Guest Posted April 9, 2016 Report Share Posted April 9, 2016 Hello. Not vehicle specific. What is the current thought about the position of a thermal switch situated in a radiator; whereby it is used to control on / off for an electric fan. Should the switch be mounted at the top of the rad or at the bottom ?? There appears to differing views, but how are those views collated ?? If you take a Kenlowe fan for example their thermal probe fits in to the top rad hose. I have also seen other set ups and I have one on my Vitesse (see pics) where an inline thermal switch is within an alloy coupling cut in to the top hose. Others suggest that by fitting the thermal switch at the bottom of the rad, the fan cuts in less often and only when really required. Basically when the rad is no longer able to cool water without assistance - for example slow moving traffic. So should the thermal switch be monitoring the heat of water coming out from the thermostat (probably at its hottest) or when the water is about to leave the rad to re-enter the engine (probably at its coolest) ?? Is the opportunity to reduce the coolant temperature at the earliest moment, when it comes out of the top hose in to the rad more beneficial than having the coolant temperature monitored at the bottom of the rad ?? It may be the case that there is not much in it; to be honest I do not know. It might just be a case of personal preference, but I do not think that really determines the thermal switch correctly. We need to assume for this that the coolant system is working correctly and all associated items are in good condition. I would be interested in any views / experience of such. Many thanks. Richard. Link to comment Share on other sites More sharing options...
rogerguzzi Posted April 9, 2016 Report Share Posted April 9, 2016 Hello Richard Mine is fitted in the bottom of the rad and is a dual temperture switch as I have 2 fans. switching on at 82deg/78deg and 88deg/82deg To me it seems the correct place, why would you wan't to cool the water before the rad had done its job? Also is it not better for the engine to have water coming back into it at not much lower temperature than it runs at for maximum efficiency? Roger Link to comment Share on other sites More sharing options...
Guest Posted April 9, 2016 Report Share Posted April 9, 2016 Hello Roger. Many thanks for that and interesting points. I can see both sides of the argument and one has to wonder why companies like Kenlowe, who obviously spend a fair bit of money on R&D, place their electric fan probe in the top hose. On my Vitesse I placed the thermal coupling in to the top hose as the company I purchased it from made it super clear on their brief that that was the only position. No rationale given for the direction. I've had my set-up in place for about 18 months and works well, but I'm starting to think if a bottom positioned thermal switch may be even more effective. Be good to hear some other views. Your double rad set-up looks spot-on and of course split switching is very advantageous. Best wishes. Richard. Link to comment Share on other sites More sharing options...
Pete Lewis Posted April 9, 2016 Report Share Posted April 9, 2016 this seems chicken and egg syndrome, maybe a sensor halfway down the rad, like on a modern but they have a siamese rad ( flows in a U ) with vertical tanks to co join them , or put a engine driven plastic fan back on and save all the hassle Pete Link to comment Share on other sites More sharing options...
rlubikey Posted April 9, 2016 Report Share Posted April 9, 2016 Yes, I used to think that it would be better in the lower position. But I've come back to thinking that a top mounted switch is preferred. With a top position, the switch should trigger a few degrees above the thermostat opening temperature. Provided you're moving, there will be plenty of air flow through the rad and, as the thermostat opens, the bottom of the radiator should deliver plenty of cool water back to the engine and the thermostat should find a balance point to maintain a stable temperature. (Thermostats don't snap open at one temperature, they open progressively over a small temperature range.) However, if it can't, either because you're stationary (or nearly so) or because of enthusiastic exercise of the loud pedal, then the temperature will rise past the point where the thermostat is fully open. There is no more cooling available unless conditions change. Somewhere at, or just before this temperature, the fan switch should trigger and the fan will cut in. Exactly where depends on how often you want the fan to come on, versus how stable you want the engine temperature to be. Compare this with a bottom mounted switch. When the thermostat is closed, the water in the rad is bound to be cold, even when stationary. As the engine temperature rises and the thermostat opens, there is going to be a time delay before the hot water hits the switch. I think this means the engine temperature is bound to cycle because of this delay in the feedback. Worse still, the water back to the engine needs to be cool enough at a given flow to absorb the excess heat based on how many horses the engine's recently been producing. So, what temperature switch do you choose? Say, after giving your car a bit of a ragging, you have to stop for a queue or lights or whatever. The engine's ticking over but there's still lots of heat from the cylinders being absorbed by the cooling system and the temperature rises rapidly. But you've set your fan to come on (which it does) and go off at a certain temperature which is a compromise - see above. But the engine's ticking over (low flow) and the fan cuts out even though your engine is crying out for cool water - and lots of it please! I think that you'll get a temperature spike (that damn time delay again) while things slowly sort themselves out. Me? I've got a fan switch in the top *AND* the bottom of the radiator (asked the man when I had it re-cored). The top switch runs things nicely and the bottom is my insurance policy! Cheers, Richard Link to comment Share on other sites More sharing options...
