ᐅ Heat Pump Domestic Hot Water Storage Tank – What Size Is Recommended?
Created on: 30 Jan 2018 10:01
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Tommes78Hello everyone,
How large are your domestic hot water tanks? We are planning a heat pump system with a separate 300 L (79 gallons) domestic hot water tank. Our installer wants to install a unit with an integrated tank of only 170 L (45 gallons) and says that would be 100% sufficient.
We have two bathrooms, one with a bathtub measuring 180 x 80 cm (71 x 31 inches) plus a rain shower, and the other bathroom has only a shower. Ideally, we will have four people living here eventually: two adults and two children.
What are your experiences, and how large are your domestic hot water tanks? Do you think the smaller tank would be enough?
How large are your domestic hot water tanks? We are planning a heat pump system with a separate 300 L (79 gallons) domestic hot water tank. Our installer wants to install a unit with an integrated tank of only 170 L (45 gallons) and says that would be 100% sufficient.
We have two bathrooms, one with a bathtub measuring 180 x 80 cm (71 x 31 inches) plus a rain shower, and the other bathroom has only a shower. Ideally, we will have four people living here eventually: two adults and two children.
What are your experiences, and how large are your domestic hot water tanks? Do you think the smaller tank would be enough?
I am facing exactly the same situation. My installer says that is more than enough; generally, people tend to go for smaller storage tanks. I trust his judgment. I don’t really have another option, since otherwise I would have to install an additional storage tank. I don’t want to incur those extra costs.
I’ve mentioned this somewhere before, as we had the same issue. For us (5 people who basically shower almost simultaneously) with 2 showers each delivering 9 liters per minute (about 2.4 gallons per minute), the 170-liter (45-gallon) tank offered by Vaillant would only last for about 4 minutes of shower time per person. In addition, we currently bathe around 3-4 times a week, and with 3 women in the house, this is likely to increase.
The installer’s recommendation: Increase the domestic hot water tank temperature to 60°C (140°F) and use water-saving shower heads. With gas heating, the supply temperature is not an issue, but with an air source heat pump and a design outdoor temperature of -15°C (5°F), it becomes less favorable.
You’ll have to look up the exact calculations yourself or, better yet, have a plumber work them out for you: 170 liters at 55°C (45 gallons at 131°F), mixed down to about 40 to 42°C (104 to 108°F) at the shower, yields roughly 220-240 liters (58-63 gallons) of water. At a flow rate of 9 liters per minute (2.4 gallons per minute) — which you can find in the shower’s datasheet — a 5-minute shower uses 45 liters (12 gallons). Multiply that by the number of people basically showering at the same time.
Simply do a test with a stopwatch to find out how long you usually shower. It’s easy to misjudge. But if I read “rainshower,” the hot water demand is going to be tight at best.
As an alternative to a larger domestic hot water tank, an electric tankless water heater (solar compatible) might also be an option. If the tank meets your needs 95% of the time, the tankless heater could kick in during the 5% of times when there isn’t enough hot water. This can be a cost-effective alternative to a bigger storage tank.
The installer’s recommendation: Increase the domestic hot water tank temperature to 60°C (140°F) and use water-saving shower heads. With gas heating, the supply temperature is not an issue, but with an air source heat pump and a design outdoor temperature of -15°C (5°F), it becomes less favorable.
You’ll have to look up the exact calculations yourself or, better yet, have a plumber work them out for you: 170 liters at 55°C (45 gallons at 131°F), mixed down to about 40 to 42°C (104 to 108°F) at the shower, yields roughly 220-240 liters (58-63 gallons) of water. At a flow rate of 9 liters per minute (2.4 gallons per minute) — which you can find in the shower’s datasheet — a 5-minute shower uses 45 liters (12 gallons). Multiply that by the number of people basically showering at the same time.
Simply do a test with a stopwatch to find out how long you usually shower. It’s easy to misjudge. But if I read “rainshower,” the hot water demand is going to be tight at best.
As an alternative to a larger domestic hot water tank, an electric tankless water heater (solar compatible) might also be an option. If the tank meets your needs 95% of the time, the tankless heater could kick in during the 5% of times when there isn’t enough hot water. This can be a cost-effective alternative to a bigger storage tank.
A rain shower head uses a significant amount of water. I’ve noticed this myself... especially combined with my wife’s shower habits ;-)
A heat pump cannot heat the water as quickly as, for example, a gas boiler.
A bathtub also requires a considerable amount of water.
If I were you, I would definitely go for the 300L (80 gallons) storage tank.
A heat pump cannot heat the water as quickly as, for example, a gas boiler.
A bathtub also requires a considerable amount of water.
If I were you, I would definitely go for the 300L (80 gallons) storage tank.
A bathtub can obviously hold a significant amount of people. So how much water is actually in it, and how quickly do you need to refill it?
A heat pump with 7 kW heats water just as quickly as a gas boiler with 7 kW. The heat pump also includes the mentioned instantaneous water heater in a “light” version, the electric heating element. Its power, often between 6–8 kW, adds to the heat pump’s capacity. This is obviously not economical in operation, but it can cover peak demand without the risk of running out of hot water.
Example: A heat generator with 6 kW heats the 45 liters (12 gallons) of water needed for a shower from 10°C (50°F) to 42°C (108°F) at 80% efficiency within 21 minutes.
The reheating process can already begin during water draw-off, if desired.
These are two different approaches.
A heat pump with 7 kW heats water just as quickly as a gas boiler with 7 kW. The heat pump also includes the mentioned instantaneous water heater in a “light” version, the electric heating element. Its power, often between 6–8 kW, adds to the heat pump’s capacity. This is obviously not economical in operation, but it can cover peak demand without the risk of running out of hot water.
Example: A heat generator with 6 kW heats the 45 liters (12 gallons) of water needed for a shower from 10°C (50°F) to 42°C (108°F) at 80% efficiency within 21 minutes.
The reheating process can already begin during water draw-off, if desired.
These are two different approaches.
Alex85 schrieb:
A 7kW heat pump heats water just as quickly as a gas boiler with 7kW. That should be clear to everyone.... However, here in our new housing development, I don’t know a single neighbor whose heat pump has more than 5kW. Our gas heating system modulates between 3 and 15 kW, so it can heat water at 15kW.
Fortunately, our plumber advised us to choose a larger storage tank, even though we have a gas heating system, because he heard that we often take longer showers or bathe the child. Depending on the size and model, a rain showerhead can easily use 20 liters per minute (or even more!).
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