Truma Spaceheater the Answer ?

[SuperMike1]

So away over Easter in our super duper new Swift 645 we suffer a problem. Very frosty outside and a little chilly inside. Yes the age old problem of the heater cutting out after a while and taking an age to come back on again.

Picture this. Swift with all their years of making caravans have still not yet worked out that the remote sensor needs to be somewhere other than where it is fitted in our 645. That is to say not directly above the fire and about 6ins to the right of the fitted car radio. So the fire lets out hot air which rises to the remote sensor and off it goes. Or even if the fire is turned down low, play the radio which gets quite warm and off it goes.

Management gets very excited. Michael rings dealer who tells us that if I reach in and unplug remote sencor then fire goes back to normal operation. So I unplug sensor and then what, the fire still cuts out, why ?

Well apparently if you unplug the remote sensor then the original thermostat which is in the rotary control comes back into operation. And where is this rotarty control, yes of course diectly above the fire. Ahhhhhhhhhhhhhhhhhhhhhhhh !!!

We switch to Gas operation and stay warm. But I need a fix, because I'm a stubon old git, so when we get home much thought goes into the new location of the remote sensor. I fit it to a long lead and experiment around the caravan, with little success. So a different solution is called for.

I've measured the thermistor they use as a thermostat and find that in the fridge it measures 25k ohms and in the sun 8k ohms. Answer one 25k, half watt resistor soldered across the terminals of a 3.5mm jack plug. The remote is now unplugged and the thermostat in the rotary knob is kept off line by my new gizmo. But of course there is no remote control, if it is too warm or cold you have to get off of your backend and turn it up or down.

An answer or what, cost 97p. Have I done something realy naughty. Polite answers only please

Regards SuperMike

 

[BrianStAlbans]

Thats great SuperMike.

ps I've just put a copyright on it and it will be in the shops by xmas. ;O)

 

[ProfJohnL]

Hi SuperMike,

Essentially what you have done is to fool the heater into thinking its cold by replacing the thermistor with a fixed high value resistor - a fact which from your description I think you understand.

Truma can supply a remote sensor extention, but I think you could easily manufacture one. I would suggest leaving the control box where it is, but put a long lead on the thermistor sensor, then by trial and error establish where the thermistor could be located to make your system work.

Having found your ideal spot, make the instalation more permanaent and tidy up the wire. The aditional cable resitance will be very small by comparison to the value of the thermistor which still shows 8K when hot. This should not upset the control system.

 

[RogerL]

You don't need to buy Truma's extension cable for the remote sensor.

When I modified my Bailey by adding a remote sensor (because the main control was heated by the fire!) the instructions include clear information that the cable can be cut and extended using any 2-wire cable.

The remote sensor needs to be positioned AS FAR AWAY as possible from ANY heat sources in order to mange the room temperature.

It's a great pity that caravan manufacturers don't read PC Forum, they'd gain lots of FREE valuable technical information to include on their production models!

 

[JohnKS]

We have had exactly the same problem in all our swift caravans (4) and I have moved the thermostat in them all except our new Sterling which is not near any heat soource other than the kettle.

I agree, why don't they think about this problem. Only goes to show that none of the designers ever use the van they design!!

 

[SuperMike1]

Thanks for all your replies. As stated in my original posting I tried walking the remote thermostat around on an extension lead, but was unable to find anywhere, high/low/front/back, where it worked as it is ment to do. The historiesis (bet thats spelt wrong)of a thermistor is terrible, so by definition its usless. Really Truma should rethink the whole idea. And so Michaels gizmo will remain, for now at least.

Regards

SuperMike

 

[AnthonyVJ]

Hi

I thought i'd tell you what my husband does when we stay in self catering places and the heating system keeps going off before you're really warm enough! He finds something that will hook over the thermostat and puts it in the freezer for an hour then clips it over the thermostat, the heating comes on quite quickly and usually stay on for some time!! Only a temporary fix of course.

Helen

 

[ProfJohnL]

Hello SuperMike,

The Thermistor is passive sensing element that changes its resistance according to temperature (and some self heating due to current) It does not determine the hysteresis of the control system, that is down to the design of the monitoring circuit.

That said, I used to be involved in the testing of heating appliances, and there is considerable difficulty of finding a suitable hysteresis that is comfortable to all. Domestic central heating thermostats typically have 0.5 to 1.5C hysteresis. Any less and the system will be switching on and off quite rapidly and becoming a noise nuisance, or in some cases it becomes unstable and because of supply line variations when loads are turned on and off the contacts can chatter. This does no good to the contacts or any valves that may be attached.

Caravan heating poses even more difficulties, the thermal mass of the caravan is less so it tends to change temperature more quickly, when a door is opened a much larger proportion of the caravans heat is lost than in a normal domestic house, There are legal requirements for the amount and distribution of fixed ventilation, so significant stratification will occur unless the air is mixed, I have seen caravans with internal floor temperature of as low a 5C but a roof temperature of 35C, that is very uncomfortable.

One of the biggest helps is to keep the air mixed well by moving it, Stratification is reduced. In the best case I saw it was down to 19 at floor and 22 at roof, and the air flow was not excessive or noisy.

It is also the case that for most simple on/off control systems, the first few cycles tend to be less well controlled than subsequent, so the hysteresis tends to narrow after the first few cycles - in a caravan I would allow a good hour to pass before passing judgement on a sensor position, and ideally nearer two or more.

Part of this problem is over shoot. If you have a very powerful appliance then it builds up heat with in it self when it is turned on. When the thermostat is satisfied, the input power is turned off, but the residual heat in the appliance continues to feed into the living space which raises the temperature above the desired cut off point. A factor such as this can increase hysteresis by a significant amount.

It is better to try to match the heat input to the heat loss, and that way excessive over shoot can be avoided. By all means use the full power setting to get up to temp, then try switching to a smaller heat source to maintain the temperature.

Gas heaters that use an oil filled expansion regulator are much better at maintaining a desired temperature than a bang bang - on off electric heater. There are temperature regulators for electric elements that use proportional or phase control of the power to improve temperature regulation, but these tend to be quite expensive and designed for commercial use.

 

[gary]

I get confused with the spelling, I didn't think the 'te' was in the middle??

Anyway the 'hysteresis' is with the IC used in conjuntion with the thermistor, the thermistor is a passive component.

The old Carver systems used a LM358 op-amp as the comparator which works well, however the Truma uses a LM714 which is well known to suffer with hysteresis.

The thermistor used with the truma fire is 10k at 25C negative, as for position I would leave it at floor level under the heater where the coldest air will be drawn across it.

However you are still up against the hysteresis problem.

 

[GStockton]

Another problem with this heater that you may not be aware of,is that the element control pcb at the rear of the heater has on it 3 change over relays 12volt dc coil with 10amp rated contacts(later ones were uprated to 16 amp by Truma) once they realised the 10 amp ones had a problem with high resistance contacts.

These 3 relays are configured to put the 2 1000watt elements in series or parellel to give 500w both in series 1000w 1 element in use or 2000w both in parellel.

What happens in vans up to 2004 when the heater shuts down on its Thermostat the current is cut,when the stat calls for heat again the relay coils pull in ok but you find a high resistance across the relays No current then flows to the elements until the relays have gone cold (could be at least 1 hour before it works again)

I have spent several hours on these heaters and pcbs and have rebuilt about 15 or so pcbs with new relays for some caravanners on another forum

If you have a heater fault with a dud Pcb they can be repaired for a materials cost of about