The short answer to this question is ‘no’ although it can make a significant contribution depending on the configuration of your system.
On initial consideration, it might be hard to understand why this should be the case. Most heat pumps are designed to heat water (to run your central heating system) to 45-50°C and domestic hot water is used at 38-42°C. However, unlike a combi boiler, a domestic heat pump cannot produce hot water quick enough to provide an adequate flow of water to baths, taps, and showers. Also, most domestic heat pumps are not designed to be able to provide hot water at the same time as providing space heating, with the demand for hot water taking priority.
One way to get around this problem is to store hot water in a cylinder until it is needed. However, care is needed when storing water between 20 and 45°C due to the risk of legionella, particularly if the water remains in the cylinder for long periods of time. Heating the water in the cylinder to 60°C for at least 30mins or 55°C for at least 5-6 hours is the conventional way to kill legionella bacteria in domestic properties, although for safety and simplicity, the usual advice is to keep the cylinder thermostat set to 60°C.
The simplest way to achieve 60°C is to use an electric immersion heater, fitted into the top of the hot water cylinder. For maximum efficiency (unless you are having a hybrid system installed) it is likely that the existing hot water cylinder will need to be replaced with one that has a much larger coil capacity, to compensate for the fact that a heat pump will deliver water at a lower temperature to the cylinder than a conventional boiler would do. The greater surface area of the larger coils aids the transfer of heat from them to the water in the cylinder.
If you don’t have space for a hot water cylinder (with a diameter of 55cm or more) then there are other options available. These include:
A hybrid system
A hybrid system is designed so that the heat pump provides your space heating needs and a boiler (which could be your existing gas or oil boiler) provides your hot water. Another reason for having a hybrid system is to cope with a home with a high heat demand. This is because the largest size of heat pump that can be installed on a standard (single phase) domestic electrical supply is 14kW (although an upgrade to a three-phase supply may be possible, at a cost)
Further information and advice on hybrid heat pumps can be found on the Energy Saving Trust website
A heat battery
A heat battery can store spare heat (or electricity) in a material when it changes phase from a solid to a liquid. These materials are referred to as phase change materials. When hot water is needed the phase change material change back to a solid, releasing heat. As heat batteries are generally smaller and lighter than a cylinder or thermal store they can be a good option if space is limited.
Instantaneous water heaters
An example of an instantaneous water heater is an electric shower – it uses electricity to produce hot water only as you needed it. Instantaneous water heaters can also be fitted to supply a single tap and can be economical because the hot water is only generated as it is needed. However, an instantaneous water heater is not as well suited to providing hot water to a bath, due to the volume and flow speed needed for a bath.
And potentially, a high-temperature heat pump,
A high-temperature heat pump can produce heat between 60 and 80°C by using a different refrigerant to a more conventional heat pump. However, this type of heat pump is more expensive to buy and run. High-temperature heat pumps are reported to provide 2.5 units of heat for every unit of electricity consumed; a conventional heat pump is more likely to produce between 3 and 4 units of heat for each unit of electricity consumed.
Summary
This blog is intended as a general guide only to the options available for the delivery of your hot water supply if you have an air source heat pump installed. The sizing and design of a heating system (whether or not this includes an air source heat pump) is a matter for a professional installer who will be able to discuss the options through with you. To find one go to the Microgeneration Certification Scheme website.
If the idea of enhancing your home’s energy efficiency through strategic retrofitting and delving into the realm of low-carbon alternatives intrigues you, then our SuperHomes project could be a perfect match. Our community members are at the forefront of redefining sustainable living. For further details, feel free to visit www.superhomes.org.uk
It was two years ago this week we had the nasty old fossil gas natural draft water heater removed. (That’s it in the photo below.) Outside of running a gasoline powered generator inside a house, this kind of water heater is probably the number one source of carbon monoxide in homes. Yes, the hot water is cheap, but how much is your health worth? So my replacement for this fossil was a heat pump water heater (HPWH). It’s a Rheem Performance Platinum, and I love it!
I am not an expert in heat pump water heaters, and I don’t know the range and availability of products all that well. But I have friends who keep up with that stuff, so I asked one of them. John Semmelhack of Think Little is a big proponent of all-electric homes, and he recommended the Rheem model that I ended up buying. Manufacturers are always changing their models, though, so my generation 4 has evolved into the generation 5 model (I think). Also, when you’re searching for information, Rheem uses the term “hybrid water heater” because its main source of heat is the heat pump, but it also has built-in electric resistance heating.
One of the choices you have to make when buying any water heater is size. I sprang for the 80 gallon model for one simple reason. If I’m going to use a heat pump water heater, I want it to use the heat pump all the time. I don’t want to use the electric resistance heating element at all unless the heat pump fails. The smaller the tank you buy, the more likely it is you’ll have to use some electric resistance heating.
The bigger tank has the other advantage of having more hot water available. My 80 gallon model has an 89 gallon first-hour rating. Did I go overboard? Yeah, probably. I do that sometimes. This weekend we’ll get to put it to the test as we’re having family in town for my 120th birthday party.* That larger tank should come in handy.
The Rheem HPWH connects to wifi and has a nice app for your smartphone. The screenshot below shows the main screen for the water heater, with the basic info about the generation (4), water temperature setting (120° F), mode (HP only), and links to other data (schedules, usage report, and wifi settings, to the left of the temperature).
