i am looking at flow meters this mo stargatemonkey, since one in the hot circuit and one in the rad return pipe would tell me two things.. the typical flow level in the hot circuit before and after the thermostat opens.. and the typical flow rate through the return pipe from the rad, when the thermostat starts tp open...
it would be interesting to know what the difference between the two circuits actually is... is there any bias? does coolant choose the easiest route or is it balanced?
my prime interest is devising a simple method of knowing the rad flow is outside a window limit.. its likely that if the rad or thermostat are not open properly.. the hot circuit will take preference.... ie there is a difference in flow between the two that can be measured using flow meters.
that knowledge could tell you what the thermostat is doing... and related, the flow through the rad..
knowing what those are in normal operation.. will tell you when they are not.
a simple experiment of what i mean.. connect your garden hose to a tap... at the outlet... give it three pipes.. one small two large.. turn on the tap... thats your car engine pump pumping... close off one large exit... and the small one... what happens?
all the flow is through one big exit.
now open the small exit... what happens? the flow through the big exit changes, its less, and now you have two streams of water.. one fast, at high pressure and low quanity... and one at low pressure but higher quantity..
now slowly open the third... what happens? now you have two lower pressure high quantity outputs... and one not so high pressure lower quantity outputs in equilibrium...
close the small exit... what happens?
open the small exit, and partially close one big exit... what happens?
a flow meter can measure those differences and tell you what it can see...
if one is in the return pipe from the rad... it will see the thermostat open.. and providing the rad is clear.. you will get a known increasing rate as it opens to full rate.. if not.. either the thermostat isnt opening fully or the rad is blocked..
if you open the heater valve...( small pipe) you will see the two flow meters register a drop.. in theory if the pipes are all clear... both the meters should see the same rate.. it is balanced.. but if one still sits low. and the other is high... something is wrong.. i would be concerned if i dont see the hot circuit drop ... or the cold circuit rise... and the two balance...
of course it could be that the hot circuit is always favoured... its got the least resistance, it dont care about the thermostat or the rad or even the heater... the flow meters will tell you that too.