Drain water heat recovery is recognized by the DOE as an effective means to reduce energy consumption in a home for heating water.
Any hot water that goes down the drain carries away energy with it. That’s typically 80%–90% of the energy used to heat water in a home. Drain-water (or greywater) heat recovery systems capture this energy from water you’ve already used (for example, to shower, wash dishes, or wash clothing) to preheat cold water entering the water heater or going to other water fixtures. This reduces the amount of energy needed for water heating.
ENERGY.GOV makes a strong case for using drain water heat recovery to reduce the cost of heating water.
Drain-water heat recovery technology works well with all types of water heaters, especially with demand and solar water heaters. Drain-water heat exchangers can recover heat from the hot water used in showers, bathtubs, sinks, dishwashers, and clothes washers. They generally have the ability to store recovered heat for later use. You’ll need a unit with storage capacity for use with a dishwasher or clothes washer. Without storage capacity, you’ll only have useful energy during the simultaneous flow of cold water and heated drain water, like while showering.
Some storage-type systems have tanks containing a reservoir of clean water. Drain water flows through a spiral tube at the bottom of the heat storage tank. This warms the tank water, which rises to the top. Water heater intake water is preheated by circulation through a coil at the top of the tank.
Non-storage systems usually have a copper heat exchanger that replaces a vertical section of a main waste drain. As warm water flows down the waste drain, incoming cold water flows through a spiral copper tube wrapped tightly around the copper section of the waste drain. This preheats the incoming cold water that goes to the water heater or a fixture, such as a shower.
By preheating cold water, drain-water heat recovery systems help increase water heating capacity. This increased capacity really helps if you have an undersized water heater. You can also lower your water heating temperature without affecting the capacity.
Prices for drain-water heat recovery systems range from $300 to $500. You’ll need a qualified plumbing and heating contractor to install the system. Installation will usually be less expensive in new home construction. Paybacks range from 2.5 to 7 years, depending on how often the system is used.
One other item I want to cover today is drain water heat recovery. Basically it’s a method of capturing waste heat that’s leaving the building, running down the drain, and I would like to capture some of that heat and reuse it before I let it leave the building. This is very clever. It’s buildable. You need height to do it. So those slab-on-grade houses, one-story, this is not going to be particularly useful for most of us. People who have vertical height and master bathrooms upstairs, there’s room to work with. These are a very, very good idea to capture waste heat.
OK, thanks, Gary. We have a number here, so we’ll try to get to a few more. Here’s one regarding drain water heat recovery. Is there a difference between drain pipe type, cast iron versus PVC?
They act differently as drains, but the drain water heat recovery devices I’m familiar with basically break into whatever plumbing you already have for your drain lines and insert a copper to copper heat exchanger.
The material’s sort of taken out of the equation, then.
That’s correct. We’re not wrapping pipe around an existing drain line. We’re creating a section that’s premade, intended to work a certain way. And so far all of those exchanges are copper to copper, that I’m aware of. By the way, I showed you a vertical one. There is a four-foot-long horizontal for shallow sloped drain lines that’s become available from another company in Canada.
Great. That remark actually addresses another one of the questions. I’m going to shift over here to temperature settings for the hot water system. Gary already spoke — spoke to this on your performance metrics slide. But one of our attendees cites a recent article that advocates that only set the hot water temperature only to the temperature needed for a shower without any additional regard for Legionnaires.’ This is a practice, in your opinion?
So, if you set a storage water heater at the set point you’d like, and you have long showers, half of the event probably won’t be very warm. Because the way storage tanks work, it just won’t happen that way. You have a tankless water heater where you have got continuous flow through, then you can set the temperature down to what you need, but you need to set it to whatever temperature you need plus whatever it takes to get — overcome the losses in the plumbing between the water heater and the use. So if the piping is very long and not very insulated and very large in diameter, you need to set the temperature somewhat higher. The question has to do with Legionella; it turns out that the 105-, the 110-degree temperatures that we like to take showers at are pretty close to what I would call to high fever temperatures, and pretty much all pathogenic bacteria just love them. I say it’s probably the most beneficial bacteria — why? It’s warm body temperatures. They’re comfortable there. If the water’s flowing through, less of an issue than if it’s not. But most of the recommendations that I’ve heard recently related to Legionella is for flow through devices like tankless, where you’re not storing any significant amount of water if any, 125 to 130 and mix it down where you need it. And for storage tanks, 140 is still a good recommendation, even though we haven’t been doing that in a while.
NATIONAL GREEN BUILDING STANDARD – NGBS SINGLE FAMILY 2012
National Green Building Standard – NGBS Single-Family 2012
Potential credits: up to 6 points
Application: NGBS scoring Performance Path, practice #702.2.1, up to 3% reduction from IECC 2009 Energy Rating Index (ERI) using HERS pathway.