rogerguzzi Posted April 9, 2016 Report Share Posted April 9, 2016 Hello Richard If the switch operates early surly the fan is just trying to cool already cool water. We are all assuming the radiator and system are clean through out in which case I can not see why the top is better? I took Spitty to Spain last year and after early nervousness by me and switching the fans on early! when I left it to do its own controlling only one fan ever came on and we were in 30/35deg + down in Jerez Roger Link to comment Share on other sites More sharing options...
68vitesse Posted April 10, 2016 Report Share Posted April 10, 2016 Some years ago I fitted a revotec fan switch in the bottom hose on my Vitesse, the switch can be adjusted for cut in temperature. This setup works very well, l also fitted an override switch which I have never felt the need to use. Also fitted a light which comes on when the fan is running and was suprised how seldom the fan cut in. Regards Paul Link to comment Share on other sites More sharing options...
Guest Posted April 10, 2016 Report Share Posted April 10, 2016 Hello. All of the replies are very interesting and informative; so many thanks for that. Out of interest is the thermal switch electrical connections specific or can either of the 2x terminals be + or - ?? The one I used on my Vitesse was already part wired so not an issue; but many others just have the 2x terminals based on the assumption that it is being used as a replacement part. Not having one to hand I do not know if they are marked +/- Many thanks. Richard. Link to comment Share on other sites More sharing options...
rogerguzzi Posted April 10, 2016 Report Share Posted April 10, 2016 Hello Richard I do not think it matters whether you switch the live or ground it is just a thermostatic switch. The switch I have has got 3 terminals so there is a common + or - and a high and low temperature. I have the model number written down some where? Photo shows the override switch which is centre off and left 1 fan right 2 fans with leds to show what is on Roger Link to comment Share on other sites More sharing options...
Guest Posted April 10, 2016 Report Share Posted April 10, 2016 Hello Roger. Thanks for that and useful to know. Neat set-up with the 2x LED's. Regards. Richard. Link to comment Share on other sites More sharing options...
SpitFire6 Posted April 10, 2016 Report Share Posted April 10, 2016 Hi, A switch is normally not polarity sensitive and I guess a car temp switch is the same. A switch would only be polarity sensitive if it incorporated a diode for external protection. Put a multimeter set on diode test and the switch should be open both ways to confirm it is not polarity sensitive. You could use a higher temperature top switch and a lower temperature bottom switch wired in parallel, as long as both are normally open, and close on temperature rise you will get the best of both? Cheers, Iain. Link to comment Share on other sites More sharing options...