And it has one annoying notice I wish Rheem would change. That green bar is ALWAYS there for me because I’ve set the mode to heat pump only. Their so-called Energy Saving mode would actually use more energy because it would use electric resistance heat sometimes.
We fired up (so to speak) our new water heater on 14 September 2019, so we just passed the two-year mark. How much electricity have we used in that time? The chart below shows our first full year of data. The total was 486 kilowatt-hours (kWh). The lowest month was August at 23.75 kWh, and the highest was December at 68.6 kWh.
Notice the pattern? Water heating energy use is low in summer and high in winter. The main reason for that is the entering water temperature. As everything cools down in the fall, the municipal water supply cools down, too. That means it takes more heat pumped into the water to raise the temperature to 120° F.
A second reason is that we use more hot water in winter, although I actually haven’t measured that yet. Next year, however, I’m going to set up flow meters and temperature sensors on my all-new hot water plumbing when I remodel my basement. Then I’ll have the data to prove it, at least in my household. Those data will also tell me how many kilowatt-hours it takes to get a gallon of hot water, by each month of the year and averaged over the whole year.
A third reason we use more energy in the winter is that the HPWH pulls heat out of the air, and our basement air is cooler in winter. In my case, we currently don’t heat or use the basement much so it’s not a big deal. In cold climates, it can be a big deal.
Another nice feature of the Rheem app is that it can show your current usage compared to usage for the previous period. The chart above shows it by month for 2021 so far compared to the corresponding months in 2020. It also does daily by hour, weekly by day, and monthly by day.
Since it’s been just a couple of days over two years since I started up the heat pump water heater, I can tell you how much energy we’ve used in that time and how much it has cost us. From 14 September 2019 to 14 September 2021, we used 1,002 kWh. I also track our electricity use in a spreadsheet (doesn’t everybody?), and over that period, our average rate from Georgia Power has been almost exactly $0.10 per kilowatt-hour (excluding taxes and fees). So we’ve paid just a bit (two bits actually) over $100 for two years’ worth of hot water.
How does that compare to our old fossil gas water heater, you ask? We lived with it for less than three months, so I don’t have full data. But based on the little that I do have, it looks like the gas water heater would have cost us close to the same amount (again, excluding taxes and fees). So, there’s no direct economic benefit to switching from a gas water heater to a heat pump water heater…when I look at the cost based on rates only.
I had already made the decision to get rid of our gas furnace, however, and that meant our only remaining gas appliance would be the water heater. At about $38 per month (screenshot above), the fees just to have gas would make the heat pump water heater a big money saver. Based on a marginal cost of $1,200, the extra $450 per year in gas fees ($38 x 12) would give me a simple payback of about 3.5 years!
If I had replaced an electric resistance water heater, however, we still would have saved money; just not as much. The efficiency of water heaters is rated by something called the Uniform Energy Factor (UEF). Higher numbers are better, and my HPWH has a UEF of 3.7. (You can see it on the EnergyGuide above.) A standard electric water heater has a UEF of about 0.9, making my HPWH about 4.1 times as efficient.
Instead of 501 kWh per year, I would have used about 2,000 kWh per year with an electric resistance water heater. At $0.10 per kWh, I would have saved about $150 per year on water heating. With a simple payback of about 10 years in this case, it wouldn’t be hard to justify this purchase for economic reasons, assuming a 15 or 20 year lifetime and minimal maintenance costs.
One thing people have complained about with some heat pump water heaters is that they’re too noisy inside the house. Some probably are, so I measured mine using the NIOSH SLM app on my phone. Here are my results:
That’s pretty quiet. For reference, here are some other noise levels on this scale:
Another nice feature of the Rheem model I bought is that it allows for the intake air and exhaust air to be ducted. You can see the exhaust port in the lead photo. It’s to the right of the digital display screen. My water heater is in the basement, so I could run a duct up to the encapsulated attic to bring in warmer intake air and make the heat pump more efficient. But it would be more trouble than it’s worth in my case.
I will need to run a short duct to the mechanical room wall, though. After I remodel the basement and put a door on the mechanical room, the HPWH won’t have much air in that space so the duct will bring in air from the rest of the basement.
Even if you don’t duct air from another place, the ability to duct intake air allows you to put a better filter on the system. The filter that comes with it is one of those paper-thin, see-through filters that mostly catches larger stuff. I haven’t done it yet, but when I remodel and move the water heater, I’ll also add a decent filter to the system and get it all sealed up to eliminate bypass. I want to keep that coil as clean as possible.
In my case, it certainly was worth it. We have low rates for fossil gas here in Georgia, but they come with high fees. Getting rid of the backdraftable gas water heater was a high priority for me. Not having to worry about low-level carbon monoxide poisoning (or worse) brings a peace of mind that’s priceless. And I’m doing my part to decarbonize my existing home by going all-electric. I could have saved on first cost by going with a standard electric resistance water heater. Had I been ready to put photovoltaics on my roof, I might have done that.
I love my heat pump water heater and am really happy with it. It’s quiet, efficient, and gives us all the hot water we desire. It even provides a little bit of cooling and dehumidification. And that’s the topic for an article coming later.
Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and the founder of Energy Vanguard. He has a PhD in physics and writes the Energy Vanguard Blog. He is also writing a book on building science. You can follow him on Twitter at @EnergyVanguard.
* OK, it’s not my 120th birthday party alone. My wife and I will be a combined 120 years old on Saturday. I turn 80.5 and she 39.5…or something like that.
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