Tag Archives: heat exchange

Welcome waste (energy)

Today, we are not pinching nickels, but degrees.

I mentioned in the last post that it took us until November 17 before we turned the heat on, whereas other Chicagoans fired up their furnaces in early October. Why were we still comfortable several weeks into the cold weather?

Waste heat!

Boiling the kettle, cooking dinner, baking banana bread … Then add in all the electrical appliances that produce waste heat: running the fridge, TV, laptop and desktop computers, having the lights on … all this and more produce some level of waste heat which is welcome during this season. Not so much during the dog days of summer, though.

But wait! There’s more. Let’s not ignore the four critters occupying the space. Two of them two legged, and the other two four legged. Believe me, they all have a healthy metabolism going, based on the heat they throw off! Seriously, body heat from building occupants is not to be ignored – not in the context of a deep energy retrofit.

Let’s think of these heat sources as miniature radiators. Individually, they don’t do much. But cumulatively they begin to matter, if – and this is a big IF – the building is well insulated  and as good as airtight. Because now this waste heat doesn’t escape. It lingers around and keeps the building interior at a comfortable temperature when others have long reached for their thermostats.

In this context, your furnishing and the actual interior of your building begins to act as a heat sink – it becomes thermal mass. Your oak dresser, your hardwood floors, your drywall, your bathroom tiles, you name it – they all store heat to some degree, which adds to the comfort.

Another gadget that helps us to delay the start of the heating season in the Energy Recovery Ventilator (ERV). It delivers fresh air into our airtight building envelope, but does so with the help of a heat exchanger. This allow us to recover most of the precious waste heat and yet still get fresh air.

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Cool ideas?

Sometimes I get these seemingly insane ideas, only to find out later that they were somewhat rational. The subject matter of those ideas is typically not my strong suit. That’s why I tend to be infuriatingly quick to dismiss them.

Air conditioning is on my list of subjects that are not my strong suit. I grew up in Germany, where air conditioning is virtually non-existent. I ran into active cooling for the first time while visiting the Caribbean, and again years later when I moved to Chicago.

I regard summer air conditioning in the Midwest as a necessary evil and have a pronounced dislike for the temperature extremes we have to tolerate between the outdoors and conditioned indoors. I dread stepping into a grocery store or office building that feels like an ice box.

Since we started our deep energy retrofit, I’ve had to wrestle with the question on how we will keep cool during the dog days of summer without creating an ice box. I quickly learned that “cool” is somewhat secondary. The primary problem to tackle is how to keep the relative humidity at a comfortable level, i.e. under 60% (preferably at 50%).

We already have an Energy Recovery Ventilator (ERV) that supplies fresh air into the house, yet keeps the outside heat and mugginess at bay. My seemingly insane idea was to install an air-to-water heat exchanger in the fresh air supply duct to remove humidity through condensation, and deliver pleasantly dry air into the house through the ERV duct system.

Simply put, I wanted to run the muggy outside air across a cold air conditioning coil to dry it out. But I was told that there was no such air conditioning device that could be combined with an ERV. Yet, I clearly wasn’t alone with this idea.

Just in the past couple of years I started reading about ‘Magic Boxes’ that basically combine the function of an ERV, or HRV (Heat Recovery Ventilator), with that of a small air-to-air heat pump, i.e. a small air conditioner.



Unlike our ERV, which uses and enthalpy wheel as a heat exchanger, a Magic Box uses an integrated air-source heat pump to transfer thermal energy between air streams. But they also can provide some additional limited conditioning – additional cooling (and drying) as well as heating.

One company (Build Equinox), located two and a half hours south of us, brought the CERV (Conditioning Energy Recovery Ventilator) to market just a few years ago. The CERV has an advertised heating capacity of 3,850 Btu/h, and cooling capacity of 2,400 Btu/h. These numbers vary depending on outdoor temperatures. One article listed a price of $4,500 for the CERV.

A competing product is the Boreal 12000 by Minotair out of Quebec, Canada. This Magic Box is more compact than the CERV and, in heat pump mode, has a listed heating and cooling capacity of 9,400 Btu/h and 8,700 Btu/h respectively. I read one article that pointed to a price of around $3,200 for the Boreal 12000.

Either the CERV or Boreal 12000 could be used instead of an ERV or HRV.

Going back to my original idea, combining an air-to-water heat exchanger with our existing ERV, I came across what looked like a promising option. An article on GreenBuilidngAdvisor.com described a variety of air-to-water heat pumps that could provide chilled water to the air-to-water heat exchanger.

In this function, the air-to-water heat pump would basically function as a chiller. But it could also reverse its cycle and produce hot water for domestic hot water consumption or a hydronic heating system.

The challenge would be to find a right sized unit that I could combine with our ERV, and that would be affordable. If you thought the CERV or Boreal 12000 were expensive, prepare yourself for a sticker shock while shopping for air-to-water heat pumps.

Nevertheless, I feel vindicated that my idea wasn’t that insane after all. But it has only brought me somewhat closer to a solution that would provide cooling and a comfortably low relative humidity during the dog days of summer in our deep energy retrofit.

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ERV croaked – Part 4a

I am having fun with my sequential blog post titles … maybe to compensate for the inconvenience of having a sequential problem with our Energy Recovery Ventilator (ERV).

Both of our ERVs (both RecoupAerator by UltimateAir) have been running flawlessly for the past two years. But earlier this month, our 1st floor ERV showed symptoms that were reminiscent of the problems we had during the winter of 2013/14 with our basement unit.

When we ran the ERV, the building cooled down rather quickly. That indicated that something was amiss with the heat exchanging process. Based on our past experience, I knew that there were two probable causes:

  1. The enthalpy wheel stopped running.
  2. One of the blower motors and/or control boards croaked.

Well, it took no time at all to determine that it was the motor and/or control board. I made a quick call to UltimateAir and a few days later we received the replacement parts. It was time to start tinkering again:

I have a suspicion that the problem may lay with the heat sink on the control board. Two years ago, when I went through the same process in the basement, I noticed that the replacement board had a significantly bigger heat sink than the original board. I also recall vaguely that Matt at UltimateAir pointed out that the board on our 1st ERV may give us the same problem.


Or was it the fibers from the enthalpy wheel that started clogging the impeller that did the motor in? I am curious to know that the experts at UltimateAir think.

On a side note – this blog begins to pay off! Because everything is documented, it’s easy to look up a problem of the past to remind myself on how to fix things – like the ERV.

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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%.


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.


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.


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)


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


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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