small heat pump to improve efficiency of gas furnace?

[star882]

Could it work to have a heat pump with an evaporator in the exhaust line (after the exhaust blower) and a condenser in either the return air duct or after the furnace (whichever works better) to recover some heat?
Of course, there will be a PWM control and some sensors to prevent the evaporator from freezing. The heat pump could be turned on using a relay connected in parallel with the gas valve.

[Peter_1]

I've done it a long time ago in the exhaust tube of ...yea.. how do you call that thing above the kitchen fire to pull away the fumes and odors... a fume hood?

Is the exhaust not corrosive? If your energy source is gas, then it's worth a try.

Will the natural convection of teh fumes not be disturbed if you cool them to much or are the fumes blown out by a small exhaust fan like we mostly do here?

Anyway, it will extract the blown off heat at a high COP.

[US Iceman]

What you are describing is a heat recovery device for exhaust air. Yes it will work, if everything is engineered correctly.

In some installations this is very important, and there are several methods that can be used.

Heat Pumps
Heat Pipes
Thermosiphon systems

As Peter said, make sure the exhaust air stream is clean and non-corrosive. All materials should be compatible with the air streams.

In some operations, the inlet air supply can be low enough to cause icing on the coils, so you should be aware of this also. Some of this can be minimized by how much coil surface is used for the heat exchangers.

[Rfcont]

Hi star882!

Ohh, I can't approve this idea.

At such decision you can expect increase SSH (suction s/heat) which will lead to burn of the compressor
However you can use special ventilating units - heat recovery units -which have heat exchanger or heat recovery wheel.
You can recover 50 - 60 % heat.

[chillin out]

However you can use special ventilating units - heat recovery units -which have heat exchanger or heat recovery wheel.

I remember the 1st time I seen one of those (heat recovery wheels). I thought it was an evap at first!!! then I seen it had a motor and belt attached!!! now im thinking, thats a clever sod that made that!!!

Then discover its only a big drum of aluminuim fins.

Looked so neat and clean, just had to touch it. Walked away with a cut finger and thinking how $hit it was now.

Chillin

[Abe]

Im intrested in this concept.
Would any member have a photo?

[US Iceman]

At such decision you can expect increase SSH (suction s/heat) which will lead to burn of the compressor

I agree on the basic fact mentioned above. The operating conditions and refrigerant will have to be carefully reviewed to ensure such things DO NOT occur.

This may not be an application for any old heat pump.

This can be a very exact selection process for equipment.

[KETTLER]

A condenser coil can be used to raise the temperature of the COMBUSTION AIR entering the burners. This will result in less energy expended during combustion.

I have a oil-fired boiler and I'm experimenting with collecting heat energy from the air surrounding the stack through an evaporator.

[Pooh]

Guys
it would be a lot easier and efficient to do away with the furnace altogether because if you take the overall effiency of running both the inefficency of the best furnace will make it a waste of energy and time. The only real reason for using a combination system is if there is insufficient heat to collect or extreme temperature variations at the evaporator. The other issue I can see is the high temperatures experienced in the exhausts of most furnaces.

Ian