirements for Scottish homeowners.