ᐅ Is it advisable to connect the washing machine/dishwasher to the hot water supply?

Hello everyone,

I recently came across the topic mentioned in the title.

Here’s my thought: If I use a ground-source heat pump (brine-to-water heat pump) in a new build, with a seasonal performance factor of around 4-5, it would make a lot of sense to connect the washing machine and dishwasher to the hot water supply, wouldn’t it? This would provide a larger flow, reduce issues related to Legionella bacteria, and allow for a water temperature of around 45-48°C (113-118°F), which benefits the heat pump’s efficiency.

The only downside I see is that if I want to wash wool at 30°C (86°F) or do a cold wash, I’d need to switch the connection. But that only happens 2-3 times a year for us.

Also, the additional hot water demand would need to be taken into account when sizing the heat pump system.

My thought is that this could actually save quite a bit of energy, since heating water is the main energy use for dishwashers and washing machines. Or am I mistaken here?

I’d appreciate your opinions.

Best regards,
Specki

First of all, your washing machine must have a hot water connection and be able to use it. This is the case for very few machines.

Normally, you won’t save anything with this because the machines regulate their own heating efficiently.

In addition, it is the hot water preparation that reduces the efficiency factor (COP) of heat pumps.

Specki schrieb:

because most of the energy in dishwashers/washing machines is used to heat the water. Or am I mistaken?

Well, since you only need to wash your clothes (except towels) at 30°C (86°F), you can skip that for the washing machine. Does it have a hot water connection? The same applies to the dishwasher, which uses between 6 and 12 liters (about 1.6 to 3.2 gallons) of water.

Hello

That is not a problem at all and actually makes sense. The annual performance factor (APF) of the washing machine would be 1. The heat pump’s efficiency can’t be worse than that. There’s no need to rewire anything either, as pre-control units are available even for standard washing machines.
We have been using these for about 20 years. Search for "Alfa Mix"



Olli

T_im_Norden schrieb:

First, your washing machine must have a hot water connection and be able to handle it.
That is true for very few machines.

Why does it need that? You can simply connect the hot water to the regular inlet. Then 45°C (113°F) warm water flows in, and the machine doesn’t need to heat it since it’s already warm. I assume it has a temperature sensor and doesn’t just heat based on time.

T_im_Norden schrieb:

Normally, you don’t save anything with that because the machines control their heating well themselves.

Yes, they regulate heating well, but they use about four times as much energy to heat the water compared to a heat pump.

T_im_Norden schrieb:

Also, it is the preparation of hot water that lowers the COP (coefficient of performance) of the heat pump.

Yes, you would have to check the heat pump’s COP when heating water to 45°C (113°F). But it should still be significantly more efficient than direct electric heating.

nordanney schrieb:

Well, since you only need to wash your laundry (except towels) at 30°C (86°F), you can skip that for the washing machine. Does it even have a hot water connection?

We normally wash at 40°C (104°F). But even at 30°C (86°F), the water still needs to be heated from about 12°C (54°F) to 30°C (86°F).

nordanney schrieb:

The same applies to the dishwasher, which uses between 6 and 12 liters of water.

That would actually be an argument against it. Both machines use little water. And presumably, the water isn’t only heated at the start but reheated throughout the process. So the effect would be reduced again.

@wrobel
Well, I’m not going to buy a €260 (approx. $280) unit per machine. Is it really worth it?
I roughly estimated the savings at around €50 to €70 (about $55 to $75) per year. So the unit would have to last 4–5 years just to break even.

Take a look here:

https://www.hausbau-forum.de/threads/spuelmaschine-an-den-warmwasseranschluss-anschliessen-sinnvoll.31159/

and here

https://www.hausbau-forum.de/threads/solarertrag-ist-nicht-ablesbar.27724/