Saturday, April 24, 2010

On Again

Forrest arranged for us to visit a house he is working on with a heat recovery ventilation (HRV) system, so that we could get an idea how noisy HRV is. On Friday, we drove up to the place. The house is a job that Forrest has almost completed. There are still a few items yet left to finish, and the owners had graciously consented to let us check the HRV out since they are away.

Surprisingly, the HRV system is almost silent, even on the highest setting. The only time I could hear anything from it was when I was standing directly under the vent in the bathroom, and that was just the sound of the air coming out of the duct. Forrest also turned on the forced air furnace, and that was much noisier, I guess because the amount of air being pushed through the ducts is larger.  Paul, Forrest's partner, opened the attic and let us take a look at the HRV in operation. With the attic door open, you could hear a distinctive hum, but the vibration was not transferred to the structure of the house, unlike a forced air furnace, because the HRV is mounted from chains connected to the frame of the house with springs. This acted as a sound damper, removing any vibration. Sound propagation through the HRV ducts is also limited by putting in lots of right angle turns. These break off the paths through which sound waves can easily propagate.

We still have some concern about the geothermal heat pump. The big pump that sends the heat exchanger fluid into the ground and the compressor that either extracts the heat from the fluid or puts it in are both likely to generate vibrations. Just like a forced air furnace, they can't be hung from the frame of the house because they are too heavy, though there are some measures that can be taken to isolate the heat pump from the frame of the house. We discussed putting the heat pump in the garage and running an insulated line to the heat exchanger in the mechanical closet instead of mounting the heat pump in the mechanical closet. We need to have the heat exchanger in the mechanical closet because that is where the manifold is for the radiant heat system. Of course, we would also need to figure out how to engineer the air conditioning. The air conditioning is never likely to be quiet, since, just like a forced air furnace, it will be shoving large amounts of air through the ducts. Putting that in the mechanical closet seems unavoidable if we will be using the old forced air ducts for air conditioning. They all converge on the mechanical closet because that is where the old forced air furnace was, though the ducts that were under the house have been removed and will probably need to be replaced.

I also asked Forrest if we could connect the air conditioning to a ventilation system with a heat exchanger. This would allow us to get fresh air into the house in summer through the air conditioning rather than opening the windows or running the HRV. He said that this typically isn't done, that the warm air return for the squirrel cage blower on the air conditioning comes from within the house, just as with a forced air furnace. In fact, the air conditioning (or forced air furnace if you have that) and the HRV are two separate sets of ventilation ducts. Somehow, one would think that somebody would have been smart enough to combine the two.

I continue to be amazed about the lack of efficiency in planning HVAC (heating, ventilation, and air conditioning) systems. Stuff that seems obvious to me as an engineer with a systems orientation never seems to have crossed the minds of the people who designed these systems. Every function is in a separate box. It is no wonder that over 40% of the energy in this country is used by buildings. Without proper integration of these systems, there is redundant energy use. For example, the squirrel cage blower on a forced air furnace and the HRV system are essentially doing the same thing: pushing conditioned air through ducts. Why not combine them, and run the cold air return through a heat exchanger on the furnace and out, and draw in outside air through the heat exchange and into the furnace?

Anyway, we've told Forrest to move ahead with the geothermal planning. We signed the design contract two weeks ago, but put it on hold pending an opportunity to experience how much noise an HRV system really makes. This means the final design will be pushed out by at least two weeks (and if we are lucky, only two weeks), meaning the start date is likely to be late June. Four months puts us into October. If the project is delayed (as usually in is the case in my experience) we could end up moving back in around Christmas. Considering we are planning on living in the back bedroom and also mostly outside during the summer, things could get a little tight come October when the weather starts to get cold again.


  1. You've got the noise issue somewhat right. Most of the noise is created by the fans. This noise travels up and down the ductwork. Flexible, insulated ductwork (the kind mostly used in North America), aka acoustic duct, is great at attenuating most of this noise. There can be structurally transmitted noise, which is really the sound of the structure vibrating - that's where the chains come in.

    Most equipment just does not vibrate that badly.

    FYI, we have a few articles on-line that touch on this subject of acoustics:

    Your comments on manufacturers not being forthright with acoustical info is noted. It's most likely a result of being burned by consumers looking for a single parameter to represent a complex phenomenon.

  2. Thanx for the info.

    I suspect that the reason the European units have acoustical info and that American ones don't is because the European governments require it and the American government doesn't. The general opinion in America is something like "if people really cared about this, they would ask manufacturers and then it would be come a competitive issue to see who could offer the quietest unit", which is true as far as it goes. Since most American homes have forced air heating (which I find noisy after experiencing radiant heat), people really don't seem to care about any additional noise from HRV.

    I guess the subjective aspect of noise is difficult to quantify. It sounds as if the way the units are installed makes a difference too. But there are other areas where subjective measurements have been developed for sound. I'm most familiar with something called the "mean opinion score (MOS)" that is used to subjectively quantify the quality of voice telephone calls. This has nothing to do with son or dB ratings, which are objective ratings that the Japanese and Europeans use for noise. It has to do with asking people what they think of the quality of a telephone call having a particular amount of electronic noise in it. I suppose one could try something like that for HVAC noise, but you would probably get different answers from Europeans and Americans, or people with radiant heating v.s. people with forced air heating.