Rules of thumb for choosing the right heat pump

Horror scenarios of incorrectly sized heat pumps and homes that do not heat up keep many installers from recommending a heat pump even in 2016, knows Sander Rutten, head of Product Marketing & Training at Vaillant Netherlands. "The central heating boiler is still the safest route for many installers. Because a central heating boiler always fits. With heat pumps this is a little trickier, especially if you opt for all-electric."

The use of heat pumps has gained momentum in recent years, he says. "Certainly new-build homes can be ideally heated with a heat pump. For this, we basically recommend ground-source heat pumps. After all, these heat pumps have a much higher efficiency at low temperatures because the source/ground temperature remains around 10˚C. In addition, this allows passive cooling."

In renovation homes, it is often not desirable to install a ground source heat pump. Not only because it requires digging up the garden, but also because the heat demand is greater, which means greater sizing and investment. "In these cases, a hybrid air source heat pump outcome, where peak demand and domestic hot water heating are covered by a hr boiler," Rutten says.

To determine the power required for the house, Vaillant has developed a handy tool that shows the estimated power in kW at -10 ˚C for each house type and year of construction. This table shows that a moderately insulated 1970 mid-terrace house has a heat loss of 130 watts per m². A comparable house from 2000 requires less than half this power at about 60 watts. In corner houses, that requirement is about 25 percent more; in detached houses, the requirement increases by about 40 percent. So a 170 m² detached house from 2000 requires a heat output of about 170 m² x 60W/m² x 1.4 = 14kW.

When opting for a hybrid heat pump system, Rutten recommends assuming 50 percent of the required capacity when sizing. "With this, 85 percent of the total heat demand can be met. So for that 2000 detached house, you end up with an 8 kW hybrid heat pump."

Vaillant has also come up with a rule of thumb to calculate the exact savings. "If you multiply the peak demand by the average of 1,600 running hours per year, you have a good indication of how many kW/h you consume on an annual basis," Rutten says. "In homes from 2000 onwards, floor heating is usually used with a delivery temperature of 35˚C. Then you can assume a COP of 4.5 with the Vaillant heat pump. When you divide the heat capacity (85 percent of the total capacity) by that COP and multiply it by the current electricity price, you know your electricity costs. You can do the same calculation for your gas consumption."

The 2000 detached house uses about 1,600h x 14kW = 22,400 kWh/year for heating, he says. "When 85 percent is supplied by the air source heat pump, this costs in electricity 22,400 kWh/year x 85 percent / 4.5 X €0.22 = €930. In gas, this would have cost about 22,400 kWh x 85 percent / 10 kWh/m³ x € 0,65 = € 1,238, which amounts to an annual savings of € 300. Add to that the government's ISDE subsidy and you'll recoup the air source heat pump in five to seven years."

Although Vaillant became known for its geysers and boilers, the company has also been developing and manufacturing heat pumps itself for many years. The range is very broad, with 3 to 15 kW hybrid and all-electric air-source heat pumps and 5 to 45 kW all-electric ground-source heat pumps with associated buffers and boilers. Especially for installers, Vaillant has developed a certification and training program to meet the necessary knowledge, skills and government requirements, including those related to the sizing of heat pumps. "Using our rules of thumb, installers can make a good initial estimate," Rutten says. "Our Project Advice department is then happy to support them in making a detailed calculation to ensure the right heat pump is installed."