Attic Heat Pump

Hello All,

I'm currently working on a new project: ATTIC HEAT PUMP. The idea is to use refrigerant to pump heat from the attic of my home to a water cooled heat exchanger (condenser). It's an exciting concept; however, the endless little installation details make the project tedious!
I am using a 2 ton REFRIGERATION UNIT (R22) salvaged from a walk-in cooler. The EVAPORATOR is in the ATTIC, and the CONDENSER is WATER-COOLED (actually GLYCOL - closed loop).
The GLYCOL LOOP then heats potable water through a second exchanger. I wasn't comfortable running potable water through the used (dirty) condenser.
ATTIC temperatures are presently rising above 100F! Do I need to run the EVAPORATOR FANS at these very high temperatures? :eek:

Hi Kettler

You need to get the warm air through the Evap, though I would imagine not quite as quickly as in a cold store.

I would suggect that you try a smaller and therefore quieter fan unit and take some readings -

1. No Fan
2. Normal Fan
3. Low speed fan

Perhaps even look at putting a speed controller on the exising fan.

HTH

Paula

Thanks Paula,

I was hoping not to need the FANs because they're going to require FILTERs to keep dust from building up inside the EVAPORATOR. Another one of those little time-consuming details I mentioned. :eek:

KETTLER

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Originally Posted by KETTLER

I was hoping not to need the FANs because they're going to require FILTERs to keep dust from building up inside the EVAPORATOR.

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So how do you propose getting the heat transferred from the attic into the evaporator?

adding fan and filter is not a diificult task which is normal for home use air conditioner. Air cooled evaporator is low efficient without forced air through it.

The attic is so hot, 120F +, that I believe the gas will flash-off once it hits the EVAPORATOR.

OK, so the gas will flash off at that temperature but, and again but, how are you planning to get the heat transfer from the attic to the gas inside the pipes.

Are you making a contact plate for the pipes to be fixed onto in an attempt at conduction heat transfer?

Simple solution ; run system without fan for a few days ,then put a portable fan for forced air and measure the difference

Also {just in case this is to be overlooked} suggest you get a drip tray under the evaporator, and run a condensate line to waste somewhere, to avoid a wet ceiling or two.

Quote:

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Originally Posted by KETTLER

The attic is so hot, 120F +, that I believe the gas will flash-off once it hits the EVAPORATOR.

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I am working on a different project which also faces unique challenges.

If your goal is to extract as much heat as possible with the evaporator that you have, then you will realize that your arrangement will generate a lot of superheat and that will transfer less heat compared to a situation where you can manage to keep your superheat relatively low.

You can lower the superheat by not overloading your evaporator with heat or by increasing the amount of ***** you are prepared to send to the evaporator.

You can reduce the overloading on the evaporator by slowing down your attic fan or you can increase the ***** flow without overloading your compressor if you can place your condensor where it is exposed to cold water.

What I have read is that a small compressor which has its evaporator and condensor coils replaced with coils designed for a larger compressor will allow that smaller compressor to transfer much more energy.

The fact that your condensor is going to be sunk in water and the fact that your attic is a strong source of heat, together are the equivalent of having larger coils, so your compressor is going to end up moving a lot more heat per hour than it ever did in that walk-in cooler installation.

Do you plan on having a TEV on your evaporator? If so, you may need to recalibrate it or buy one that lets you play with the set screws. If not, you may be able to simulate the missing TEV by getting your fan to run at a variable speed as a function of the temperature measured on your evaporator's outlet.

Getting a fan that is rated for a hostile environment (hot) might be a problem. If it is a shaded pole or PSC type motor, then an inexpensive fan motor speed control (dimmer) might be a good idea. You need to get that fan speed controlled by a thermistor attached to your suction line at the evaporator. That calls for approaching someone who knows some electronics.

There are thermistors that increase in resistance with temperature. Using the thermistor, you need to turn on a small lightbulb (like found in the ceiling light of a car). This light falls on a CDS photocell which has a resistance proportional to the amount of light. The CDS photocell is used to replace the reostat in the fan speed control (dimmer).

