Internal Drain Water Heat Recovery
M5MoreM
| Posted in General Questions on
It seems to me that the most efficient drain water heat recovery “device” would be one where the preheating water lines are run inside the drain pipe instead of wrapped around the outside of the pipe. This means the drain pipe would not have to be copper, only the heat exchanger water pipes inside the drain would need to be copper. Does anyone make a drain heat recovery device like this?
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Replies
A copper pipe isn't really cutting into the efficiency of the heat exchanger -- it's probably actually helping by increasing contact time with the waste water. Copper is a VERY good conductor of heat. You have other issues too in terms of the flow of fluid and contact time between the "sides" of the heat exchanger that come into play too.
I'm not aware of any drain water heat recovery heat exchangers with the coil inside the drain pipe. I don't think you'd want one built that way, either -- there would be much more chance of clogs and gunk buildup on the irregular surface of the coil compared with what you'd get on the smooth surface inside of a regular drain pipe. You'd also have a lot more chance of damaging the heat exchanger with a snake while clearing the clog in such a device.
Bill
I've always been curious why heat recovery couldn't go the other way around though. Maybe impractical and adding complication but theoretically in the summer tap water is anywhere from 40-60°F right? Couldn't you run the tap water supply through lets say like a PCM thermal storage tank and reject some of the heat to the water and store "coolth"? Seems like for a mild summer climate that could be beneficial.
Or additionally using a split system heat pump water heater and blowing the air from the "outdoor unit" over cooling coils for a cold storage. So basically the cooling potential was driven by the hot water usage and tap water usage.
The way these work is to pre-heat the water going into the water heater, which recovers some "waste" heat from the drain water to pre-heat the cold supply water going into the water heater. Doing this reduces the energy required to raise the supply water to the water heater's setpoint temperature since you're starting with warmer water. Recovering "coolth" doesn't help your water heater, because you're going the wrong direction.
Another thing that comes into play is that the efficiency of these simple heat exchangers is better with larger delta T's, which means bigger differences between the temperature of the waste water and the supply water. If you have very hot waste water and very cold supply water, you recover more of the otherwise wasted heat in the waste water than you would if the temperatures were pretty close together.
What you could conceivably do is use cool waste water to precool the gaseous refrigerant line for a heat pump or air conditioner. The down side in this case is that those systems run much higher pressures than water lines, and you'd need a lot more refrigerant charge in many cases to loop the refrigerant lines to wherever the heat exchanger was located. There are systems that let air condioners use their waste heat to heat pool water though, which both heats your pool and improves the operating efficiency (lowers energy requirements) of your air conditioner compressor.
Bill
Sorry I guess I didn’t fully explain but I was thinking of more “cold” thermal storage for hydronic cooling in the summer and sort of trying to benefit from water usage and DHW production. So as you use more water you reject more heat to the 40-60°F which in turn pre-heats the water going to the tap or water heater and store “cooling potential/water” to be used for hydronic cooling. Essentially a mini thermal storage buffer tank for a cooling loop was the thought. Might not be practical but was thinking of rejecting heat to a Parrafin or PCM storage since you don’t really need the cold tap water at your faucet.
The other thought is that in the summer yes you could duct the split system inside so blow “cold” air and help with cooling when you run the heat pump for water heating but that seems impractical and complicated to set up a damper system or something for that. Additionally its like “hey go take a shower its hot in here” and so would be driven off water usage. Instead my thought was just if you could use that cold air that is rejected t be blown across cooling coils you could benefit.
I’ve always felt mixed about drain water heat recovery from showers etc except maybe an apartment complex or hotel where I think ThermoDrain shines right? Not necessarily for residential or tanked systems.
I hadn’t considered the smaller ∆T and the HX efficiency loss so thank you for that. Additionally interesting about the pool I hadn’t heard of that before but totally makes sense.
During the cooling system, a heat pump water heater is essentially free to run -- all of the heat it adds to the water is removed from the house, which is that much less the AC has to run. What I've wondered about is whether it would make sense to run the incoming water through a radiator with a fan to get it up to room temperature -- and cool the room -- before it goes into the water heater.
The COP of a heat pump depends on the temperature difference, and I think what you'd find is that when you're pumping heat from 70F air into 50F water the COP is very high and it's not worth doing anything else. But I don't know.
I understand that copper is an excellent heat conductor. However, when the heat exchanger coil is outside the drain pipe most of the circumference of the pipe that forms the heat exchanger coil is not in contact with the drain pipe. In our home I must run the drain heat recovery pipe horizontally due to the design of the house. In this case, rather than using a coil, using several straight pipes running in "the bottom of" and inside the drain pipe might be more efficient as the water travels on the bottom of the drain pipe and would completely surround the exchanger pipes. Cost may be reduced as well because the drain pipe would not have to be copper. This does require a high quality installation of the heat exchanger pipes so that the drain pipe will not leak around the entrances/exits of the exchanger pipes through the drain pipe wall. However, I do share your concerns about drain clogging and maintenance. Although, the shower I am thinking of converting has been in use for 30 years and has never had clogging or build up problems.
Typical drain water heat recovery heat exchangers are installed vertically, and water in a vertical pipe tends to travel as a film along the inside surface of the pipe (known as "laminar flow"). This has the effect of greatly increasing the thermal interface area between that waste water and the inner surface of the heat exchanger. Putting pipes, linear or coiled, inside that drain pipe are likely to disrupt this laminar flow and actually decrease the amount of water in contact with the heat exchanger surface since some of the water will now fall freely inside the drain pipe. This will also greatly increase noise of the heat exchanger while water is flowing in the drain pipe.
Typical heat exchangers have the coiled copper tube soldered or brazed to the copper waste pipe. This results in an excellent thermal interface, and combined with the high thermal conductivity of copper, the contact between the two metal pipes isn't as important as the contact area between the inner pipe surfaces and the water flowing through them. The drain pipe has a lot of contact area due to laminar flow and the large inside area of the relatively large drain pipe. The smaller coiled supply water tube achieves high inside surface area by a long length of the relatively small pipe.
A horizontally installed heat exchanger will likely be less efficient due to the waste water traveling only along the lower side of the pipe, resulting in much less contact area between the water and the heat exchanger. In this case, the turbulent (think "disorganized", like rapids in a river) flow would help instead of hurt you, but I think overall you would at best get close to the efficiency of the conventional design and installation.
Bill
Drain water heat recovery doesn't make sense for a typical homeowner. There's more fruit elsewhere on the tree offering better payback and improvement in comfort. This topic has been discussed on GBA a plenty.
Drain water heat recovery seems like one of the best payback and comfort improvement options for many homeowners who want low cost, high reliability, and high hot water capacity improvements for showers, especially as your incoming water temperatures decrease and heating fuel costs increase.
I read through the other discussions and it seems that when the heat exchanger water can be fed directly into a tankless electric on demand water heater that is located very close to the drain that the benefits may be worth it. If I misunderstood the other conversations, please let me know.
As to the laminar flow issues mentioned by Zephyr7, I am not able to install a vertical system. My questions were meant to be considered in a horizontal flow only situation. That is why I thought that exchanger pipes inside and on the bottom of the horizontal drain might be effective.