Heat Pump Installation Explained: Sizing, Installation Time, and Using Your Existing Radiators

An increasing number of homeowners are considering heat pumps as part of renovation projects, extensions, or complete heating upgrades. Increased energy prices, ageing boilers, and a move toward lower-carbon solutions have each played their role. Heat pumps can be a great choice, but only when done well. This is particularly relevant in places like Southern California, where a large portion of the housing stock consists of older homes and renovation projects rather than new builds.

One of the biggest mistakes is to treat a heat pump as a drop-in boiler replacement. It doesn’t work like that. Having the right-sized system, understanding what a typical installation entails, and confirming whether your current radiators meet the requirements all contribute to a successful installation.

This article outlines the key points so you can enter the process with appropriate expectations.

Why Correct Heat Pump Sizing Matters More Than Brand

There’s considerable attention paid to heat pump brands, model numbers, and efficiency ratings when choosing the right equipment. Equipment quality matters, but it isn’t what makes a system perform well in your home. Correct sizing matters far more.

An undersized heat pump will have difficulty heating the home as effectively in cold weather. It will have to work harder and longer than necessary, which increases operating costs and places unnecessary stress on the system. Conversely, an overly large heat pump can be just as problematic. Oversized units are prone to cycling (turning on and off), which negatively impacts comfort, durability, power consumption, and the environment.

An appropriately proportioned heat pump runs continuously for longer but at a lower output. That’s precisely how the technology is supposed to work, and it’s where some of its efficiency advantages derive from.

Even the priciest unit on the market will not perform well if it is too small for your house.

How Installers Calculate the Right System Size

Professional installers know not to size a heat pump solely based on floor space. Instead, they use a heat-loss calculation to determine how much heat your house requires.

This calculation considers several factors, including:

• Gross floor area and height of the ceiling
• Installation of wall, roof and floor insulation
• Window size, type, and glazing
• Air leakage and draughts
• Home layout and room usage
• Local climate conditions

The heat requirement for each room is calculated and summed to determine the total load for the entire system. This method results in the heat pump being sized for the home, rather than the other way around.

Online calculators and rules of thumb can provide ballpark figures, but they often lack critical details. You can determine this through a qualified heat-loss calculation, which should always be performed before any equipment selection.

Typical Heat Pump Installation Timelines

There is often speculation among homeowners about how long a heat pump installation will take. It depends on the complexity of the project and what you already have in place that can be reused.

Simple Installations

An installation, assuming the home is prepared correctly, can be completed in two or three days. This typically applies when:

• Electrical capacity is sufficient
• Existing pipework is in good condition
• Radiators are compatible or require minimal changes
• Outdoor unit placement is straightforward

In these cases, only a minor disassembly is required, and the machine can be put into service quickly.

More Complex Installations

Larger homes, older homes, and renovation projects may take longer­­; three to five days is not unusual. The following may take extra time:

• Electrical upgrades
• Radiator replacements or additions
• Pipework modifications
• System flushing or upgrades
• Coordinating with other renovation work

The point is, there’s no urgency in the installation. Commissioning, testing, and tuning are necessary phases to ensure the system’s functionality.

Can you use Your Existing Radiators?

One of the most common questions homeowners ask is whether their current radiators can be used with a heat pump or if they need to invest in a new heating distribution system. The short answer is: Sometimes yes, sometimes no.

Heat pumps operate more efficiently at lower water temperatures than traditional boilers. Radiators designed for high-temperature systems may not sufficiently heat when operating at  lower temperatures.

But that doesn’t necessarily mean they should be replaced; they should be evaluated appropriately.

When existing Radiators Can be Reused

Existing radiators may be suitable if:

• They are generously sized for the rooms they serve
• The home has good insulation and low heat loss
• The system is designed to operate at appropriate flow temperatures

You may only need to upgrade some radiators; others can remain unchanged. That could mean lower costs and less disruption without sacrificing performance.

When Radiator Upgrades are Recommended

You may need a new radiator if:

• Rooms have high heat loss
• Radiators are small or outdated
• The system previously relied on very high water temperatures

That’s not to say an upgrade requires replacing every radiator. But strategically placed larger panel radiators or other types of heat emitters can be used to balance the system and maintain comfort.

How Professionals Assess Radiator Suitability

Assessing radiator compatibility isn’t guesswork. Installers determine how much heat each radiator can deliver at lower temperatures and compare that to the room’s heat loss.

This involves:

• Calculating room-by-room heat demand
• Reviewing radiator size and output ratings
• Matching emitter capacity to system design temperatures

This approach delivers just enough heat to all rooms without overloading the heat pump.

Planning Installation During Renovations or Extensions

It is often easiest to install a heat pump when the building work has been planned. During a renovation, there is an opportunity to improve insulation more broadly and implement layout changes at a reasonable cost without creating additional inconvenience.

Early planning allows installers to:

• Coordinate pipe routes and unit placement
• Design systems around new layouts
• Avoid compromises caused by finished surfaces

Homeowners who are already making home improvements will ultimately benefit the most, as they can integrate every aspect of heat pump design from the start, reducing later retrofitting.

A System, Not Just a Product

Heat pump installation should always be considered a system, not just a single component. The interaction of sizing, radiators, pipework, controls and commissioning determines performance.

And a good installer who correctly evaluates systems and provides adequate capacity is far more critical than the specific model you buy based solely on marketing.

At Reliable Heating and Air, the process centres on proper sizing, realistic planning, high-quality installation and thorough commissioning of heat pumps in LA & Ventura County. The hope is that by attending to individual homes one at a time, heat pump systems will operate as designed without being intrusive or causing unnecessary discomfort for homeowners.

When the fundamentals are correct, a heat pump is not only an upgrade; it’s an enduring improvement to how your home feels and works.