As the superheat goes up, the resistance of the thermistor goes up and the power to the lightbulb goes down and the light output falls and the resistance in the photocell goes up and the speed of the fan goes down so less heat goes into the evaporator and the superheat begins to fall and eventually you have an equilibrium.

Getting the right amount of light from the bulb for a given temperature calls for a small DC amplifier between the thermistor and the lightbulb with a gain control on the amp that allows you to set the calibration for the right amount of superheat.

Kettlers last activity on this forum wan on 24 June 2006, I think he took some of teh iedeas and is still busy with it.
Thanks for the help Kettler and for the helpfull info afterwards. :o

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Originally Posted by Peter_1

Kettlers last activity on this forum wan on 24 June 2006, I think he took some of teh iedeas and is still busy with it.
Thanks for the help Kettler and for the helpfull info afterwards. :o

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I hope he is subscribed to this thread. That way he will receive an e-mail notification and come back and read my information. I will send him a personal message if possible.

Because it is now nearly winter, he may not be as interested in his project right now.

Thank you for your reply! Very informative.

I'm thinking that the refrigerant will pick up plenty of heat energy as it flashes-off in the attic evaporator and therefore will not need fans.

This heat energy will be transferrred to a water cooled condenser and used to preheat potable water.


Thoughts?

Quote:

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Originally Posted by KETTLER

I'm thinking that the refrigerant will pick up plenty of heat energy as it flashes-off in the attic evaporator and therefore will not need fans.

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I keep forgetting that you are pumping heat from a hot spot to a cold spot, which is in the opposite direction to the way heat is usually pumped either in winter using a heat pump or in summer using an air conditioner.

If you supply too little heat to your evaporator, you risk flooding your compressor. If you supply too much heat, then your compressor will have to struggle with a large quantity of superheated gas to be compressed.

If your condensor is kept cool by the water, then the compressor will not have to work that hard. But as your water begins to heat up, then the compressor is going to work against a greater load.

Perhaps a better solution than the one I originally proposed would be to monitor the amps drawn by the compressor and use this measurement to vary the speed of the evaporator fan so that you keep the compressor running under a constant load.

Once the water temperature in the reservoir reaches the desired temperature, then you could shutdown the compressor and evaporator fan until the next cycle.

Quote:

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Originally Posted by KETTLER

The attic is so hot, 120F +, that I believe the gas will flash-off once it hits the EVAPORATOR.

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Why not duct the air and air condition you living space.

taz

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Originally Posted by montreal

Perhaps a better solution than the one I originally proposed would be to monitor the amps drawn by the compressor and use this measurement to vary the speed of the evaporator fan so that you keep the compressor running under a constant load.

Once the water temperature in the reservoir reaches the desired temperature, then you could shutdown the compressor and evaporator fan until the next cycle.

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:rolleyes: Wow! What a terrific idea. :) Thanks!

Quote:

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Originally Posted by taz24

Why not duct the air and air condition you living space.

taz

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There's a lot more energy to heat water in the attic.

Than ks,
KETTLER

Quote:

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Originally Posted by montreal

If you supply too little heat to your evaporator, you risk flooding your compressor. If you supply too much heat, then your compressor will have to struggle with a large quantity of superheated gas to be compressed.

Perhaps a better solution than the one I originally proposed would be to monitor the amps drawn by the compressor and use this measurement to vary the speed of the evaporator fan so that you keep the compressor running under a constant load.

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:rolleyes: Perhaps a pressure switch to cycle the fans could be used to govern the load on the compressor?


KETTLER

Get a voltage controlled oscillator Use a thermistor to sent it a varying voltage based on monitoring point of your choice. Sent the signal to a power fet and connect to the ground side of a dc fan. Now you have a temp speed modulated fan. You can make a VCO out of a 555.

As I understand it, you are going to use a refrigeration system to transfer heat from the attic air to a glycol loop and then pump the heated glycol through a water heater.

Why not eliminate the refrigeration system and simply pump the glycol through a coil in the attic? It would be far more efficient.

Quote:

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Originally Posted by Gary

Why not eliminate the refrigeration system and simply pump the glycol through a coil in the attic? It would be far more efficient.

