Designed and built hot water heatpump

[ChillTechnican]

Hi all, i have managed to put together a heatpump for domestic hot water, and have it plumbed up and running ok but have some questions about "tuning it".
It is on R410a , using a rotary compressor .It is a split unit with evaporator in the roof space and compressor and tube in tube heat exchanger under the floor directly beneath hot water cylinder.
When running i have

head pressure 485 psi @ 55 c
liquid out of heat exchanger 45 c
water out 64 c
discharge temp 78 c

evap 120 psi @ 4 c
air on 16 c
air off 11 c
suction temp 11.8 c

the ambient temperatures have dropped a bit in the last week , down to 8 or 9 deg c and the unit will run for 5 mins before any hot water gets to cylinder. Do you think that is a little bit under charged with the super heat??
Also i was wondering about in the summer time when temps get into 40s in the roof space would it be a good idea to have a fan speed controller to slow the fan and drop evap pressure a bit?.(i have one lying around)
thanks for any advice
take care CT

[Gary]

Is the metering device TXV or cap tube?

[chemi-cool]

It takes a few minutes for rotary compressors to build up pressure.

[ChillTechnican]

It has a txv and there is no sight glass in system .

[Gary]

head pressure 485 psi @ 55 c
liquid out of heat exchanger 45 c
water out 64 c
discharge temp 78 c

evap 120 psi @ 4 c
air on 16 c
air off 11 c
suction temp 11.8 c

Without water in temp we cannot calculate cond TD or dT.

Approach is 64 - 55 = 9K/16F. This should be no more than 11K/20F, so the heat transfer is good.

The subcooling is 55 - 45 = 10K/18F. This should be no more than 8.5K/15F, so the system is slightly overcharged.

Evap TD is 11 - 4 = 7K/12.6F. This could change considerably as the attic warms in the summer.

Evap dT is 16 - 11 = 5K/9F, indicating good airflow.

The superheat is 11.8 - 4 = 7.8K/14F. Expect this to rise a little as the attic warms up.

All in all it looks very good.

You are right to be concerned about the effect that a warm attic temp will have on the system. The heavy load could in fact overload the compressor.

Possible solutions for limiting the SST would be TXV/w MOP charge or CPR valve. Or as you mentioned, slowing the fan could drop the SST to a safe level. How would you propose controlling this?

[Tesla]

Hi Chilltechnician
Good work, not sure about the other gas I would have used r22 with a scroll. May I suggest adding ( considering DHW ) an additional heat exchanger in your waste pipe - something like a GFX Gravity Feed Heatexchanger (Perrys chemical encyclopedia)would increase capacity or efficiency by around 45%. You could use 3 times /38 tube spiralled with diameter of 5" about 2' long in waste line. Then it seams you would need an electronic controller to monitor and log your heatpump accuratly. If you consider water from say a shower goes down the drain at about 36 degC, it's like money going down the drain. For free you can recoupe upto 50% of the differance between water in (11degC) and waste (36degC) ie 12.5K
Cheer Bro

[ChillTechnican]

Thanks Gary and Tesla for your replys ,
Gary the fan speed controller is adjusted by the user, i was just going to leave fan at full speed for winter and then reduce speed in summer time. Im not quite sure how much to slow it or what settings i should be trying to achieve, i guess it is to try and keep the evap TD as close to 5 to 7k or so??
Tesla , yes i have often thought about all that hot water going straight down the plug hole and was toying withthe idea of a HX .