Boiler to Heat Pump Migration Checklist: Radiator Survey, Pipe Sizing, Cylinder and Grant Eligibility

Quick Answer: Migrating from a gas/oil boiler to an air source heat pump is not a like-for-like swap — because heat pumps run at lower flow temperatures (typically 35-50°C vs a boiler's 60-80°C), the existing emitters and pipework often need upsizing. The migration checklist is: room-by-room heat loss calculation (to MCS/BS EN 12831), radiator survey against the new low flow temperature, pipework sizing check, hot water cylinder sizing (heat pumps need a cylinder, even replacing a combi), electrical supply capacity, and grant eligibility under the Boiler Upgrade Scheme (BUS, £7,500 in England & Wales) which requires MCS certification and EPC with no outstanding insulation recommendations. Skip the heat loss calc and the system will underperform — the single biggest cause of unhappy heat pump customers.

Summary

The most expensive mistake in heat pump installation is treating it like a boiler swap. A boiler delivers heat at 60-80°C, so small radiators sized for that high temperature work fine. A heat pump is most efficient at low flow temperatures (35-50°C), and a radiator's output falls sharply as flow temperature drops — a radiator giving 1,500W at 70°C might only give 700-800W at 45°C. If you bolt a heat pump onto a system designed for a boiler without checking the emitters, the house won't heat in cold weather, the heat pump runs hard and inefficiently, and the customer is left cold and angry. Every credible migration starts with a heat loss calculation, not a heat pump model number.

This is fundamentally a system design job, not a product swap. You're redesigning the heat distribution: recalculating heat loss room by room, surveying every radiator against the new flow temperature, checking the pipework can carry the required flow, sizing a hot water cylinder (heat pumps almost always need one — they can't do instantaneous DHW like a combi), and confirming the electrical supply and the property fabric. Get the design right and a heat pump delivers comfortable, efficient, low-carbon heat for decades; get it wrong and it's a noisy, expensive disappointment.

This checklist walks through each migration step, the standards and the grant framework (Boiler Upgrade Scheme, MCS certification, MCS 3005/MCS 007). It's a cross-reference hub linking the detailed articles on each topic. For the heat loss method see heat pump sizing heat loss; for radiators see radiator sizing for heat pumps; for cylinders see heat pump cylinder sizing; for the grant see bus grant scheme guide.

Key Facts

• Heat pump flow temperature — typically 35-50°C (vs boiler 60-80°C); efficiency (COP/SCOP) rises as flow temperature falls
• Heat loss calculation — mandatory; room-by-room to BS EN 12831 / MCS methodology; sizes the heat pump and every emitter
• Radiator de-rating — output drops sharply at lower flow temp; a radiator may give ~50% of its 70°C output at ~45°C
• MCS 3005 — the MCS standard for heat pump installation (design, install, commission)
• MCS 007 / MIS 3005 — heat emitter and system design requirements
• Cylinder required — heat pumps need a hot water cylinder (often a heat-pump-specific cylinder with a large coil); a combi-to-heat-pump migration means adding a cylinder
• Cylinder coil area — heat pump cylinders need a much larger heat exchanger coil (e.g. ~3m²+) than a boiler cylinder, because of the lower flow temperature
• Pipework — 22mm/28mm primaries often needed; microbore (8/10mm) frequently inadequate for heat pump flow rates
• Electrical supply — heat pump needs a dedicated circuit; check main fuse (often 60-100A), consumer unit capacity, and possibly a DNO notification for larger units
• Boiler Upgrade Scheme (BUS) — £7,500 grant for ASHP in England & Wales (Home Energy Scotland grant/loan in Scotland)
• BUS eligibility — property must have a valid EPC; any "loft/cavity wall insulation" recommendations on the EPC must usually be addressed first
• MCS certification — required to claim BUS; the installer must be MCS certified and issue an MCS certificate
• Permitted development — most domestic ASHPs are PD subject to siting/noise conditions (MCS 020 noise assessment), but check for listed/conservation/flats
• Weather compensation — heat pumps run continuously with weather compensation, not on/off like a boiler; controls strategy differs

Migration Checklist (Quick Reference)

Detailed Guidance

Step 1 — Heat Loss Calculation (Do This First, Always)

Everything flows from the heat loss calculation. You measure each room (dimensions, fabric, windows, exposure) and calculate the heat loss at the design external temperature (typically -2 to -3°C depending on region) and the desired internal temperatures, to BS EN 12831 as required by MCS. This gives:

• The total peak heat demand → sizes the heat pump (oversizing causes short-cycling; undersizing leaves the house cold)
• The per-room heat demand → tells you the output each radiator must deliver at the chosen flow temperature

A "rule of thumb" or boiler-size match is not acceptable and won't pass MCS. See heat pump sizing heat loss.

