Tag Archives: heat exchange

DWHR follow up

As I said in the last post, the primary benefit of running the DWHR performance test was the discovery that it wasn’t operating as intended. It gave me a chance to fix the problem and have the heat exchange and heat recovery process run at its full potential, which I measured at 46.7%.

DWHR-07

That in turn should help me save some money, which is important to make this investment pay off. There is a lot of copper in the DWHR, and it doesn’t come cheap. We bought ours for $617.00 a few years back.

DWHR-01

To sweeten the investment, Renewability provides an energy savings calculator on their website, to give the consumer an idea what level of savings could be expected.

Our DWHR (the R2-60 PowerPipe) serves the first floor and second floor bathroom showers. I assumed an average occupancy of 3 people per apartment, 0.75 showers per person per day and a shower time of five minutes. According to the energy savings calculator, we could expect annual savings of

  • 388,605 Btu (or 4.1 gigajoules),

which would translate into

  • $67.36 savings per year.

If I increase the shower time to 10 minutes, the expected annual savings increase to

  • 767,732 Btu (or 8.1 gigajoules),

which would translate into

  • $132.96 savings per year.

I guess the savings will lie somewhere in between the five and 10 minute shower time scenarios, and so will be the payback time for the DWHR, which would fall somewhere between four and a half to nine years.

That “M” word!

Using the calculator is not as straight forward as you may think – as I found out. First, it appeared to be down quite a lot, displaying an error message. This may just be a temporary issue, or so I hope.

Secondly, using the calculator, I was reminded that it is us (or should I say US) against the rest of the world. I think we must be the only culture left that doesn’t use the metric system. Canada does use the metric system (bless the Canadians!), and Renewability, the manufacturer of the DWHR, is a Canadian company.

The use of the metric system becomes relevant in the energy calculator if your fuel type is natural gas. The input field ‘Cost of Fuel’ uses the unit $/cubic meters natural gas – and not therms! A subtle detail that makes a difference in the calculator output.

How do you determine the cost of fuel?

And I am not talking about unit conversion – yet. Should I just use the cost per therm and ignore all the delivery charges and other add-ons?

I opted for what would I call the true cost. I added up the total volume of cubic feet of natural gas delivered over the past 12 months and converted it into cubic meters. I also added up the bill totals for the past 12 months and divided it by the total cubic meter volume. That gave me an average fuel cost of $0.55 per cubic meter of natural gas.

Closing comments

The $0.55 fuel cost is a snapshot. It is on a sliding scale depending on the occupancy of the building and natural gas prices.

I also have to take the calculator output at face value. I have not cross-checked the results through my own calculations. The fact that the advertised effectiveness of the DWHR was right on par with my own test results gives me some confidence into the energy savings calculator results.

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DWHR performance test – good data

Although I would have liked a heat recovery rate of close to 90% for our drain water heat recovery (DWHR) unit, I knew that I couldn’t trust that number. Some good troubleshooting led us to the problem, thanks to some expert advice.

The pressure differential between the supply line to the domestic hot water tank and pre-heated water line from the DWHR prevented the setup from working properly, as did our less than perfect plumbing layout. Fortunately, we were able to resolve the issue with a quarter turn on a shut-off valve.

DWHR-13

Another test

It’s time to run the performance test once more to see if I could get some credible readings. I attached the temperature probes again to the three data points on the DWHR:

  • Cold potable water in (Tci)
  • Pre-heated potable water out (Tco)
  • Hot drain water in (Thi)

DWHR-09

As before, I took readings every 20 seconds while Cathy was taking a shower upstairs. Once I punched the readings into the spreadsheet, I saw some good data emerging.

The data

DWHR-07

The heat recovery rate for our PowerPipe R2-60 maxes out at 46.7%, which is a smidgen above the published performance rating of 46.1% by the manufacturer Renewability. Still, this took me by surprise.

