Heat pump radiators and hot water in rented homes: sizing, cylinders and legionella

Do heat pump radiators need upsizing?

Some usually do. A gas boiler typically sends water round the radiators far hotter than a heat pump should, and a radiator gives out less heat when the water in it is cooler. A radiator that keeps a bedroom warm on a hot boiler flow can leave the same room cold on a heat pump flow.

The answer for any one property comes from a room by room heat loss calculation. It works out how much heat each room loses on a cold day, then checks whether the existing radiator can deliver that much heat at the design flow temperature. Rooms where the radiator falls short need a larger or double panel radiator, a fan assisted unit, or a different emitter. Rooms where the radiator is already oversized, or where insulation has cut the heat loss, can keep what they have.

So the honest answer for a landlord is: expect some radiator changes, but do not assume every radiator goes. Ask the installer for the calculation in writing, room by room, and for the list of radiators to change, keep or move. If a quote says “all radiators replaced” without showing the room figures, ask why.

Insulation changes the answer. Loft insulation or draught proofing cuts heat loss in every room, which can let more of the existing radiators stay. That is one reason fabric work often comes first. See before you install a heat pump in a rental.

Flow temperature: the number that sets the running cost

Flow temperature is the temperature of the water leaving the heat pump for the radiators, and it is the biggest single lever on what the tenant pays. The lower it is, the more heat the pump produces from each unit of electricity.

The DESNZ Electrification of Heat demonstration project, analysed by Energy Systems Catapult, measured this in real homes. The median heat output per unit of electricity was 3.50 at a flow temperature of 35°C, 2.38 at 55°C and 2.02 above 65°C. The mean operating flow temperature across the monitored heat pumps was 39.7°C.

The table turns those measured medians into a cost per kWh of heat at the Ofgem price cap for 1 October to 31 December 2026: electricity 26.32p per kWh and gas 7.97p per kWh.

Flow temperature	Median measured efficiency	Cost of 1 kWh of heat	Compared with gas boiler (90%) at 8.86p
35°C	3.50	7.52p	Cheaper
55°C	2.38	11.06p	Dearer
Above 65°C	2.02	13.03p	Much dearer

Formula. Heat pump cost per kWh of heat = 26.32p ÷ efficiency. Gas = 7.97p ÷ 0.9 = 8.86p. The efficiency figures are medians across many homes, not a promise for any one property, and prices change every quarter.

The point for a landlord is simple. A heat pump that keeps the old radiators and runs hot to compensate can cost the tenant more than the boiler it replaced. Spending on radiators is often what makes the running cost work.

What the building guidance says

Approved Document L, the official guidance on energy efficiency for dwellings in England (2021 edition incorporating 2023 amendments, paragraph 5.10), covers a wet heating system that is newly installed or fully replaced, including the heating appliance, emitters and pipework. It says the system should be sized to meet the home’s heating needs at a maximum flow temperature of 55°C or lower. Where that is not feasible, for example where there is not enough space for larger radiators, it should be designed to the lowest temperature that still heats the home. This is guidance on how to meet the Building Regulations, not a rule that every radiator must change, and it applies to full replacements.

What a radiator survey should tell you

A good design pack lets you check the installer’s thinking before any money is spent. Ask for:

• The heat loss for each room and for the whole property.
• The design flow temperature, and the expected seasonal efficiency at that temperature for this property.
• A list of radiators: keep, replace with a larger one, or add a new one.
• Whether the existing pipework can carry the flow the new system needs, and what changes if it cannot.
• Where weather compensation will be set up, and whether it stays on.
• The total cost of the radiator work as a separate line, so you can compare quotes.

Larger radiators take wall space. In a rented home that matters: a wider radiator under a window may clash with furniture the tenant owns. Agree positions with the tenant before the installer orders anything.

The hot water cylinder: space in flats and terraces

Most air to water heat pump systems store hot water in a cylinder rather than heating it on demand, so a home with a combi boiler usually needs space found for one. That is often the hardest part of a rental installation.

