DIY ventilation heat exchanger

[ham789]

Found this dew point calculator.
It's the first one I've found that actually addresses
the issue of mold growth.
These guys are in the document preservation business.

Dew Point Calculator



While I've got your attention...I've been catching up on the
active HRV issue. I'm puzzled by the talk about cobbling
together a fridge or dehumidifier.

The action of an HRV loses heat that must be replaced.
If your primary heat source is as efficient as you know how
to make it, why not use that to replace the heat?
Conversely, if it's not, put your effort into improving the
efficiency of the primary heat source.

What am I missing?

[Fornax]

Originally Posted by ham789

The action of an HRV loses heat that must be replaced.
If your primary heat source is as efficient as you know how
to make it, why not use that to replace the heat?
Conversely, if it's not, put your effort into improving the
efficiency of the primary heat source.

What am I missing?

You are correct, an HRV loses heat indeed.
What you are missing is that places where people recide need ventilation.
Now in short you have 3 options (I don't want to sound blunt but I want to keep it short, I know you are not stupid.)
1) a leaky house where wind blows through the walls while it is freezing outside.
2) a well sealed and insulated house with 2 windows open, while it is freezing outside.
3) use a somewhat complicated apparatus that inserts and extracts roughly equal amounts of air and recovers a substantial amount of energy from the warm air extracted, again while it freezes outside. (The inbound air is now almost roomtemperature, this saves energy and raises comfort, if done well.)

I agree a focus on the efficiency of the primary heatsource is important, it's good for the environment and the wallet but it is another subject. For maximum profit one should consider both: don't let the warmth of your primary heatsource slip through the walls or an open window, keep it where you want it while also refreshing the air you breath.

[AC_Hacker]

Originally Posted by Fornax

You are correct, etc...

I think that an active heat exchanger can have a place, but the niche is a pretty small one.

If your house is so tight that it requires ventilation, AND your insulation is sufficient that your heating needs are extremely low, then the heat that would be lost from a very efficient HRV could be salvaged by an active HRV. In fact, a mathematical formula could be developed to determine the range where an active HRV could be useful.

I have also seen them used in industrial settings where ventilation requirements, due to airborne pollutants & particles, are many times higher than in a house. Active HRVs are useful there.

-AC

[Fornax]

Originally Posted by Piwoslaw

Not sure if this is technically possible in your house, but maybe the HX could be hooked up to the cooking hood so that it sucks up the stale air directly from the kitchen?

Well, actually you hit the hammer on the nail (pun intended).
While cooking you produce some seriously bad air with excessive moist and fat, like Kostas mentioned, but that can be filtered, if done right.

Then again while cooking you produce some (a lot) extra heat that could be reclaimed, instead of being extracted to the outside by the cookinghood.

So Piwoslaw, it's not only possible to do but I designed my system to be the cookinghood. We have a livingroom with open connection to the kitchen, smells from the kitchen always reached the livingroom (and the rest of the house). Now with the HRV in action that is much and much less.

I didn't post for a few days because of work etc., but in the off-hours I did some work on my apparatus/surfing the web/playing with Google's Sketch Up:



You see the ground floor is is 44m^2 in total (490-ish sqf) of which around 33sqm or 75% is livingroom, it's volume is 86 cubic meters (3000qf), the kitchen is about 1/6th of that.

More details later. Point is: I hooked up the ducts temporary without thermal insulation, and expected realistic results. Not true.
I was disappointed by primary figures but things were working fine, in the situation. Hooking things up proved the concept of ventilation, as expected but the heat regained was lower than expected.

Playing with it the results got better, without changing the HX-core or the cabinet, just the ducting.

More details will follow later. Today while preparing dinner the temperature in the livingroom was 20C, the FAI was 23C (Reclaimed heat from the kitchen, pans on furnace with boiling vegetables and potatoes) so quite soon it felt a bit warm.

[ham789]

Originally Posted by AC_Hacker

I think that an active heat exchanger can have a place, but the niche is a pretty small one.

If your house is so tight that it requires ventilation, AND your insulation is sufficient that your heating needs are extremely low, then the heat that would be lost from a very efficient HRV could be salvaged by an active HRV. In fact, a mathematical formula could be developed to determine the range where an active HRV could be useful.

I have also seen them used in industrial settings where ventilation requirements, due to airborne pollutants & particles, are many times higher than in a house. Active HRVs are useful there.

-AC

Answers don't seem to address my question, so I'll try again.
I'm addressing the advisability of adding an active element
to your HRV.
Start with the tightest, most insulated house you can manage.
If I build the most efficient passive HRV I can, I still lose some heat.
Say, that I lose 1BTU/HR.
That 1BTU/HR has to be made up somewhere.
Option one is to use the primary heat source to replace that
energy loss. If my primary heat source uses the most efficient
technology available, how can adding what amounts to a heat
pump in the HRV be more efficient...even ignoring the
cost of the additional plant?

Stated another way, wouldn't improving the efficiency of the
primary heat source give me more bang for the buck?
Any increase in efficiency applies to the entire heating load,
not just the amount lost by the passive HRV.

Stated yet another way, I can't understand why anyone
would consider cobbling a dehumidifier or the guts of
a refrigerator onto a passive HRV. Seems like the cost/benefit
ratio is higher than your other alternatives.

[Fornax]

Originally Posted by ham789

Answers don't seem to address my question, so I'll try again.
I'm addressing the advisability of adding an active element
to your HRV.
---

If my primary heat source uses the most efficient technology available,
how can adding what amounts to a heat pump in the HRV be more
efficient...
---

Stated yet another way, I can't understand why anyone
would consider cobbling a dehumidifier or the guts of
a refrigerator onto a passive HRV. Seems like the cost/benefit
ratio is higher than your other alternatives.

