ᐅ Is a Cistern Economical with a Pumping System?

Hello everyone,

Here is the situation:
- Single-family house in the southern Hamburg area; 545 m² (5,856 ft²) plot; 110 m² (1,184 ft²) house footprint; approximately 150 m² (1,615 ft²) roof area (rough estimate)
- Cost for a cistern according to the earthworks contractor’s offer, 5 m³ (1,320 gallons), including delivery and installation, excluding the "interior components" (What does that mean???): €2,150
- Funding from IFB Hamburg: 40%, up to €1,500 --> Net cost: €1,290

Questions:
- Does such a cistern make sense under these conditions?
- At which planning stage should the decision be made? Before construction or possibly only afterwards?
- Does it make sense (also in terms of cost-effectiveness) to supply the toilet flush and washing machine with water from the cistern?

Best regards!

Schnubbihh schrieb:

- Does it make sense (also economically) to supply the toilet flush and washing machine from the rainwater tank?

For that, you need a separate water system for rainwater. In my case, it was quoted at 3000€ (about $3,200). As long as you install it officially and pay wastewater fees for it, it barely pays off within your lifetime. You can ask your construction company for a price. The cistern itself seems reasonably priced to me, so maybe the overall cost is manageable.

Schnubbihh schrieb:

- At what planning stage should the decision be made? Before building the house or perhaps only afterwards?

Beforehand. You have to install the rainwater drainage system anyway, so diverting it into the cistern initially adds almost no extra work. Digging everything up later to retrofit will cost a lot more. Not to mention the piping inside the house. You have to plan the rainwater system inside the house early on; otherwise, you’ll have to open up the walls again.

For a 1000-liter (about 260 gallons) cistern, I would definitely install it, even if you just use it for garden irrigation and washing your car.

Sounds logical. The costs for piping into the house and the separation between potable water and rainwater are probably several times higher than the cistern itself. Accordingly, it likely makes sense to use the cistern only for garden irrigation, and with the offered subsidy, it hopefully pays off within a few years (and not decades).

I have a rainwater cistern with a capacity of 10m³ (13,100 gallons) that is fed by a 200m² (2,150 sq ft) roof area. It’s practically impossible to empty because it gets refilled quite well after an average summer rain. The water is used exclusively for garden irrigation. I did not install a second system for toilet flushing for three reasons: a) based on friends’ experiences, b) cost considerations, and c) to keep the system simple.

Cisterns are just very practical, as they buffer both inflow and demand. I really enjoy watering the garden this way and even being quite generous with the water. Next year, an automatic irrigation system will be installed, which will be supplied by the cistern. When building new, the additional costs are very manageable, and even if it didn’t pay off financially, I would do it again without hesitation.

We have never had any unpleasant water issues, nor any other problems. Maintenance is limited to opening the tap in spring and closing it in winter. After a few years, I removed some sediment by intentionally emptying the cistern, then rinsing it with a water jet and pumping out the stirred-up sediment water with a wastewater pump. Otherwise, water is always drawn from the surface.

I’d like to join in with a question, which might be silly, but apparently, such things don’t exist 😉

Our groundwater level is quite high, we already have a driven well, and our garden borders a field, with the Isar river behind that field...

What would be the purpose of having a rainwater harvesting tank here? Or can I safely do without it since the well is already sufficient for the garden and the water drains away or soaks into the ground anyway?

hausbau_phobos schrieb:

I’m joining in here with a question that might seem silly, but supposedly such a thing doesn’t exist 😉

Our groundwater level is quite high, we already have a driven well, and our garden borders a field, behind which is the Isar River...

What would be the point of having a rainwater tank here? Or can I safely do without one since the well water is enough for the garden and the water drains away anyway?

In my opinion, a rainwater tank rarely pays off, at least without subsidies. Especially if you already have a well. The problem is you don’t get rainfall evenly. Even if you have 10 m³ (about 2,640 gallons), it will be empty after a few days. If it doesn’t rain for three weeks, the best rainwater tank won’t help you. With a well, you have water available at any time.