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:rolleyes: Gary,

I believe the R22 refrigerant will absorb more heat energy.

Quote:

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Originally Posted by KETTLER

:rolleyes: Gary,

I believe the R22 refrigerant will absorb more heat energy.

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Ok what is the specific heat capacity per kG off R22, much less that water you will find;)

Kind Regards Andy:)

Kettler
a system has been available in the UK since the early eighties but has never taken off. The only reason for using refrigerant is to get the required water temperatures required to elliminate the chances of legionella growing, however this can be done using an electric heating element set to boost the temperature above 60degC once a week. This method is used on comercially available heatpump systems.

Ian

Quote:

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Originally Posted by KETTLER

:rolleyes: Gary,

I believe the R22 refrigerant will absorb more heat energy.

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You believe transferring heat from air to refrigerant to water is more efficient than transferring heat from air to water?

Air>Refrigerant>Water

VS

Air>Water

Gary
I agree with you that air to water would be more efficient however you will struggle to reach the required temperatures for houshold water taking that most of the world take water at 55 def C then mix it with cold water to wash their hands.

Ian

good point that considering @ 55 deg C water would burn most people:)

Here's a better way: Assuming you are in an area that isn't going to freeze the attic, run the supply water through a fan/coil unit in the attic and then down to the water heater. This will preheat the water before it enters the water heater.

Thanks Gary,

I live in the Northeast and freezing is a problem. Also, the water would have to be pumped 20+ feet vertically which would be a significant electrical cost compared to the energy payback.

I think refrigerant is the way to go because it effectively gathers enthalpy even at low temperatures.


KETTLER

Quote:

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Originally Posted by Andy

Ok what is the specific heat capacity per kG off R22, much less that water you will find;)

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Andy,

Yes, I agree, water does have a greater heat capacity; however, R22 will absorb heat more readily and at lower temperatures. I can collect heat energy at temperatures just above 40F using R22 whereas the well water comes from the ground at 56F.


:rolleyes: KETTLER

Quote:

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Originally Posted by Gary

As I understand it, you are going to use a refrigeration system to transfer heat from the attic air to a glycol loop and then pump the heated glycol through a water heater.

Why not eliminate the refrigeration system and simply pump the glycol through a coil in the attic? It would be far more efficient.

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Some one has had a swimming pool heater on the market like that. A fan coil grabs heat from the attic to heat a pool.

http://www.solarattic.com/pools.htm

http://www.ecosmartinc.com/productdocs/1-saph.pdf

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Originally Posted by Pooh

Gary
I agree with you that air to water would be more efficient however you will struggle to reach the required temperatures for houshold water taking that most of the world take water at 55 def C then mix it with cold water to wash their hands.

Ian

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I don't think we want to run the head pressure that high anyway, so we are talking preheater here.

The question then is whether it is more efficient to use mechanical refrigeration to heat a preheater heat exchanger...

or...

to pump glycol through a fan/coil in the attic to heat a preheater heat exchanger.

We can move more heat with the refrigeration system, but is it worth it?

Quote:

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Originally Posted by KETTLER

Andy,

Yes, I agree, water does have a greater heat capacity; however, R22 will absorb heat more readily and at lower temperatures. I can collect heat energy at temperatures just above 40F using R22 whereas the well water comes from the ground at 56F.


:rolleyes: KETTLER

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We don't want to chill the attic below room temperature or we are fighting the house heating system. And we need to think about condensation. This would be much easier to handle with a fan/coil unit than bare pipes with no fan.

Gary,

Yes, I agree with you about the evaporator fans - they should be used for greater heat transfer. The condensation will be piped outside.

I'm not concerned about the heating loss because most of the preheating will take place in the summer when I don't want to run my boiler (especialy to heat water).


KETTLER

Quote:

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Originally Posted by Andy

Ok what is the specific heat capacity per kG off R22, much less that water you will find

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Don't forget the latent heat. R22 boils and absorbs latent heat whereas the glycol is purely sensible heat.

if you (or anyone else) needs a variable speed fan EBM makes a nice reverse impeller type, 0-10vdc control. available in various cfms & pressure drops with ball bearings. very efficient & readily available on ebay.
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