Where a cylinder can go depends on the home:

• Houses with an existing cylinder. An airing cupboard that already holds a cylinder is the simplest case, although a heat pump cylinder may differ in size from the old one. Ask the installer to measure.
• Terraces with a combi boiler. Common options are a bedroom cupboard, under the stairs, or a utility space. Each takes storage from the tenant, so agree it before work starts.
• Flats. Space is usually the binding constraint. A compact cylinder may fit a cupboard, or it may not. In a block, the lease and the freeholder may also have a say over external units and pipe routes. See heat pumps for flats.

Ask the installer for the cylinder volume, its dimensions, its location, and how the volume was chosen for the number of people the home is let to. A cylinder too small runs out of hot water and leads to immersion top ups, which cost 26.32p per kWh of heat. A cylinder too large wastes space and heat.

An air to air heat pump heats rooms but does not make hot water, so the hot water question stays open with that route. See air to air heat pumps for rental properties.

Legionella and hot water temperature: the landlord’s duty

Landlords have a duty to assess the risk from legionella to their tenants and to control it. HSE guidance for landlords puts the legal basis in section 3(2) of the Health and Safety at Work Act and the Control of Substances Hazardous to Health Regulations (COSHH). HSE also says:

• The assessment does not need to be in depth. Most landlords can assess the risk themselves and do not need to be professionally trained or accredited.
• Landlords are not necessarily required to record the findings, although keeping a record is sensible.
• Health and safety law does not require landlords to obtain or produce a legionella test certificate.
• The assessment should be reviewed periodically in case anything changes. A new heat pump and cylinder is that kind of change.

HSE’s control measures for landlords include flushing the system before a let, setting the cylinder so water is stored at 60°C, removing redundant pipework, keeping cold water tanks covered with tight fitting lids, and cleaning showerheads. HSE’s wider guidance on hot and cold water systems is that hot water should be stored at least at 60°C and distributed so it reaches 50°C within 1 minute at the outlets.

Why this matters more with a heat pump

A heat pump makes hot water less efficiently than it makes space heating, because hot water is hotter. Storing at 60°C therefore costs more to run than storing cooler. Do not let that tempt anyone into a lower setting without thought. Ask the installer three questions:

1. How does the design meet the HSE guidance on stored and distributed hot water temperature?
2. Is any part of that done by the immersion heater, and on what schedule?
3. What does it add to the tenant’s running cost?

Then put the answer in the tenant handover note. HSE guidance for landlords says tenants should be advised not to adjust the cylinder temperature, and to report problems with the hot water system.

Disruption with tenants in situ

Many rental heat pump installations happen with a tenant living there, so plan the works around them. A full job can involve radiator changes in most rooms, a cylinder in a cupboard, an outdoor unit, pipe runs, and electrical work at the consumer unit.

A practical plan:

1. Agree dates in writing. Explain what happens, in which rooms, and on which days. Keep a copy.
2. Keep heating and hot water available. Ask the installer how long the home is without each, and arrange temporary heating if the works fall in cold weather.
3. Avoid the coldest months where you have a choice. A failing boiler may not give you that choice.
4. Clear the paperwork. Book the commissioning record, the electrical paperwork and the handover in the same visit plan.
5. Hand over properly. A short demonstration of the controls and a written note avoids most early complaints. See heat pump running costs for tenants.

On access, section 11(6) of the Landlord and Tenant Act 1985 gives the landlord a right, on 24 hours’ notice in writing, to enter at reasonable times to view the condition and state of repair. That right is about inspection. For installation works, agree access with the tenant rather than relying on it.

When this is not the right answer

A heat pump is a poor fit when the home needs a high flow temperature and there is no room to fix that. Typical cases:

• A small flat with no cupboard space for a cylinder and no practical place for larger radiators.
• A solid wall home where insulation has not been done and the heat loss is high, so the design would need to run hot.
• A property where the tenant cannot accept the works this year and the boiler still has life in it.

In those homes, insulation first, a later heat pump, or a different heating route may serve the tenant better. The Energy Saving Trust puts an air source heat pump at around £12,000 before any grant, so it is worth getting the radiator and cylinder answer right first. See is a heat pump worth it for landlords and the cost page.

Next step

To test running costs for your own property, use the heat pump landlord calculator. Or send an enquiry and relevant independent firms will contact you, with no charge to you and no obligation.