Help solve H.R.V. puzzle

Ken Caverly

I need someone to explain my H.R.V. readings to me.

It is 30 degrees F outside my HRV air intake and yet I am getting air coming into the rooms at 70 F. I took some readings.

Temp of the ducting carrying the outside air just before it enters the HRV 53F.
Temp of the ducting of the exhaust to the outside 62F
Temp of the ducting bringing house exhaust air to the HRV 72F.
Temp ofthe ducting supplying fresh air to house 70F
Temp of the air coming out of the room exhaust duct. 69F.

Questions
How can the outside temp be 32F and yet the pipe entering the HRV only show a temp of 53F. how did I get a 21F increase in temp just having the air travel 12' down an insulated pipe.

How can the air from house and the air to house only have a 3F degree difference. If it is a 40F difference in inside and outside temp then shouldn't it show at least a 20F drop after they pass each other in the HRV.

What is the usual return temp coming from a HRV room vent. This is a house with radiant floor heat and so the air isn't passing through ony other appliances.

Ken C.

Ericjeeper

what brand do you have?

whatever brand you have ... I want one like it.. I have been researching HRV's myself..

Ken Caverly

It is a bryant model VA3B.

My post isn't so much to promote this model as to see if these results were possible. These high vent temperatures seem to foul up the thermostats in some of the cooler rooms and then keep the floors from coming on.
The owners manuals don't give any indication of these temps.
Ken C.

Constantin

Thermodynamically,

the above scenario does not yet make sense. Energy has to be preserved, so what is warming the incoming air besides the HRV?

For one, the exterior duct temperature may or may not be representative of the temperature of the air flowing inside it. From the sounds of it, the HRV is in a cold location but it's probably much warmer than the exterior of the home.

Secondly, there may be a duct heater/defrost cycle/whatever that is causing the exterior air to be warmed. How does the unit defrost?

Lastly, you might inquire with the manufacturer to send you the efficiency charts to review just how much sensible heat your HRV is supposed to recover at specific fan speeds. Cheers!

Brad White_33

And to add to that

might you have the ducts connected to the appropriate collars? Dumb question but it would not be the first time that happened... Your measurements and expectations may be off too.

I agree with Constantin (surprise) that the information you stated is contra doxas...

Unknown

How far apart are your intake and exhaust hoods?

Could you be pulling in some of your exhaust air (warmer air than outside ambient)?

Just a thought.

Ken Caverly

I took new readings this morning and moved all the sensors inside the HRV. That made a considerable difference but still doesn't explain the results.
The outside the house temp at the intake was 20F.The intake inside the HRV was still 34F, a 14degree difference. I then opened the damper on the intake ducting used to balance the in and out airflows. Opening the damper reduced the intake inside the HRV to 28F. This seems a little more reasonable in an 8 degree difference between outside air and the air entering the HRV. The HRV is located in the mechanical room that maintains an 80F temp.

Now my new readings.
Intake in HRV 28F.
exhaust to outside. 41F. A 13 degree difference.
From house to HRV 71.6F
From Hrv to house 63F. An 8 degree difference.
This still seems like an incredible heat recovery.

The 63degree fresh air supply temp is better suited to not fouling up my thermostat control but I can't leave the damper open or it is pressurizing the house.

Is this reasonable compared to the results of anyone elses testing. What sort of fresh air to room temps would be expected. Remember it is only 20F outside.

The outside intake isn't close to any heat sources.

Ken C.

Ken Caverly

Also

The unit does have a defrost cycle but I had put a weak piece of tape over it to see if it moved and it hadn't.

Ken C.

Mike T., Swampeast MO

I found the manual rather informative.

The defrost scheme is to stop incoming air and recirculate room air--perhaps there is a fault.

The control description may offer a clue as well. If you don't have some form of mechanical humidification it appears highly probable that you're getting more recirculation than actual air exchange from indoors to out. Your statement about over-pressurization seems to point to this.

Why such high ambient temp in the mechanical room? If the unit is in any way sensing the temp and humidity in that room then I'd guess there could be severe problems as the RH in such a warm room will be significantly lower than the rest of the house and it could really mess with both the humidity-related control system and adjustment of such...

Constantin

Hmmmm....

I'd measure the incoming and outgoing airflows and see if they actually balance.

From what I gather, the incoming air temperature goes up 35°F, while the exhausting air temperature drops only 31°F. It shouldn't be that way. An imbalance in the air flow could explain it, however.

jwade55_3

I agree with Constantin,

Balanced air flows are very important. I would venture to guess that you have more exhaust air, than intake air. This could be a bad situation as you are creating a negative pressure in your house envelope, I'd rather see a netural pressure. Get yourself some magnahelic gauges, and check statics. See the attached.