rlubikey Posted April 10, 2016 Report Share Posted April 10, 2016 Hello Richard If the switch operates early surly the fan is just trying to cool already cool water. We are all assuming the radiator and system are clean through out in which case I can not see why the top is better? I took Spitty to Spain last year and after early nervousness by me and switching the fans on early! when I left it to do its own controlling only one fan ever came on and we were in 30/35deg + down in Jerez Roger Hi Roger - if you're referring to the top-mount switch then, if the vehicle is stationary and the thermostat begins to open, first the water already in the radiator is sent back to the engine, then the water coming through the 'stat. If this is insufficient cooling for the engine then the temperature will continue to rise, the 'stat will open more and, at some point, the fan switch will be triggered. Hello. All of the replies are very interesting and informative; so many thanks for that. Out of interest is the thermal switch electrical connections specific or can either of the 2x terminals be + or - ?? The one I used on my Vitesse was already part wired so not an issue; but many others just have the 2x terminals based on the assumption that it is being used as a replacement part. Not having one to hand I do not know if they are marked +/- Many thanks. Richard. Hi Richard - the only thing I would add to the other comments about the switch is to control the fan via a relay. This is because the fan will have an inrush current much higher than its continuous rating and it will kill the switch after a while. Guess how I know this!! Cheers, Richard Link to comment Share on other sites More sharing options...
Guest Posted April 10, 2016 Report Share Posted April 10, 2016 Richard. Absolutely correct and one already installed on the Vitesse; but always worth mentioning, especially for owners looking to embark on such a conversion. Ditto for the headlights as well. Regards. Richard Link to comment Share on other sites More sharing options...
rogerguzzi Posted April 10, 2016 Report Share Posted April 10, 2016 Hello All You correct about relays I run both my fans through relays and seperate fuses(built into the relay's) Roger ps and headlights Link to comment Share on other sites More sharing options...
rogerguzzi Posted April 10, 2016 Report Share Posted April 10, 2016 Hi Roger - if you're referring to the top-mount switch then, if the vehicle is stationary and the thermostat begins to open, first the water already in the radiator is sent back to the engine, then the water coming through the 'stat. If this is insufficient cooling for the engine then the temperature will continue to rise, the 'stat will open more and, at some point, the fan switch will be triggered. Hi Richard - the only thing I would add to the other comments about the switch is to control the fan via a relay. This is because the fan will have an inrush current much higher than its continuous rating and it will kill the switch after a while. Guess how I know this!! Cheers, Richard But surely the thermostat starts to open early and by the time it gets to 82deg(or 88deg) it is fully open so the flow would be ok? because before that it does not need the extra cool water? as it is still coming up to operating temperature. Assuming the system it up to scratch. Roger Link to comment Share on other sites More sharing options...
Clive Posted April 10, 2016 Report Share Posted April 10, 2016 I have thought a lot about this malarky, and come to the conclusion that once yu have got over the theory and start using actual results either position will work just fine as long as the switch is suited. ie lower temp at the bottom of the rad. Which temp is the tricky bit, and I have a collection of switches off modern cars, the dual temp ones are handy!. (both my triumphs use VW rads, so easy to fit temp switches, and did I mention cheap/efficient) 1 Link to comment Share on other sites More sharing options...
SpitFire6 Posted April 11, 2016 Report Share Posted April 11, 2016 Hi, You cold fit two sensors top and bottom. Only switch on when no air flow thru fan or flow is not doing good? Cheers, Iain Link to comment Share on other sites More sharing options...
SpitFire6 Posted April 11, 2016 Report Share Posted April 11, 2016 Cold is could. Link to comment Share on other sites More sharing options...
Mjit Posted April 12, 2016 Report Share Posted April 12, 2016 Technically top is best. The fan and radiator are there to stop the engine getting too hot and in the top hose it means the fan kicks in as soon as the water coming from the engine gets too hot and doesn't kick out until the temperature of the water throughout the engine has come back down. In the bottom hose you have to wait for all the water in the radiator to get hot (and so that in the engine to get even hotter) before the fan kicks in and it will kick back out as soon as the water in the radiator get cool-enough (and before it's actually made it to the engine). That said I've had mine in both at one time or another and can't say I noticed any practical difference! Link to comment Share on other sites More sharing options...
