Heat Recovery Unit for shower

[C Williams]

I love showers. I love long showers. Let's say I take an "hour power shower".

With that in mind I've been thinking of installing a heat recovery drain system for an upstairs shower. I'm currently thinking of "Power Pipe" but I'm sure there are others.

First, does anyone here have any experience with these good or bad. And second: Typically one would install this in line with the cold supply for the hot water tank but since my tank is a good distance from where the heat recovery unit would be, it would be simpler to install this in line with the showers cold supply. I'm not sure if I would need a fancy mixing valve to make this feasible. Any thoughts?

Thanks.

[johnfrwhipple]

I think you should try it. What size is your hot water tank?

I am going to install a second hotwater tank in my mechanical room and strip it of any insulation. My plan is to use the warmth from the room to pre-heat the water overnight to about 24 degrees C.

When we have demand for hot water the water will first enter the secondary hot water (holding tank) and then feed the proper hot water tank. I think this will save me hundreds of dollars if not thousands over the next ten years.

An investment like this will take years to break even. Don't expect to see a return on your investment any time soon.

JW

[jimbo]

You need to rely on recommendations from the manufacturer about install options. Their engineers should have the answers to those questions.

What are you wanting to accomplish with this? Certainly some energy will be recovered. How much...check their research. Figure that the water cools considerably as soon as it starts running off your back. It gathers on the tile floor, and eventually into cold uninsulated drain pipes. So if you expect this system to give you longer showers, I don't think that is going to happen.

I assume you have a tankless water heater?, since if would take about a 200 gallon tank-type to provide an hour shower@!!

[hj]

The recovery amount will be so miniscule that it is doubtful you would ever notice its effect. To get heat transfer heat source and the water have to be in contact long enough for the transfer to take place. Usually this is done with a twisted "labyrinth", a long transfer pipe, such as a flue, or multiple small tubes with a lot of surface area. Your shower unit will have none of these.

[johnfrwhipple]

Here in North Vancouver the water temperature entering the hose is icy cold. I would think even a 10' section of cast pipe with the copper feed line twisted around would improve things somewhat.

Is there any test data on the subject that anyone has found?

I would love to know more about this subject.

JW

[C Williams]

Here is a 2007 study by the Canada Mortgage and Housing Corporation.

Drainwater Heat Recovery Performance Testing at CCHT

The in situ effectiveness testing looked at how long the shower could be run before the water temperature dropped below 37°C (98.6°F). All DWHR devices resulted in significantly longer hot water availability times than the benchmark time of 28 minutes. Configuration A results ranged from 39 minutes to 62 minutes. Configuration B results ranged from 53 to over 75 minutes.

Some Canadian provinces are offerring rebates for these.

[jimbo]

On that link, the $$ figures are fairly impressive. I don't know what the units cost, but with installation, I see a 10 year payback.
Apparently the "technology" is that the drain water tends to flow in a thin film on the inner surface walls of the drain pipe, providing a lot of surface area for thermal transfer.

As with all research, the data raises as many questions as they answer. It seems they are using a starting shower temp of about 105º, but run until the shower temp drops to 98º?? That is not acceptable to most people. If anything, I personally tend to dial UP the temp after a bit of time. I normally take a pretty short shower, but if I feel like a "steamer" then 98 doesn't cut it!

All in all, it does look like there is some savings worth pursuing. The websites do seem to show that the system must be vertical, so retrofit may be difficult.

[DonL]

I think that a Hot Tub would better satisfy your need for a "hour power shower"

Or a hot air blower in a nice bathtub.

[jadnashua]

Heat transfer depends on numerous factors, but the delta T, or difference in temperature is a big factor. Water does tend to run down the sides of the pipe, even when vertical, rather than falling down the middle of it, so you end up with a thin film on the maximum surface area. Then, by splitting the supply water into multiple square channels like one of the recovery systems does, you tend to maximize the heat transfer. By warming the inlet water to the WH, you're decreasing the load and making it look like a larger WH. If you warm the cold into the shower, you could use slightly less hot and achieve the same temp, but you would have a much bigger variation and still likely have to start with a hotter input. This is a situation where a thermostatically controlled valve would have advantages, even as the hot cooled off, the valve would be adjusting the hot/cold mix to try to maintain the set temperature. I think I'd put it in the cold supply to the WH and then make sure I'd insulated all of the piping to retain the heat gained.

[Runs with bison]

The recovery amount will be so miniscule that it is doubtful you would ever notice its effect. To get heat transfer heat source and the water have to be in contact long enough for the transfer to take place. Usually this is done with a twisted "labyrinth", a long transfer pipe, such as a flue, or multiple small tubes with a lot of surface area. Your shower unit will have none of these.

Incorrect. This is the most common layman's misunderstanding of how heat transfer works: "in contact long enough." It's not the residence time but the heat transfer coefficient and area, along with the driving force (delta T). Liquid film coefficients are quite high.

If you look at the designs of the drainwater heat recovery units they can achieve reasonably good recovery. The layout is typically one of countercurrent, falling thin film exchanger, often with a multi-parallel path coil in close contact on the other side. Combine these with the excellent heat transfer properties of water and you have a functional design.

