Dave Liquorice wrote:
On 13 Aug 2020 11:38:08 GMT, David wrote:

Can only some systems be run in reverse?

The heat pump will only work one way.

I assume that there is some extra plumbing?

Simplest form is just running chilled coolant from outside through the
fan unit inside.

Simplest is to have the all the heat pump bits outside,ie compressor,
condensor and evaporator and a couple of dampers that determine which
of the condensor or evaporateor the air flow into the room travels
via. Condensor to heat the air, evaporator to cool it. May even have
options to recirculate or use fresh air.


Heatpump with reversing valve in its two positions.
Heating in winter, cooling in summer.

http://www.elephantearshvac.com/?p=288

At -20C external air temp, with an air-source heatpump,
you switch off the heatpump and go to your aux heating
system. You don't allow the heatpump to use the
clever electric element it uses to augment output
at the extremes. I think people leave the heatpump
off, even when the outside air temp returns above
-20C and just use their other system instead. Until summer.
I don't see evidence of people flipping around their
system, all winter long. Even if it was -4C in March,
nobody would bother to go back to the heatpump.
They probably like the feel of the higher
quality heat anyway. It's probably heat quality,
that drives them away from that heatpump in winter.

*******

There are some heatpumps in my neighborhood. There's
a lower limit in winter, when it's no longer possible
to extract heat from the out-of-doors air, to heat
the house interior. That's when a person might
install a ground-source heatpump, where the source
of heat is less variable compared to outdoor air.
I'm not aware of any ground-source installs in
my neighborhood, but it's pretty hard to see because of hedges.

Some ground-source heatpumps are connected to adjacent
waterways - to do that here requires permits, and isn't
a sure thing. They don't like it if you drop a couple 2 inch
lines into a stream, and dump your heat (or cold) into
that water.

And heat has "quality". People using oil-fired furnaces
with air circulation (air handler), if you come in
from the outdoors in winter and put your feet next to
the airflow, the air is pleasantly warm. Lots of delta_T
to warm "frost-bitten" feet.

Whereas a heatpump, the air is barely above ambient,
makes a lousy footwarmer, and will take substantial hours
of operation to heat the house. But you're "winning",
since from an energy perspective, most of the heat
or cooling is being extracted from somewhere. It would
not use nearly as much electricity as pure electric heat.

The same goes for air-conditioning. It works best
if left thermostatically controlled. If you have
time-of-day electricity billing, you can run your
air conditioning in "batch mode", but this is what
happens if you do.

7PM 86F lying naked on the bed, sweating
3AM 70F pulling covers over, because you're cold.
system now switched off - you don't pump
below 70F because it might "freeze the coil".

Noon ?? Still comfortable because of the 40% RH
inside the house. Will be miserable at 5PM.
Cursing and waiting for switch-on at 7PM.

About two degrees F per hour, is a good rate to
properly dehumidify the air. The next day, with the
dried air, you're comfortable until noon, then start
to suffer from noon until 7PM (if that's when the
cheap electricity arrives).

*******

The older systems with R12, were good because the gas
didn't tend to leak out of them. My old system had been
run by the previous owner for some number of years,
and when I got the house, I didn't add any gas over
a period of 20 years operation.

The newer systems that don't use R12, are more
prone to leakage. Pinhole leaks in the A-coil
are common. You might need a recharge after four
years. The very latest gas-substitute was obviously
selected to bankrupt people.

If you had half a dozen split-units running crappy
materials like that inside, you would be in for
a world of hurt, in terms of maintenance calls.

Even new portable units on wheels, you can leave
one of those in storage until the next hot spell,
switch it on and... no gas. Why, it's enough
to make you want to place *combustible* organic gasses
in there :-/ Lower pressure, less leak prone. Whether
they're doing that, you'll want to check the specs
carefully to see if they've moved to the Dark Side.
They've put combustible gases in fridges - there is
a precedent.

On the R12 system, electric bill was $200 per month during
summer. On the spiffy-gas system, with an external
unit with a much higher SEER rating, electricity is
$100 per month (using the 7PM electricity). However,
when the system needs a recharge, you could easily
burn up all the savings in recharge gas, new A-coil,
and so on.

Paul