I am somewhat suspicious of the performance ratings you find in product literature. Maybe the performance is inflated to help in selling the product. Maybe the laboratory test set up is so removed from the real world that test results don’t translate.

Yet, the DWHR results were right on the mark, as were the results for the ERV testing. Maybe I need to adjust my attitude?

The real value of the DWHR performance test was the discovery that the setup didn’t work as intended. I would have had pre-heated water sitting in the DWHR, doing a whole lot of nothing, whereas it should have fed into the domestic hot water storage tank. That could have gotten expensive, because next to no heat recovery translates into next to no savings!

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Troubleshooting test results

I ran a first performance test on our drain water heat recovery (DWHR) system, and got readings that were too good to be true.

Because I was at a loss about what could have caused the wonky readings, I picked up the phone and called the manufacturer of our DWHR. Joel, the technical manager, volunteered to look at my test data and look over the plumbing diagram. That led to some very helpful troubleshooting.

Reversed hot water flow?

DWHR-11

My close to 90% heat recovery readings could be caused by hot water flowing from the domestic hot water storage tank towards the DWHR, and not the other way around. This is a very unlikely scenario, but still worth testing.

I attached the temperature probes along the pre-heated return line to the domestic hot water storage tank and started running the shower. I could trace the hot water flowing from the DWHR toward the domestic hot water tank through the readings on the temperature probes.

Good! It was flowing into the right direction. But – it was flowing very, very, very slowly!

The pressure issue

Joel from Renewability reminded me that there is a certain pressure loss associated with the DWHR. The copper spiral around the outside of the DWHR creates a flow resistance that causes some amount of pressure loss. To be precise, the rated pressure loss for our R2-60 DHWR module is 1.4 psi at a flow rate of 2.5 gallons per minute (gpm).

To manage the pressure loss and maximize the heat recovery of the DWHR, Renewability recommends a certain plumbing set up, called the equal flow configuration. A diagram of that setup is provided with the installation instructions.

And because our DWHR installation was half an afterthought, our plumbing set up doesn’t comply with the equal flow configuration. Bingo!

DWHR-12

It’s all in the plumbing, baby!

Our domestic hot water storage tank has a cold water supply that feeds into the bottom of the tank. The pre-heated water line from the DWHR is connected into that cold water supply line.

The pressure in the cold water supply line is greater than the pressure in the pre-heated water line (remember the pressure loss issue?). That pressure differential slows the flow in the pre-heated water line almost to a halt. I basically have water sitting in the pre-heated water line and DWHR spiral, rather than flowing. And that is the reason why that water was picking up all that heat from the inner tube of the DWHR.

DWHR-13

To balance the flow, Joel recommended to shut off the existing cold water supply into the tank, and instead force all the supply water to the domestic hot water storage tank through the DWHR. And I have just the right valve in the right spot to do that.

Let’s see if that will yield more realistic test results. Can you wait ‘til the next post?

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Test results to dream of

That heat is mine, and I plan to keep it!

DWHR installation

Test results to dream of

Winter appears to be out of sight. But before it gets out of mind, let’s look at the performance of one of our heat recovery gadgets – the drain water heat recovery unit (DWHR).

DWHR-10

We installed this beauty in the basement, at the bottom of the sewer stack that drains the 1st and 2nd floor showers.

While I want to let the shower water go, I would love to keep the heat that is in that water. The DWHR allows me to recover and reuse a large chunk of that thermal energy.

Water running down a drain stack tends to cling to the pipe walls. While the hot drain water is running down the inner copper tube of the DWHR, it transfers its thermal energy to the cold potable water that is flowing up in the outer copper spirals, effectively pre-heating it.

DWHR-08

That is the theory. But what I really want to know is if the heat exchanger works as promised and what percentage of the thermal energy I actually recover. The technical term of what I measure is “effectiveness” or “empirical heat transfer rate.”

