This is actually a really important topic if you’re thinking about buying an air-to-water heat pump for your home.

In fact, I come across situations almost every week with my heat pump customers where an air-to-water heat pump has been incorrectly sized.

And often the problem isn’t noticed until it starts getting really cold outside.

That’s when people start to wonder why the electric heater is running constantly, why electricity usage is so high, or—in the worst-case scenario—why the house isn’t staying warm enough.

Let’s go over what usually causes this and what you should keep in mind if you’re thinking about buying an air-source heat pump.

Watch the video or read the blog post below 🙂

One of the most common mistakes is made as early as the design phase

Let’s take a simple example.

You have an oil-heated single-family home, and you tell the heat pump salesperson that you use about 1,600 liters of oil per year.

The salesperson calculates the size based on 1,600 liters of oil and concludes, for example, that a heat pump with a capacity of about 8 kW is suitable for your home.

At first glance, that sounds pretty reasonable.

But here comes the really important question.

Do you heat your home with anything other than oil?

For example, you might have an air-source heat pump, a fireplace, a baking oven, a wood-fired sauna, or even a dual-chamber boiler that you use to burn wood during the winter.

If you actively heat with a fireplace and use an air-source heat pump for heating at the same time, they will reduce your oil consumption.

In that case, the 1,600 liters doesn’t tell the whole story about how much heating energy your home actually needs.

And this is where the sizing can go terribly wrong.

Instead of that 8-kW unit, your home might actually need an 11- or 12-kW unit, for example.

So what does an undersized heat pump actually mean in practice?

The problem may not be immediately apparent.

In the fall, everything might work out just fine. The same goes for light frosts.

But when the cold snap sets in, the situation changes.

As your home’s heating needs increase, the heat pump’s performance changes in response to the conditions.

If the unit is already too small to begin with, the electric heating element will have to kick in earlier to provide heat.

And this can easily lead to a vicious cycle in which electricity consumption increases dramatically.

In other words, even if you have an air-source heat pump, during severe cold spells, a large portion of your home’s heating may actually be provided by direct electricity.

At its worst, the house starts to get cold

Let’s take another example.

If your home requires about 11.5 kW of heating power during extreme cold, and the heat pump can no longer produce the necessary heat at that temperature, the electric heating element must take over.

In many systems, the power of an electric heater can be, for example, 9 kW.

If the house needs 11.5 kW and the system can only generate 9 kW in that situation, then that simply isn’t enough.

That’s when the temperature in the house starts to drop little by little.

I’ve come across cases where, during severe cold spells, the temperature inside the house has dropped to around 10, 11, or 12 degrees.

And at that point, we’re already talking about a real problem.

In a home with radiator heating, you need to consider more than just the unit’s output

This is another really important point.

It’s not enough to just look at how many kilowatts the heat pump is rated at on paper.

You also need to know how your home is heated.

Does it have underfloor heating or radiators with a water-circulating system?

This plays a major role in determining what kind of water-source heat pump you should choose for your home.

In a house heated by radiators, very hot domestic hot water may be needed during severe cold spells.

When it’s, say, 20 or 25 degrees below zero outside, the radiator system may need water at a temperature of 55 or even 65 degrees to keep the house warm.

In that case, you need to see if the heat pump can actually produce hot enough water even in severe cold.

Not all air-to-water heat pumps are the same

This is also something that’s really easy to forget.

Different heat pump brands and models have different features.

A single unit can stop heating with a heat pump, for example, when the temperature drops to minus 20 degrees.

The other one can still operate at temperatures as low as -25 degrees.

Another system might be able to generate heat even in colder conditions, within certain operating limits.

And this can have a really big impact on electricity consumption.

Consider, for example, a house with radiator heating, where a high supply water temperature is needed during severe cold spells.

If you choose a unit that can no longer produce the necessary heat in colder temperatures, the heating will rely more on the electric heating element.

That is why you shouldn’t base your choice of the right machine solely on the device’s rated power.

We’ll have to see what that machine is really capable of when it’s really freezing outside.

It also matters where in Finland you live

The same heat pump isn’t automatically the right solution for every home.

The location of the home should also be taken into account.

In southern Finland, near the coast, the design conditions are different from those in the interior. And the farther north you go, the more severe the freezing temperatures the system must, in principle, be prepared for.

For this reason, a heat pump should not be chosen based solely on the square footage of the house or previous oil consumption.

You have to look at the big picture.

The cheapest heat pump isn’t necessarily the most cost-effective

This brings us to what I think is a really important point.

A system that is incorrectly sized may look good in a proposal.

Why?

Because a smaller machine is usually cheaper.

If you’re offered a smaller, less expensive plane, the offer can easily seem very tempting.

But what happens after that?

If the unit is too small and much more electrical resistance is needed, the money saved may end up costing you more on your electricity bill later.

And that electricity bill gets paid year after year.

A properly sized system may cost a little more up front, but if it can provide the heat your home needs more efficiently, your operating costs could be on a whole different level.

That’s why I wouldn’t just look at the price when considering a heat pump offer.

I’d also like to ask:

How is this machine sized for my home?

And even more importantly:

How does this machine work when it’s 20 or 25 degrees below freezing outside?

So what should be taken into account when determining the correct dimensions?

When sizing an air-to-water heat pump, you need to consider much more than just a single figure on your electricity bill or oil consumption.

You need to know how much the current primary heat source is being used.

It’s important to know whether the home is also heated with wood, a fireplace, or an air-source heat pump.

You need to determine your home’s heating requirements.

You need to know whether the place has underfloor heating or radiator heating.

We need to figure out what temperature the hot water supply needs to be during severe cold spells.

And you also need to take into account where in Finland your home is located.

Only after that can we really start figuring out what kind of air-source heat pump would be best to install there.

Be sure to also tell the salesperson about the fireplace and the air-source heat pump

If you’re in the market for an air-to-water heat pump and the salesperson asks you about your current energy consumption, don’t just tell them, for example, that you use 1,600 liters of oil a year.

Tell the whole truth.

If you heat your fireplace just a few times a year, let us know.

If you have an air-source heat pump that’s basically on all winter, let us know that, too.

If you use wood in any other way to heat your home, please let us know about that as well.

This information can have a significant impact on the actual capacity of the heat pump your home needs.

An air-to-water heat pump is a great solution when done right

I think an air-source heat pump is a really good option for both radiators and underfloor heating systems.

But that’s not a good solution if you install the wrong type of machine or one with the wrong power rating in your home.

The right sizing and proper installation make a huge difference in whether you’ll still be satisfied with your air-to-water heat pump years from now.

At Omavatti, we want to size our projects correctly.

We don’t want to start by looking for the cheapest possible appliance, but rather by finding the one with the right power output for that specific home and the one that best meets the home’s needs.

When a heat pump is properly sized from the start, it has a much better chance of performing as it should, even when the Finnish winter really shows its teeth.