Ah, I see I misinterpretated your question.
The main heatsource is used to provide heat when needed, and thus it will not run continiously. Usually it will produce large amounts of heat in bursts.

An HRV will run 24/7. If it freezes hard then on the cold side of the heatexchanger ice will build up and that has to be removed or prevented. Removing the ice requires running defrostingcycles during which no heat is recovered.
Preventing the forming of ice would require heating up the incomming cold air to a point where no frosting will occur. This requires less heat than the main heating source provides but demands a continious heatsource.
Rather than putting a resistive electrical heater there people get creative with heatpumps etcetera since they are more efficient than plain electrical heating.

I hope this is a more suited answer to your question.

[kostas]

I agree with Fornax.
As a preheating stage (and even for a post-heating one, in order to gain the lost temperature of the HRV) I was planning to use a reclaimed car heating radiator like this one:



It could be plugged ti the main heating apparatus, or better more, to a closed circuit that will use the heat of the gas boiler's exhaust pipe



A small 12V pump and a thermostat will do the job
The only think I do not know yet is the flow/pressure drop it causes.

[AC_Hacker]

Originally Posted by ham789

Answers don't seem to address my question, etc...

I spent a lot of time looking into Passive House design and the rationale behind some aspects of the design.

Passive House decided NOT to opt for a ZERO energy house because the additional amount of sealing/insulation to get from Passive definition to Zero Energy was too high to offset by saved energy in a reasonable time.

Usual ASHP COPs are in the neighborhood of 2.5 to 3.7 average.

However, when the source temperature is raised, the efficiency rises considerably. I measured efficiencies of up to COP=6 with elevated temps, in my experimental setup.

An active HRV would exploit this kind of elevated temperature.

There aren't many heating sources with a COP=6.

By the way, your 1 BTU/hr loss example could be offset by keeping a pet reptile, maybe a Komodo Dragon (a cat would certainly be be too much heat).

-AC

[kostas]

Originally Posted by Fornax

Well, actually you hit the hammer on the nail (pun intended).
While cooking you produce some seriously bad air with excessive moist and fat, like Kostas mentioned, but that can be filtered, if done right.

Then again while cooking you produce some (a lot) extra heat that could be reclaimed, instead of being extracted to the outside by the cookinghood.

You could use a cooking hood with a recycling function, usually economic models have that. Adding some good active carbon filters will certainly help, so you will have some clean and warm air to feed your HRV.
I also noticed that you haven't installed an outlet duct in the bathroom, I guess it's for the constructing difficulties but extracting the stale air from there will definitely help further more.

[ham789]

Originally Posted by AC_Hacker

I spent a lot of time looking into Passive House design and the rationale behind some aspects of the design.

Passive House decided NOT to opt for a ZERO energy house because the additional amount of sealing/insulation to get from Passive definition to Zero Energy was too high to offset by saved energy in a reasonable time.

Usual ASHP COPs are in the neighborhood of 2.5 to 3.7 average.

However, when the source temperature is raised, the efficiency rises considerably. I measured efficiencies of up to COP=6 with elevated temps, in my experimental setup.

An active HRV would exploit this kind of elevated temperature.

There aren't many heating sources with a COP=6.

By the way, your 1 BTU/hr loss example could be offset by keeping a pet reptile, maybe a Komodo Dragon (a cat would certainly be be too much heat).

-AC

I see, it's about freezing up.

My toy HRV has been much colder than I remember.
I had it on the ToDo list, but this thread made me go measure it.
Long story short, it was frozen solid. Never had that problem
last year, so never thought about it as a possibility.
Washed it out with hot water. Now, the inside exit temp is
up by 15F.

I can't get my head around your COP=6 argument. The heat
to get the elevated temps has to come from somewhere.
Sounds like using one heat pump to raise the temperature
for the second heat pump.
I'm not saying you're wrong. I'm just stuck back here
at conservation of energy and not able to convince myself
that the gain is worth the cost.


OK, so now, I gotta think about efficient defrosting.
Adding a heat pump is outa the question. What else?

Simplest option is to put the incoming fan on a timer.
That reduces the input flow and allows the core to warm up.
I'm still worried about the resultant negative pressure, but
that may be ok in winter???
OR
Add a fan or vane that bypasses the core for incoming air.
OR
Figure out what to measure.
If the output end of the core is what freezes and reduced
air flow is the symptom, I can measure air flow or plenum
pressure or optical ice detector as the sensor.
Maybe can sense the fan speed as it increases with higher
back pressure. I'm using computer fans with the sense wire.
THEN
I can use that to control the fans.
OR
I can use that to open a leakage path between the two
ports on the outside end. The defrost heat isn't lost,
except for that to melt the ice, but the excess heat gets
recycled. You can let the core get very warm without
wasting much during the defrost cycle.
There is less fresh air coming in.
AND
all of the above uses the primary heat source.

I have a vision of a vane in the outgoing air path.
As the flow goes down, the vane allows a leakage
path between the two ports on the outside end
to let the core
heat up for defrost. You don't want it running in
steady state at half flow, so need to have some
mechanical hysteresis to let it finish defrosting.
Or maybe hold it open with an electromagnet until
the core temp gets above freezing.

Another thought.
If you're using cross-flow heat exchangers, you
can't expect much better than 50%. So, you put more
than one in series.

Imagine several cores in series...number TBD.
In steady state, there should be at least one
core who's temperature is above freezing,
but below the dew point. You drain out most of the
water while it's still wet. Should be much less ice
to defrost.

Whatchathink?