rogerguzzi Posted April 12, 2016 Report Share Posted April 12, 2016 Technically top is best. The fan and radiator are there to stop the engine getting too hot and in the top hose it means the fan kicks in as soon as the water coming from the engine gets too hot and doesn't kick out until the temperature of the water throughout the engine has come back down. In the bottom hose you have to wait for all the water in the radiator to get hot (and so that in the engine to get even hotter) before the fan kicks in and it will kick back out as soon as the water in the radiator get cool-enough (and before it's actually made it to the engine). That said I've had mine in both at one time or another and can't say I noticed any practical difference! Hello While agree the fan and radiator are there to stop the engine getting to hot. But I still don't see why the fan has to operate before the radiator is getting to hot? after all the water going into the engine from the bottom of the radiator is colder than the engine(back to assuming the system is working correctly) I find with my Spitfire if I run the engine in the garage until it is hot and the bottom mounted thermostat operates the temperature gauge goes just over half way with an 82deg thermostat fitted and the fan cuts out after a few minutes(to get the second fan on I have to rev it a lot and get some soak heat in) Didn't MGF's suffer from thermal shock causing the head gaskets to fail due to the radiator being at the front and the water returning to the engine was to cold?(my daughters failed and I researched it) but they do not fail in kit cars with a normal set up. I think I will leave Spitty's were it is as it was ok in the heat of Spain Link to comment Share on other sites More sharing options...
John Bonnett Posted April 12, 2016 Report Share Posted April 12, 2016 Hello Richard Mine is fitted in the bottom of the rad and is a dual temperture switch as I have 2 fans. switching on at 82deg/78deg and 88deg/82deg To me it seems the correct place, why would you wan't to cool the water before the rad had done its job? Also is it not better for the engine to have water coming back into it at not much lower temperature than it runs at for maximum efficiency? Roger Very well thought out and neat fan installation Roger. Do you have a part number you could let me have for the dual temperature switch you are using. Thank you rgds John Link to comment Share on other sites More sharing options...
rogerguzzi Posted April 12, 2016 Report Share Posted April 12, 2016 Hello JohnThe one I am using is an Intermotor 50101. 83/79 88/83 I did try a Motoquip VRF327 88/82 92/87 but the engine got to hot before it cut in.Would probably be ok if fitted to the top of the radiator.Have a look on lowcostbuilders.co.uk they have a list with all sizes and temperatures. http://www.locostbuilders.co.uk/viewthread.php?tid=152082 I purchased my fans from here, there was no way I was paying £200+ for a kit and a micky mouse sensor pushed into the top hose(car manufactures dont fit things like that?) http://www.mcgillmotorsport.com/9-universal-slimline-electric-cooling-fan-reversible-268/RogerPs my rebuilt full width radiator came with the sensor tapping at the bottom 22mm x 1.5 so I assumed it was standard Link to comment Share on other sites More sharing options...
Guest Posted April 12, 2016 Report Share Posted April 12, 2016 Roger. You are correct that is the standard size and the thread is Metric Fine. Regards. Richard. Link to comment Share on other sites More sharing options...
Mjit Posted April 13, 2016 Report Share Posted April 13, 2016 But I still don't see why the fan has to operate before the radiator is getting to hot? The following numbers are made up but say fan kicks in when the water coming out the engine hits say 110C and out at 90C. When the water in the top hose hits 110C the bottom hose might still be at 90C with the average water temp in the system at 100C. If the cooling fan kicks in at this point ('stat in top hose) it's only got to cool the water by an average of 10C before it kicks back out. If it's in the bottom hose the water coming out the engine may be at 130C before the bottom hose reaches 110C, with an average temp in the cooling system of 120C and the fan needs to run for longer to get the average temp down by 30C. I think I will leave Spitty's were it is as it was ok in the heat of Spain Like I said mine's in the bottom hose too and the real temperature differences between the top/bottom hoses are probably just a couple of degrees Link to comment Share on other sites More sharing options...
rogerguzzi Posted April 13, 2016 Report Share Posted April 13, 2016 Hello All How about an electric water pump with variable speeds to suit the engine tempeature? Only thinking out loud and to get all the brains going!!!!! Roger ps 2 thermostats of course 1 for the fans and 1 for the pump? Link to comment Share on other sites More sharing options...
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