### Image Credits:

1. Alex Wilson

1.
Feb 20, 2014 1:56 PM ET

Edited Feb 20, 2014 2:34 PM ET.

Wow, that looks like a pricy addition
by Jonathan Rupp

I was thinking about about installing an HRV when i re-do my bathrooms, but your energy consumption seems really high 118 KWH over 25 days?!? That corresponds to an average of 197W, and would be a Huge energy sync in my monthly consumption.

That made me wonder how HRVs would compare vs. just opening your window for a few minutes / hours a day (As i was told to do when i rented an appartment in Germany for 6 months). I have to admit, my thermodynamics are pretty rusty, so someone correct me if i am wrong:

118 KWH / 25 days = 4.72 KWH/day
at my electric prices is Mass: = \$0.90 per day in electricity
using my oil prices, that corresponds to 0.242 gal of oil per day
With my boiler efficiency, that is 28.9 k BTU of heating
Assuming you raise the outside temperature 30 degrees (i think the rough average temperature in Mass for a heating day), it takes 0.6 BTU per cubic feet of air to raise the air 30 degrees (i just googled and saw that it takes 0.02 BTU/cubic foot / 1 degree)

That then equates to 48k cubic feet of air movement per day, or 33.5 cfm of air directly from outside, 24 hours per day?

That also doesnt include the capital costs of the equipment, and the labor to install the ducts. Plus i am using an oil boiler instead of the much cheaper natural gas alternative.

I am even rustier on my fluid dynamics of chimneys, but it would seem cheaper to leave my fireplace flue open and crack the windows in my bathrooms when i take a shower.

Does anyone else have any thoughts?

I looked up the chimney flow rate on wikipedia, and assuming 60F interior, 30F exterior, and a 20ft chimney, 33.5 cfm requires a chimny cross sectional area of 5.9 sqft... so i guess to get that much airmovement, i would have to add a window fan infront of my fireplace to blow air up the chimney

2.
Feb 20, 2014 2:22 PM ET

Response to Jonathan Rupp

Jonathan,
You're asking the right questions, for sure. This question was discussed at length in a GBA article, Are HRVs Cost-Effective?

3.
Feb 20, 2014 4:15 PM ET

Cost of HRV
by Chaz Steffen

What is it that makes HRV's in general and the Zhender in particular so expensive?

5.
Feb 21, 2014 9:50 AM ET

Response to Jonathan Rupp
by Jonathan Teller-Elsberg

I don't have an answer to your question, but it's important to consider possible nuance in the situation. Looking at your calculations, it appears to me that your math assumes a linear relationship between outdoor temperature and the HRV's electrical consumption. Because the HRV is hogging electricity specifically to defrost, answering your question requires knowing the relationship between temperature and the defrost cycle. It seems to me plausible that this is a nonlinear relationship in which increasingly cold temps demand exponentially more time in defrost. For example, if you drop from 30'F to 20'F the defrost time increases by X percent, but when you drop from 20'F to 10'F the defrost time increases by more than 2X percent.

6.
Feb 21, 2014 2:30 PM ET

defrost
by Nick T - 6A (MN)

Defrost cycles i have to imagine in a lot of instances can be done without much penalty using a recirc mode. So this need not be an issue.

The reduced air changes due to the recirc mode should likely be negated during these extreme cold conditions with an increase in stack effect passive air over the average day. Or could be accounted for in total average daily runtime/airflow.

For 50-90w a HRV seems to make sense. It can be installed very economically in most cases - no need to spend almost 10times the cost.

I really dislike the idea of sucking on a house via exhaust only... and then having -5degree air coming in via leaks and/or 'inlets'... :-/ a leaky house is bad... so is one that you put leaks into on purpose. I suppose in mild climates it wouldn't be such a big deal.

7.
Feb 22, 2014 1:59 PM ET

Edited Feb 25, 2014 2:00 PM ET.

Too Complex
by Kevin Dickson, MSME

I really don't see this type of installation as being the future of residential ventilation.
A Panasonic spot ERV can be installed for about \$600.

Let Fick's Law handle the fresh air distribution. Once the fresh air molecules are delivered, they bounce around the house at about 500 feet per second.

There's a very slim chance that the next owner of the house will maintain a complicated and expensive system since he probably can't even tell if it's not working.

8.
Feb 25, 2014 10:05 AM ET

Update: energy consumption during February
by Alex Wilson

Our Zehnder 350 has so far used an average of 2.7 kWh per day in February. During months when the defrost function isn't needed, consumption should average less than 1 kWh/day.

9.
Feb 25, 2014 10:22 AM ET

Response to Alex Wilson