I evaluated a few designs a year or so ago the same way as I did when doing heat exchanger design including--thin film types--for a living (and I've not yet had any exchanger I designed/specified underperform.) I started out skeptical of some of the claims, but when I began reviewing the drawings I achieved similar results with my own calculations of heat transfer coefficients, pressure drop, effective area and such.

Higher volume/long duration shower users will get more benefit than low volume users. For high volume, minimizing pressure drop requires more parallel coils and heat transfer efficiency declines somewhat, but that is more than made up for by the greater total Btu's recovered.

[johnfrwhipple]

Some great information shared thank you all.

I have three little girls and they all have bath and shower time after dinner. This is my biggest demand time for hot water.

What do you think of a second tank as a holding tank? I would think in a 24 hour period the holding tank would have enough hot water or at least warm water that the recovery of the main hot water tank would be quicker and shorter. And in doing so cheaper.

I have even thought of wrapping the spare tank with my hot water heat return legs. Tapping any heat out of them and making the temperature of the holding tank even warmer???

JW

[hj]

quote; Then, by splitting the supply water into multiple square channels like one of the recovery systems does, you tend to maximize the heat transfer.

Splitting it into "multiple square channels" may make it transfer heat better, but would be disastrous for a shower drain, especially when it comes time to clear the debris out of those channels. Did any of the evaluations consider the degradation caused by soap and hair buildup on the walls, which would be exacerbated by having small channels? Also, what were the respective flow rates? High flows would not allow the "hot" and "cold" waters to stay in contact long enough to get the temperature differentials the diagram shows. The discussion might be academic, because a search for "Watercycle" does not produce any results so they may not be a factor any longer.

[C Williams]

Wow. I had no idea this forum was so widely read. Thank you to all who read and replied.

Admittedly I don't take "hour power showers" but I liked the rhyme.

More accurately, I am planning a new home in a rural location on Vancouver Island so my water will be drawn from a chilly well. Without natural gas, BTUs for water heating are relatively expensive so my payback on a DWHR unit will likely be shorter than that for the majority of North Americans.

The main shower in the house presents and ideal installation as its drain immediately drops one story to the main drain below grade. Because the concept works best when drain flow matches inlet flow I wouldn't bother re-designing the main stack to accommodate one of these as the shower is the only load in my house that matches the basic criteria.

The only question in my mind is can I use the drain waste heat recovery unit to warm the shower's cold water supply or with this present a problem for the mixing valve?

Jim, You seem to suggest that I could use a thermostatic mixing valve to do this but your preference would be to warm the water heaters supply. Having thought about it more I tend to agree based on the my belief that, after the shower is over, the DWHR unit will have some latent BTUs that might get used the next time there is a call for hot water. Perhaps minuscule but the additional plumbing is not worth mentioning.


Thanks again all!

The following are some links for others considering drain waste heat recovery.

Products
http://www.watercycles.ca/
http://www.ecoinnovation.ca/residential-solutions/
http://www.renewability.com/general/residential.html
http://www.retherm.com/
http://www.ecodrain.ca/
http://www.shower-save.com/

Rebates/Grants
http://oee.nrcan.gc.ca/residential/p...retrofit/13302
http://www.fee.qc.ca/en/business/dra...ry-systems.php
http://www.gazifere.com/EN/en-reside...cuperateur.php

Research
www.cmhc.ca/odpub/pdf/65680.pdf
Northern Ireland Center for Energy Research and Technology

NRC Savings Calculator
http://www.ceati.com/calculator/

Other Threads on Terry Love Forums (My apologies for not searching first)
See the bottom of this page.

[C Williams]

Splitting it into "multiple square channels" may make it transfer heat better, but would be disastrous for a shower drain, especially when it comes time to clear the debris out of those channels. Did any of the evaluations consider the degradation caused by soap and hair buildup on the walls, which would be exacerbated by having small channels? Also, what were the respective flow rates? High flows would not allow the "hot" and "cold" waters to stay in contact long enough to get the temperature differentials the diagram shows. The discussion might be academic, because a search for "Watercycle" does not produce any results so they may not be a factor any longer.

For clarification, it is the water supply line that is split into multiple channels not the drain. The drain portion is simply a straight cooper pipe with an i.d. of 2", 3", 4", or 6" (Available PowerPipe sizes)


Try www.watercycles.ca/

It seems that most of the references I have found are Canadian so payback may be climate dependent and not applicable to your part of the world. I suspect that your cost per BTU is somewhat cheaper as well.

Regards

[C Williams]

...
What do you think of a second tank as a holding tank? I would think in a 24 hour period the holding tank would have enough hot water or at least warm water that the recovery of the main hot water tank would be quicker and shorter. And in doing so cheaper.

I have even thought of wrapping the spare tank with my hot water heat return legs. Tapping any heat out of them and making the temperature of the holding tank even warmer???

JW

Your recovery time may be shorter but I can see no cost savings here. It strikes me as an inefficent use of space and money. Given that your "holding tank" will be absorbing BTUs from a conditioned space, a space that you paid to condition, there is no net gain in BTUs. You're not capturing lost energy, you're mearly transfering energy.

Your money might be better spent investigating solar domestic hot water. The District of North Vancouver has already done you the favour of estimating your savings based on your location's solar exposure. See their mapping applications at http://geoweb.dnv.org/

Here are the stats for your house.



Solar BC has grants available and I expect you have the expertise to install it.