windowshaker heat pump controls hacking
 

I have a happy little frigidaire FFRH0822R1 - that's an 8k cap tube window mounted heat pump.

it works great down to 40F, where it switches over to the 1kW strip heat, providing less heat, and using more energy than it needs to. I can't fathom a reason they didn't implement defrost other than to save the cost of one additional motor, and an extra ten cents on the control chip.

I plan to re-do the controls (and the outdoor side fan) so that it can operate down to whatever point it is no longer useful. to do this I plan on ditching the factory controls entirely, and switching it over to 24VAC controls.
If anyone sees a flaw in my plan, please point it out to me.



I will be using a Honeywell prestige thermostat, because I like them. YTHX9421R5085WW

I have this defrost board 47D01U-843 due to a friend picking it up for his home system and then upgrading to an inverter split system, so I got a great deal.
It has a number of defrost modes, including a couple of demand modes, so I'll give it a go.

Please note that I do NOT want to open the refrigeration circuit. it's a dual cap tube currently, and has no issues working above 40F, so I see no benefit to messing with the charge/refrigerant choice. That's another idea, for another time in another thread.

Thermostat inputs
User demand
return temp
supply temp
outdoor temp

Thermostat outputs
Y (comp call)
O/B (reversing valve)
G (fan)
W (aux heat)

defrost board inputs
Y
O/B
air temp
coil temp

defrost board outputs
Y
O/B
ODU Fan relay
W for defrost heat

Logic provided by thermostat / demand defrost board
aux heat lockout
compressor lockout
defrost of outdoor coil
short cycle protection
outdoor coil temp sensing
outdoor air temp sensing

Things I won't have without further logic/sensors
protection against strip heat without fan (airflow proving)
low pressure cutout (no plans to implement)
high pressure cutout (no plans to implement)
outdoor coil fan (plan to cut off shaft and add in a 12V radiator fan(due to cost, ease of mounting, availability, and small diameter selection) that will be controlled by the defrost board fan relay.

I think they only remaining tasks will be swapping out the 120VAC reversing valve coil for a 24V coil, and adding a couple relays to trigger the 120V components.

Thoughts?

Sounds like your control system is going to be worth more than the unit it is controlling. But who am I to tell you what you can spend in your project? If you wire everything correctly, it should work like a charm!

That's quite possible - I'm doing this because I want to repurpose something I own already, it's cheaper than buying a minisplit, or a traditional split system - and it won't require me to pull any permits or get licenses, and I like to tinker, so here I am.
Even it isn't the least expensive way to go about it, I'm thinking my ROI vs resistance heat should be pretty decent. I did that a bit this winter and it was not kind to my electric bill. I might even learn a bit in the process.

Sounds like you have it all tacked down. I've considered doing something similar for a future project. Do post updates when you get it all put together!

so far the high power relays and control board/thermostat are here. I have confirmed fan behavior (on while compressor/strips active) is as needed, so no additional logic for that needs to be built.

A little progress on disassembly:
http://ecorenovator.org/forum/attach...9&d=1529269515

a picture of the heat strips:
http://ecorenovator.org/forum/attach...0&d=1529269515


and because I was wondering if it actually had limit switches:
http://ecorenovator.org/forum/attach...1&d=1529269515

I plan on removing the exterior fan blade and replacing it with a Spal type car radiator fan (not sure if I can find a 120V fan that's suitable) for defrost, and retaining the stock motor and blades for the interior section. the only minor concern is the motor is an air-over motor, so a failure of the external fan would be bad. not an issue for the winter, as i can't imagine the motor would need extra cooling, but it would be bad if the condenser fan failed to operate in the summer. I can fit about a 12" fan in there without significant modifications to the shroud. I'll set it for intake (like one would a normal full size split) so it's easier to clean the coils without full disassembly.

I've run into a minor hiccup - all of the reasonably priced 12V fans I can find are brushed motors, which means shorter lifespan. anyone have a good idea for a long life fan that might fit at a decent price?

a used SPAL fan would run me $50. a new one $200+

What about replace the indoor fan with a few 120mm computer fans? It's the indoor fan you want variable speed for optimum dehumidification.

it's already got a 3 speed fan. since it's not ECM, I assume the low speed tap without the outdoor fan load will run at a higher RPM than before, so i likely won't want it faster than that. the other issue with replacing the inside fan is fabricating a mount will be more difficult.

to entertain the idea of an indoor fan swap, i'd need a high ambient rated fan motor, since the fan is after the condenser and heat strips when in heat mode. if both are active I anticipate 50C+ air temps.

if I abandon the idea of a used fan for the outdoor half and pay full price, this may suffice:
https://www.supplyhouse.com/Packard-...ly-w-Fixed-RPM
however i will also have to abandon the idea of reversing the airflow through the condenser for ease of cleaning. it will have to push out through the condenser as it's only rated for 40C.

A lot of computer fans are rated for surprisingly high temperatures, because it can get that hot on the exhaust side of a heatsink. But maybe you could add the fans on the inlet side?

I suggest adding a SSR to control the heat strips so it's literally only used as much as it needs to be. In heating mode, you'll want the fans going at high speed because that gives better efficiency. In defrost, slow the indoor fans down so they run just enough to stop the indoor coil from freezing and apply just enough power to the heat strips to counter the little little cold air that comes out. (Or not use them at all if the room temperature is within a few degrees of the setting.)

I've been thinking of how to do a fan wall with computer fans - I need 250+cfm (factory spec) on the inside, and 500-800 cfm for the outdoor half. I think those numbers are going to be difficult to reach with computer fans.

The heat strips would be controlled by a relay, and controlled by the thermostat. they will be locked out most of the time.

the other option that would allow me to retain the factory fan in its entirety would be shutters between the fan blades and outdoor coil. I'm not sure how i would make an off the shelf unit fit - they're usually made in standard ductwork sizes. I see no downside to letting the fan freewheel for a few minutes at a time.

It's pretty easy to find 120mm fans in the 100 CFM range. (Delta is one common brand.) A few are even into the 200 CFM range. How much area do you have to work with? The little 5000 BTU unit I hacked long ago had enough area to fit 6 120mm fans, but I only put 2 because that's enough.

I like the nidec servo D1225C12B5AP and the rest of the lineup. great bearings, good static pressure, very tolerable noise. I wasn't thinking down that route but it's worth a thought or two. only problem with computer fans is they're not aimed at outdoor use (exposed windings/drivers), but they would be reasonably well protected.

the radiator measures approximately 18" x 14".

I've been thinking on a method of control for the second fan, and I think perhaps a damper is a better idea. I found a place that makes custom sized aluminum zone dampers, which means i wouldn't have to mess with the fan at all. that would things a lot simpler, and it may help defrost the coil faster on windy days (not sure what impact the fan running behind the coil might still have).