Testing the effectiveness of the DWHR is remarkably similar to that of the ERV. I look at three data points:

  1. Cold potable water in (Tci)
  2. Pre-heated potable water out (Tco)
  3. Hot drain water in (Thi)

DWHR-09

I attached my temperature probes to each data point at the DHWR unit and took readings every 20 seconds for 10 minutes while Cathy was taking a shower upstairs.

The results were excellent! Just by touching the outer coil, I could feel how the cold inlet water water got increasingly hot toward the top outlet (Tco). Once I had punched the numbers into my spreadsheet, I came close to 90% effectiveness (or a 90% heat recovery rate).

But therein lies the problem: If it sounds too good to be true…

A quick web search will tell you that the effectiveness of DWHR units typically ranges from 40% for smaller units to 70% for the largest units. Plus, the in-house test data by Renewability for my DWHR (called the R2-60) is listed at 46.1%.

Something is amiss and I would like to find out what it is.

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That heat is mine, and I plan to keep it!

DWHR installation

ERV performance test

Mechanical system explained

While writing the last post about our boiler going into lockout mode, I realized that after all these years I never fully explained our mechanical system.

I have posted a time lapse showing the utility room setup and described the PEX and radiator installation, including the difficulties we encountered along the way, but I have yet to show the components of the mechanical system in the utility room and how they work together.

utility-room-003

The diagram above does exactly that, except most of us will have some level of difficulty deciphering it. Let me try to put it into a format that is easier to follow, starting with a list of the main components (I won’t list all to keep it reasonably simple):

Boiler (Trinity LX 150)

This pieces of equipment is basically heating the water for the hydronic heating system (radiators and radiant floor slab) as well as indirectly heating the domestic hot water (DHW).

Boiler pump

utility-room-011

This pump is moving the water through the boiler into the buffer tank while the boiler is firing.

80 gallon buffer tank (insulated)

This tank is feeding the hydronic heating and also (indirectly) heating the domestic hot water system. The tank temperature is set to 140 degrees Fahrenheit.

Due to our low energy load on the space heating and DHW side, we run the risk of short cycling the boiler. The buffer tank prevents that by providing the initial thermal energy. In that process, the temperature in the tank drops and is elevated again through hot water supplied by the boiler. But at this point the energy load is large enough for the boiler to run efficiently and without short cycling.

Main manifold

This is a three zone manifold supplying hot water to (1) the radiant floor slab in the basement, (2) the radiators on the first floor, and (3) the radiators on the second floor.

Zone pumps

In the manifold are three zone pumps, supplying hot water to each zone once the thermostat of that zone turns on.

Mixing valves

Also in the manifold are mixing valves for each zone. The mixing valves reduces the temperature from 140 degrees Fahrenheit to the supply temperature of 120 degrees Fahrenheit.

System pump

 

utility-room-014

Whereas the zone pumps push the hot water into the hydronic heating system, the system pump sits right under the manifold and pushes the water from the system back into the buffer tank. This pump is activated whenever there is a heating signal from any of the three zones.

120 gallon domestic hot water storage tank (insulated)

This tank supplies domestic hot water to the kitchens and bathrooms throughout the building. The tank is heated with hot water from the buffer tank that flows through a double walled heat exchanger. This way, the non-potable water from the hydronic heating system does not mix with the potable DHW. Because the tank has no gas or electrical powered heating element, it is also referred to as an indirect water heater.

DHW mixing valve

The temperature in the DHW tank can also get up to 140 degrees Fahrenheit, which is dangerously hot. To prevent scalding, the temperature is mixed down to 120 degrees Fahrenheit, which is also the supply temperature to the various faucets and showers.

DHW pump

utility-room-016

This pump is feeding the water from the buffer tank through the heat exchanger in the DHW tank whenever there is sufficient hot water demand.

To make things a little easier to follow, I put a simplified diagram together that is animated and shows how the system is working. I hope this will do the trick.

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Connecting the unruly…

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