Why Do Heat Pump Cylinders Have Larger Coils?
Why Do Heat Pump Cylinders Have Larger Coils? Heat pump cylinders have to have bigger coils because when an air source heat pump is supplying water, it’s almost always coming in at a much lower temperature than what a traditional gas or oil boiler would give you. This smaller temperature difference between the heating water in the coil and the domestic hot water stored in the cylinder makes heat transfer a whole lot slower. A larger coil – or heat exchanger coil, as we call it – has more surface area so the heat pump can move its output into the cylinder with ease, without forcing it to crank up to unnecessarily high flow temperatures.
Why do heat pumps need a coil that’s a bit bigger?
The coil inside an indirect hot water cylinder is basically a heat exchanger.
Hot water from the air source heat pump flows through the coil while the domestic hot water is stored around it. Heat passes from the water in the coil over into the stored water, and the two water circuits remain separate.
The rate of heat transfer is affected by a few factors, but the two most important ones are:
The temperature difference between the primary water and the stored water
The surface area of the heat exchanger coil
A boiler can usually supply water at a pretty high temperature, which creates a pretty big temperature difference, so a smaller coil can still get a useful amount of heat moving.
Heat pumps work a bit differently though.
Heat pumps are always more efficient when they’re working at lower temperatures.
An air source heat pump does its best when it’s working at relatively low water temperatures.
As the required output temperature goes up, the compressor has to work harder. Electricity consumption goes up, and the COP (Coefficient of Performance) usually starts to fall.
So, it doesn’t really make a lot of sense to install a heat pump and then force it to run at too hot temperatures just because the hot water cylinder has a coil that’s too small.
A special heat pump cylinder has a bigger coil to help the heat pump transfer heat at lower primary water temperatures.
Our smallest air source water heating option is the 100 litre wall mounted ASHP hot water heater, suitable for properties where hot water demand is relatively modest.
For larger domestic requirements, take a look at our 200 litre ASHP domestic water heater or 300 litre ASHP domestic water heater.
For larger properties and commercial applications, see the 400 litre ASHP commercial water heater or 500 litre ASHP commercial water heater.
How much bigger is a heat pump cylinder coil?
Size of the coil isn’t determined by a single rule – it depends on the heat pump’s output, the cylinder’s capacity, the primary flow temperature, the required domestic hot water temperature and how quickly you want to recover hot water.
For instance, a 300-litre heat pump cylinder might have a coil with a surface area that’s a lot bigger than a conventional 300-litre indirect cylinder designed for a boiler.
What really matters isn’t just whether the manufacturer calls it a “large coil”, it’s the actual coil surface area or heat exchanger performance at the sort of temperatures you’ll be running the heat pump at. And this needs to be matched to the output of the heat pump.
What happens if the coil is too small?
If the coil is undersized, it can become a sort of bottleneck on the whole hot water system.
The heat pump might be capable of producing several kilowatts of heating, but if the cylinder coil can’t transfer that heat into the stored water quickly enough, the heat pump can’t use its full capacity properly.
This can lead to:
Longer hot water recovery times
Higher required flow temperatures
Lower heat pump COP
Longer compressor running periods
Greater reliance on an immersion heater
Which is why, if you’re changing to an air source heat pump and keeping an existing boiler cylinder, you need to think carefully about whether it’s going to work efficiently.
The cylinder might still work, but that doesn’t necessarily mean it’ll be efficient.
Does a bigger coil mean faster hot water recovery?
Generally, yes, provided the coil is the right size for the heat pump.
More surface area on the heat exchanger coil means heat can pass into the stored water more easily, helping the cylinder to recover a bit faster after hot water has been used.
Recovery time is especially important with heat pumps because their output to the cylinder might be lower than what you’d get from a boiler.
Getting the cylinder sizing right also matters. The system should store enough hot water to last through periods of high demand without needing to rely on rapid reheating or the immersion heater too much.
Is bigger always better for the coil size?
Not necessarily.
A very big coil isn’t a substitute for proper system design. The heat exchanger still needs to be matched to the heat pump’s output, flow rate and operating temperatures.
Cylinder capacity, insulation, standing heat loss, controls and hot water demand all have an effect on how well the system performs.
The aim is not just to fit the biggest possible coil – it’s to give the heat pump enough heat exchanger area to transfer its output efficiently under normal running conditions.
What about integrated heat pump water heaters?
An integrated heat pump water heater works differently.
Instead of using a separate air-to-water heat pump connected to a conventional coil, some hot water heat pumps transfer the heat directly from the refrigerant through a heat exchanger built into or wrapped around the cylinder.
In these dedicated appliances, the heat pump and hot water storage vessel have already been designed to work together, so a separate heat pump cylinder with a large coil is not needed.
Why is coil size important when choosing a heat pump cylinder then?
**The main reason heat pump cylinders need bigger coils is to transfer heat without forcing the heat pump to run at ridiculously high temperatures.**When picking a cylinder, take a closer look at the coil surface area, heat exchanger output, cylinder capacity and recovery performance rather than simply going by the storage volume.
A good match between your cylinder and heat pump coil (especially if it’s a large one) lets your air source heat pump work at lower flow temperatures, makes hot water recovery more efficient, and generally gets the complete system getting more out of the available efficiency.