to answer this question we need to get a bit of background on HVAC practice [Heating Ventilating & Air Conditioning]

there is nuthin new under the sun with AC - it was invented by Dr Carrier in 1930 or so together with its airside variants and these got applied to cars etc over the years

the type in my volvo is Single Zone Reheat [SZR] and has a heating coil [HC] on top [in series] of a cooling coil [CC] and in a car the CC is a DX [direct expansion] coil and the HC of course HW [hot water]

the type Mr BMW describes is Dual Duct [DD] or Multi Zone [MZ] and has the same HC and CC but in parallel

as you will know it is almost impossible to get the heating correct in SZR because of the inertia of a HC full of very hot water which even if you turn it off you still get hot air for 2 minutes [however in my experience Volvo engineers did a good job to overcome that problem]

however when some manufacturers opted for a DD system, which allows the driver to INSTANTLY set the correct mixture of hot and cold air, the others followed in about 1980 or so - I was Jaguar owner then and I remember Jaguar was heavily critisised for sticking with the "outmoded water valve" and finally changed also

what the marketing does not tell you is that DD does not allow humidity control hence the fogging up problems we have with later models which needs to be overcome with rear screen demisters etc - with the good old SZR you freeze the air down below the dew point and wring the water out of the sucker and dump it out the bottom of the car then reheat it to a nice temp for yr body but at only 5% RH [relative humidity] so all windows become clear within a very short time

so there are pros and cons for both systems - the DD system was popular in large prestige buildings in 1960s but got "outlawed" in the 1980 or so "fuel crisis" because it is an energy hog [same time as Volvo 5 MPH bumper got outlawed as I said before]. It remains popular where heat is "free" as on cruise ships [and cars] - in fact the last DD system I designed was for the accom module on a North Sea oil platform when I worked in UK in late 1970s

THE ENGINE

so back to the engine side ie the provider of the heat for the HC. What the engineers have done is what is called "riding the pump curve" and as the diag from Mr BMW shows the engine thermostat changes from a 3 way way valve to a 2 way valve and the ONLY water going through the engine at start is the 20% [of max flow] which circuits via the HC [which now has no CV ie control valve - this is nicely called "a wild coil"]. So we are riding at very top of pump curve with low flow and high pressure BUT the resistance of the coil is deliberately made quite high for reasons seen below

an extra advantage over the "3 way valve constant volume" system used to supply the old SZR AC setup is that you would get heating in the car marginally quicker at startup

now once engine reaches 80 C, as before, the t/stat starts to open and allow water to the radiator and is fully open at say 90 - well the rad is a very low resistance element in the circuit - so now we sit at far right on pump curve with high flow and low pressure drop [PD] and in fact the heater would only be getting about 5% of the total flow with such a reduced flow pressure but still plenty of heat there to keep those bodies warm.

so IS this as good from engine point of view ?? - well the answer is no - it is far better to have ALL the water cooled than 95% of it but I guss the engineers simply added 5% more safety margin to the radiator/fans etc

there always has been and always will be a battle between what engineers see as good design [to make it GO good] and what marketing sees as good design [make it LOOK like it goes good and so SELLS good]

THE "BYPASS"

so back to the mechanic bypassing the heater - well a straight piece of hose has bugger all resistance and maybe at full flow ie at boil over point there might be say 15% [rather than 5%] bypassing the radiator - but whatever the percentage it WAS enough to CAUSE the subject volvo to boil over

but I agree with Mr BMW that given this volvo HAS a DD system and hence ONLY bypasses via the heater, THEN is was stupid [on the part of owner or mechanic or George Bush - who cares!!!] to do ANY type of "fix" other than a PROPER fix of the PROPER HC with the PROPER resistance

in other words this type of cooling system is far more delicately balanced than the "older" version so one would mess with it at their own peril

but I agree with Mr BMW that given this volvo HAS a DD system and hence ONLY bypasses via the heater, THEN is was stupid [on the part of owner or mechanic or George Bush - who cares!!!] to do ANY type of "fix" other than a PROPER fix of the PROPER HC with the PROPER resistance





I knew an argument with The Doctor would lead to; the downfall of the world at the hands of the U.S. First it was auto safety, now HVAC and cars overheating, hey Doctor, is there anything in the world that isnt the fault of the U.S.? Let me guess, every bad mechanic in the world was trained by an American, right.

Well, I guess your going to give us some background of your worldly experience. Next youll tell us the history of refrigerant, and how Americans created it to rule the world...

I have a feeling this forum will get alot of use :grin: :grin: :grin:

so yo are saying it WAS Bush who bypassed :liebe011:

I bet Michael Moore told you :grin:

I am afraid my worldly experience on refigerants is rather scant - I am not a full bottle [joke there :banana: ] on them

reason is I was mainly involved in larger buildings/complexes where cooling is via chilled water from a chiller rather than DX systems - for sure I never understood that sub-cooling and super-heating jargon used by "fridgies"

the crossover to centrifugal chillers is typically at about 200 tons [and I am loathe to mention the only building I know in USA of that threshold size being the Murah Building in Oklahoma - the one bin Laden bombed between his 2 attacks on WTC, but it got blamed on Timothy McVeigh]

but I seem to remember that the early systems used ammonia as a refrigerant and it sometimes escaped and killed people so the flurohydrocarbons thingos got developed - R12, R22 and R500 [and chillers used R11 from memory]

so I am sorry to not be able to provide any info on refrigerants, espec as the new anti-greenhouse ones happened well after my time in the HVAC industry - but if you want truth re Murah please read "Others Unknown" by Stephan Jones the deeeeefense attorney for Timothy

Cant we all just get along?

yer right tech :beer:


:gitara: come on now, smile on yer brother
everybody get together
gonna luv one another right now :gitara:

so just to sign off on the orig question - given the apparent huge job involved in replacing the heater coil and fact the owner said he was no fussed about actually having a heater then the cheapest and most convenient way to fix would be something like that Ford heater valve I spoke of [which actually came with a cable pull to install inside car] and simply connect the flow and return hoses to that valve

so normally he would have it open so as to get some bypass before t/stat opens but if a hot day and in traffic he would pull valve closed as temp gauge started to red line

other way would be just put a shutoff valve [gate valve] there but if temp gauge started to rise he would have to stop car, open the bonnet and turn off valve

a less comfortable way I have used on English cars espec Jaguar which are badly designed on cooling side for other than a mild English summer is to open windows wide and turn heater on full blast to act as a second radiator. Not too comfortable but if it prevents an expensive engine rebuild it is worth it in an emergency

If your calling me a Michael Moore supporter, now those are fighting words. Dont even get me strted on that "person".

Regarding Bi Laden and McVeigh, check out this site and go to terrorism...

http://www.peterlance.com/