J

Ericjeeper

Ok I do not understand HRV

Lets say you have 120 cfm per hour going out..70 degree air. and you are bringing in 120 cfm per hour at 20 degrees.. what keeps an air to air heat exchanger from being a 50 percent recovery? as much cold air passes the exchanger as warm....so when Lifebreath says 82 % recovery rate... How does this work? Not trying to steal this thread.. it has been very educational so far. Thanks Eric

Ken Caverly

The bryant rep brought the equipment and balanced the flows at the time of installation to train the plumbers staff on our unit.
Model # VA3BAB20. Efficiency rating 68% at 32F. 61% at -13F.
The internet manual is informative and more thorough than the one supplied with the HRV. Thank you.
The defrost flap is wide open as it should be at this temp.
The mechanical room has a propane boiler which keeps it very warm. The humidity controls are upstairs and would not be influenced by the mechanical room conditions.

You are probably thinking ," Great recovery, what's the problem". The problem is that I have a basement with very little heat loss and mostly bedrooms. I want to keep this area cooler, perhaps64F. With the heat provided by the HRV coming in at 69F as well as the heat from the upper floors using 135F water temp and no insulation in the floor joists, The basement thermostats do not call for heat very often and thus the finished concrete floors are always very cold. Cold floors is not what radiant is all about.

I don't understand the comment that I might just be recirculating rather than exhausting the air.

Ken C.

Mike T., Swampeast MO

I don't understand the comment that I might just be recirculating rather than exhausting the air.

Not being familiar with the unit itself I can only go by that manual.

Look at page 4 under "Control Use and Description".

If I'm understanding things properly, "Continuous Mode" [might] result in low speed air exchange but normal fan operation. In other words, just mixing in a bit of fresh air with the rest being recirculated room air.

In any regard, the operation you're seeing may well be the result of the indoor relative humidity and/or the relative humidity setting in the unit.

Christian Egli_2

Thermodynamic jump

I have not tried to plug numbers into what you have but I have an inkling about where this fountain of heat of yours has its source. (leaks and imbalance excepted)

People who live in steam heated home can all vouch for what happens when you deboil water - the radiator routine. The stunning results are 1) condensate and 2) boundless heat.

Your perpetual heat machine is no doubt extracting lots of condensate water that it is squeezing out of your exiting indoor air. This is particularly true in the cold winter when dew point conditions are met, and it is compounded with the very high moisture content of vapor barriered hermetically sealed air proof homes.

Measure how much condensate is pouring out. Then for every pound, you just got yourself more than 1000 BTU of heat. The rest is all numbers.

It's no mystery. It's what condensing boilers rely on for boosting efficiency numbers. It's also what allows people in places like Seattle to heat a space economically with a simple dehumidifier.

This may be significant in your case. I don't know. It's just always amazing to watch a steam radiator flexing its iron. Not bad for the HRV.

To get the recovered heat upstairs, why not pipe the outlet somewhere else than downstairs.

Ken Caverly

I will take some time to study the manual some more. I do think it must have something to do with not exhausting all the inside air. I thought perhaps the plumber had hooked an exhaust pipe to the outside intake ducting by mistake but this recirculating theory sounds more reasonable.

As for checking the condensate, the plumbers did not attach a drain line to the HRV, so if it was producing any condensate it would be dripping all over my drying hockey equipment. It is bone dry inside the HRV and my equipment is dry.

Thank you all for the help.

Ken C.

Constantin

The idea is..

... to recover 82% of the sensible BTUs leaving the home via the HRV.

If you take a look at the recovery tables, you'll see that the recovery rate is fan-speed and HX-size dependent. For example, the DCS units from Lifebreath have two HRV cores and therefore make it up to 87% of sensible recovery on slow-speed. At higher speeds the HX drops as the air does not get to linger long enough to shed BTUs across the Al walls of the HRV core.

The HX's found in HRVs are hampered somewhat by their usual design in that they are not true counterflow HX's. Instead, the air flows pass at right angles to each other, which lowers assembly cost.

Create a large enough HX surface, combine it with good engineering practices, and you'll achieve some pretty phenomenal heat exchange. For example, some colleagues of mine at my former place of employ made a 99%+ efficient gas HX for the US government that featured electron-beam machined holes, among other things. Your tax dollars at work (for the space program, IIRC).