Can an Air Source Heat Pump Water Heater Supply A Bath?

Can an Air Source Heat Pump Water Heater Supply A Bath ? Yes – an air source heat pump water heater can easily cope with a bath, provided the hot water cylinder is the right size for the amount of water needed. A typical UK bath will use around 80 to 100 litres of mixed water, though large baths can use much more than that. What you need to think about are the cylinder capacity, the stored water temperature, the incoming cold water temperature and whether extra hot water will be needed at the same time for a shower or taps.

Our range provides several storage capacities, so the heat pump water heater can be selected according to expected hot water demand. For smaller applications, see the 100 litre wall mounted hot water heat pump.

For typical family homes requiring more stored water, explore the 200 litre domestic hot water heat pump and 300 litre domestic hot water heat pump.

For high-capacity applications, our range includes a 400 litre commercial hot water heat pump and 500 litre commercial hot water heat pump.

How much hot water is used in a bath?

It all really depends on the size of the bath and how full you fill it.

For our purposes, around 90 litres of mixed water is a rough figure for a standard bath, but this is only a guide.

An actual bath that’s quite small and only half full will use less, while a large freestanding or double-ended bath will need a lot more.

If hot water demand is going to be high, it’s worth figuring out how much water the actual bath can hold.

Is a 90 litre bath really going to need 90 litres from the cylinder?

Not usually.

When you run a bath tap, you get a mix of hot water from the cylinder and cold water from the mains.

Let’s say the hot water cylinder is storing water at 60°C, the incoming cold water is 10°C and you want a bath at about 40°C.

Under those conditions, roughly 54 litres of 60°C stored water would be needed to produce 90 litres of mixed water at 40°C.

The rest would come from the cold supply from the mains.

Performance can vary a lot because of things like the way the water in the cylinder gets mixed up, heat loss and changes in temperature as the cylinder is used, but it shows that the storage volume of the cylinder and the usable hot water volume aren’t the same.

Will a 150 litre heat pump water heater fill a bath?

In most cases it should be fine.

A 150 litre heat pump water heater with warm enough water could provide a lot more mixed water once some cold water is added at the tap or a thermostatic mixing valve, but it all depends on what else is going on in the house.

If someone’s just had a long shower beforehand, part of the stored hot water will already be gone. And if another shower is needed straight after the bath, then the remaining cylinder capacity and how long it takes to recover become very important.

That’s why heat pump water heaters need to be sized for the highest demand rather than just a single appliance.

What about large baths then?

Large baths need a bit more thought.

A bath with a big usable volume can use a lot of the cylinder’s stored energy all at once. Whirlpool baths and large freestanding baths can be particularly greedy.

If you regularly use a large bath, you might want to go for a bigger cylinder than you would have thought necessary based just on the number of occupants.

You should check the bath capacity alongside the shower flow rates and other domestic hot water needs.

Will the heat pump put hot water back in the bath right away?

The cylinder will refill with cold water from the mains as hot water is being taken out of it, but that then needs to be reheated.

A normal air source heat pump will reheat a cylinder more slowly than a high-output gas or oil boiler.

This is why recovery time is so important.

If you fill one bath and don’t need lots of hot water again for a while, recovery might not be a problem. But if you need to fill two big baths one after the other, the cylinder needs to have enough stored capacity or enough heating power to recover between them.

Does the water temperature in the cylinder matter?

Yes, it does. The higher the temperature, the more usable mixed water you can get from a given cylinder.

However, there’s a trade-off.

Heat pumps usually get a better Coefficient of Performance (COP) at lower hot water temperatures. Raising the cylinder temperature increases the temperature lift the compressor has to deal with and can reduce efficiency.

Higher storage temperatures can also increase standby heat loss.

The answer isn’t just to turn the cylinder temperature up all the time when you need more hot water.

And what about the immersion heater then?

Many heat pump water heaters come with an electric immersion heater for backup or extra heating.

This can provide a bit more heat if unusually high hot water demand occurs, but it shouldn’t be needed regularly on a properly sized system.

If the immersion is needed every time someone has a bath, it might be a sign that the cylinder capacity, heat pump output or control settings aren’t well matched to the household’s hot water needs.

What size heat pump water heater is the best for baths?

A heat pump water heater can supply a bath without any trouble, but the cylinder should be sized for the whole house, not just the bath.

Consider the bath size, number of baths, shower flow rates, number of occupants, stored water temperature, incoming cold water temperature and heat pump recovery time. For a household with a standard bath and a decent amount of shower use going on, a heat pump water heater that’s properly sized will do just fine in terms of providing plenty of hot water. However, if you’ve got a big bath, multiple bathrooms or a whole bunch of people all after hot water at roughly the same time, you might find you need a water heater with a bit more cylinder capacity in order to keep your hot water levels up to scratch.