Step 2 — Radiator Survey

This is where boiler-to-heat-pump migrations live or die. A radiator's heat output is quoted at a standard temperature difference (often Δ50, i.e. 70°C mean water temp into a 20°C room). Drop the flow temperature to 45°C and the mean water temperature falls, the Δ falls, and output drops dramatically — often to around half.

For each room, compare the required output (from the heat loss calc) against the actual output of the existing radiator at the new flow temperature. Radiators that fall short must be:

• Upsized (taller/longer/double or triple panel), or
• Supplemented (a second emitter, or underfloor heating where practical)

Many migrations need 30-60% of radiators upgraded. Underfloor heating is ideal for heat pumps (large surface area, very low flow temp) where it can be installed. See radiator sizing for heat pumps and radiator heat pump compatibility.

The honest customer conversation: "To run efficiently and keep you warm, the heat pump runs cooler than your boiler, so some radiators need to be bigger. Here's which ones and why." Customers who understand this up front are happy; those surprised by it mid-job are not.

Step 3 — Pipework Sizing

Heat pumps move more water at a lower temperature than boilers, so they need higher flow rates and adequately sized pipework. Common findings:

• Microbore (8mm/10mm) systems are frequently inadequate and may need replacing with 15/22mm
• Primary pipework to/from the heat pump and to the cylinder often needs to be 22mm or 28mm
• Index circuit (the longest/most resistant run) must be checked for adequate flow without excessive pump head

Undersized pipework starves the system of flow, causing poor heat distribution and high pump energy. The pipework check is part of MCS-compliant design. See pipe materials and pipe compatibility.

Step 4 — Hot Water Cylinder

A heat pump cannot produce instantaneous hot water like a combi boiler, so:

• Replacing a combi means adding a hot water cylinder — which needs a location (airing cupboard, loft, garage) and is a major part of the job and cost.
• Replacing a system/regular boiler means replacing the existing cylinder with a heat-pump-specific one, because boiler cylinders have a small coil sized for high flow temperatures.

Heat pump cylinders have a large heat exchanger coil (commonly ~3m² or more) so they can transfer heat into the water at the heat pump's low flow temperature, and are sized on the household's hot water demand (number of occupants/bathrooms). Reheat times are longer than a boiler, so sizing for peak demand matters. See heat pump cylinder sizing and cylinder selection.

Step 5 — Electrical Supply

A heat pump is an electrical appliance drawing significant current. Check:

• Main fuse rating — typically 60-100A; a large heat pump plus existing loads may approach capacity
• Dedicated circuit — the heat pump needs its own correctly sized and protected circuit to BS 7671
• Consumer unit capacity — spare way, RCD/RCBO protection
• DNO notification — larger units or constrained supplies may require notifying/seeking approval from the Distribution Network Operator (G99/G98 doesn't apply but supply capacity checks do)

This is electrician's work and must be designed to BS 7671:2018+A2:2022. See heat pump wiring.

Step 6 — Fabric and EPC

Heat pumps work best in reasonably insulated homes (they deliver gentle, continuous heat). Before migration, assess the fabric — loft insulation, cavity/solid wall insulation, draught-proofing — both for performance and for grant eligibility:

• The Boiler Upgrade Scheme requires a valid EPC, and any recommendations on it for loft insulation or cavity wall insulation must generally be addressed before the grant is paid (there are limited exemptions).
• Better fabric lowers the heat loss, which lets you run a lower flow temperature (higher efficiency) and may avoid some radiator upgrades.

See epc ratings, loft insulation, and cavity wall.

Step 7 — Siting, Noise and Permitted Development

The outdoor unit needs a location with airflow clearance, a stable base, condensate drainage (with frost protection), and acceptable noise. Most domestic ASHPs are permitted development but subject to conditions: a single unit, sited to minimise visual/noise impact, not on a flat roof/within set distances of boundaries in some cases, and (key) an MCS 020 noise assessment demonstrating the noise at the neighbour's window is within limits. Listed buildings, conservation areas, and flats often fall outside PD and need planning permission. See heat pump noise planning.

