daestrom wrote:

The GFX does well with its small surface...

OK, GFX doesn't help with heat recovery for a bath, but great for hot
showers, dishwashing, clothes washing.

...60% is not "great," IMO.

For a total surface area of just (4 in)*pi *60 in /144 = 5.24 ft^2, 60% is
pretty 'great'.

It might be 3 vs 4", but it's still poor overall performance.

How much surface area does your setup require?

There's no requirement... 300' of 1" pipe is a convenient design choice.

Here's what physics tells us on page 3.4 of the 1993 ASHRAE HOF:

1. E = (Thi-Tho)/(Thi-Tci) when Ch = Cmin and
= (Tco-Tci)/(Thi-Tci) when Cc = Cmin, where

Ch = hot fluid capacity rate, Btu/h-F
Cc = cold fluid capacity rate, Btu/h-F
Cmin = smaller of the two rates
Th = terminal temp of hot fluid (F). Subscript i indicates
entering condition; o indicates leaving condition.
Tc = terminal temp of cold fluid (F)...

2. Number of Exchanger Heat Transfer Units NTU = AUavg/Cmin.

3. Capacity rate ratio Z = Cmin/Cmax.

Generally, the heat transfer effectiveness can be expressed for a given
exchanger as a function of NTU and Z: E = f(NTU,Z,flow arrangement).
The effectiveness is independent of the temps in the exchanger.

For any exchanger with Z = 0 (where one fluid undergoes a phase change,
eg in a condenser or evaporator), E = 1-e^(-NTU).

For parallel flow exchangers, E = [1-e^(-NTU(1+Z))]/(1+Z).

For counterflow exchangers, E = [1-e^(-NTU(1-Z))]/[(1-Z(e^(-NTU(1-Z))],
= NTU/(NTU+1), when Z = 1.

For instance, if we use 50 gallons per day of hot water in short bursts
and Cmin = Cmax = 50x8.33/24h = 17.4 Btu/h-F and A = 78.5 ft^2 (a $60
300' piece of 1" polyethylene pipe with a 50 year guarantee) and U = 10
Btu/h-F-ft^2 (with slow-moving greywater and crud outside and slow-moving
fresh water inside), NTU = 78.5x10/17.4 = 45.2, and E = 0.98.

We've been through this before Nick. You can't calculate Cmin or Cmax using
a 24 hour 'average' flow rate.

Sure I can :-) You might enjoy calculating E if 50 gpd of hot water flows
in 1 second 1.25 gpm bursts, then 2 second bursts, and so on.

My shower is 1.25 gpm, so a 10 minute shower fills the 1" pipe.

So *while* the water is flowing, you might see NTU=78.5*10/960 = 0.818.
And *that* would give you about E = 0.45.

How long between showers?

By using an average flow rate that includes long periods when there is no
flow at all, you make it seem as though the heat exchanger is much longer
than just 300'. If you want to get your kind of performance with the
existing surface area and U, you would need to reduce the flow to 0.034 gpm
and keep it there all day/night.

I disagree, altho that might happen with continuous hot tub water exchange.

Nick