R290 undercharge in 1 ton cap tube split sytem heat pump?

Hi all,
I have a 1 ton (2.4kw) cap tube split system which was originally on R410a until the old gas escaped after a copper line failed....since the system was open to atmosphere i decided to plumb in a reversible filter dryer in the liqiud line to catch any moisture etc. After doing a fair bit of reading and cosideration I have decided to regas the system with R290. According to the literature the charge is 40% (304g) of the original 760 grams...no problem there but was unable to get the specs on the liquid capacity of the filter...comparing it with some similar models of filter I estimated that the filter capacity with R290 was around 50 grams. So after making sure the system was leak proof and evacuated, I put my charge in and tried the system. Tried it on cool and no cold air...this ststem only has a service port on the low side and was reading around 20psi...the liquid line felt very cool....reading some another post on this forum re R290 i should be reading 63-70 psi on the low side. My assumption is that I am undercharged and i will need to add small amounts of refrigerant until i get a good vent temp at the evaporator.
Thanks!

You're doing it wrong!

When changing refrigerant types the replacement needs to match the original on a PT chart (pressure temperature), go compare your two.
It needs to have a similar mass flow requirement and heat transfer capability and it needs to have a similar viscosity if its being pushed through a cap tube.

Why change from r410a? If it's designed for r410a then I'd be inclined to put that back in it. I don't understand what your trying to achieve?

If i have choice, i prefer HC as refrigerant compare to environment harmful HCF refrigerant and R410A have high GWP-value.

---

R290 give mostly case better flow in capillary compare to HCF in same pressure range (read R407C/R22) - but now going from R410A to R290/R407C pressure range you only working 2/3 of R410A pressure and capillary is too long and give low capacity.

I think you need to cut down capillary some bit if you still try with R290

(Is much ease to convert R407C-machine to running with R290/R1270 and keep cooling capacity better than try convert R410A machine to running on R290/R1270)

But before this I should test with propylene/propene (R1270) ie. have higher pressure than R290 and little closer to R410A pressure range and compare capacity


Reason to why need high pressure refrigerant close to R410A is a R410A compressor have small cc and low pumping volume compare to motor strength ie. using high pressure, high density refrigerant as R410A and with 2.4 kW cooling capacity for R410A, capacity going down with propylene to 1.73 kW and with R290 going down to 1.45 kW cooling depend of lack of compressors pumping capacity if rest of cooling circuit is tuned and working well. (simulate with coolpack)

This post contain really important points, which is very helpful. Thanks for sharing it.

if going through all this truble ditch the cap tube and put in thermal static valve (Balanced port is recomended) and gain some efficiency in it as well.

rest of the big points been covered.

Propane is closer match to R-22 as stated by the other fellow to 407, so it is a poor choice of a drop in for the cc per rev of the compressor.