Step 8 — Grant Eligibility and MCS

The Boiler Upgrade Scheme (BUS) pays £7,500 toward an air source heat pump in England & Wales (ground source higher; Scotland uses Home Energy Scotland grant + loan). Requirements:

• MCS-certified installer designing and installing to MCS 3005, issuing an MCS certificate
• A valid EPC for the property, with insulation recommendations addressed (see Step 6)
• The property replacing fossil fuel heating (gas, oil, LPG, electric storage), with limited exceptions
• The grant is applied for by the installer on the customer's behalf and discounted from the price

MCS certification is the gateway — without it, no BUS grant. The installer must be MCS registered for the technology. See bus grant scheme guide, mcs 007 heat pump standard, and mcs certification explained.

Commissioning and Controls

A heat pump is commissioned and runs differently from a boiler: continuous operation with weather compensation (the flow temperature adjusts to outdoor temperature) rather than on/off cycling to a thermostat. Setting it to mimic a boiler (high flow temp, on/off) destroys efficiency. Proper commissioning sets the weather compensation curve, balances the radiators for even flow, and educates the customer that "low and slow" is correct. See heat pump commissioning checklist and heat pump controls setup.

Frequently Asked Questions

Can I just swap a boiler for a heat pump on the same radiators?

Almost never without checking. Heat pumps run at lower flow temperatures (35-50°C vs a boiler's 60-80°C), and radiator output drops sharply at lower temperatures — a radiator may give only ~half its boiler-temperature output. You must do a room-by-room heat loss calculation and a radiator survey; typically 30-60% of radiators need upsizing or supplementing. Skipping this is the number one cause of cold houses and unhappy heat pump customers.

Do I need a hot water cylinder with a heat pump?

Yes, in almost all cases. A heat pump can't produce instantaneous hot water like a combi boiler, so it heats a cylinder. If you're replacing a combi you must add a cylinder (needing space and adding cost); if replacing a system boiler you usually replace the cylinder with a heat-pump-specific one that has a large coil (~3m²+) to transfer heat at the low flow temperature. Cylinder sizing is based on the household's hot water demand.

How much is the heat pump grant and what are the conditions?

The Boiler Upgrade Scheme (BUS) gives £7,500 toward an air source heat pump in England & Wales (Scotland uses Home Energy Scotland grant + loan). Conditions: an MCS-certified installer designing/installing to MCS 3005, a valid EPC with any loft/cavity wall insulation recommendations addressed, and replacing a fossil fuel system. The installer applies and discounts the grant from the price. Without MCS certification there's no grant.

Will the existing pipework be big enough?

Often not. Heat pumps need higher flow rates than boilers, so microbore (8/10mm) is frequently inadequate and primaries often need to be 22mm or 28mm. The pipework must be checked as part of MCS-compliant design — undersized pipe starves the system, causing poor heat distribution and high pump energy. Plan for some pipework upgrading on most older systems.

Does a heat pump need planning permission?

Most domestic air source heat pumps are permitted development, but subject to conditions: siting/clearance, only one unit, and an MCS 020 noise assessment showing acceptable noise at neighbouring windows. Listed buildings, conservation areas, and flats frequently fall outside permitted development and need planning permission. Always check the specific siting and the local planning position before committing.

Regulations & Standards

• MCS 3005 (MIS 3005) — Microgeneration installation standard for heat pump systems (design, installation, commissioning)

• MCS 007 — Heat emitter and system design guidance for heat pumps

• BS EN 12831 — Energy performance of buildings — method for calculation of design heat load (heat loss calc)

• MCS 020 — Planning standards for permitted development noise assessment

• BS 7671:2018+A2:2022 — Requirements for electrical installations (IET Wiring Regulations) — heat pump circuit

• Boiler Upgrade Scheme (BUS) — Ofgem-administered grant scheme (England & Wales)

• Building Regulations Part L — conservation of fuel and power (heat pump efficiency, system design)

• Building Regulations Part G — hot water safety (cylinder, unvented)

• G3 / unvented cylinder — competent person requirement for unvented hot water cylinders

• Permitted development (GPDO 2015) — heat pump siting conditions

• GOV.UK — Boiler Upgrade Scheme — grant eligibility and application

• MCS — Microgeneration Certification Scheme — installation standards and certification

• Ofgem — Boiler Upgrade Scheme — scheme administration

• GOV.UK — Approved Document L — heating system requirements

• Home Energy Scotland — Scottish grant and loan scheme

• heat pump sizing heat loss — the heat loss calculation that drives the whole design

• radiator sizing for heat pumps — surveying and upsizing emitters for low flow temperatures

• heat pump cylinder sizing — sizing the hot water cylinder

• bus grant scheme guide — Boiler Upgrade Scheme grant detail

• air source heat pump pricing guide — pricing an ASHP installation