12 volt DC 3 way mid position valve (central heating)

[Aidan]

Don't think they're made. All the commercial valves use synchronous AC motors. You can get 24V motors for the valves, but they're still AC, so you'd still have to go; 12V dc-inverter-240 V ac- transformer-24V ac. Which would be fairly pointless unless you particularly want low voltage actuators.

You could get a cheapo Maplin inverter for about =A315. Sunvic (& I think Danfoss) valves are wired as motor open-motor closed (mo-mo) so they only use power whilst moving (assuming they're not spring return).

Solenoid valves are generally pilot operated diaphragm valves. They do not like any particles in the water stream and so are not used for heating applications.

You might find it simpler to use 2 x 12 V dc centrifugal pumps with non-return valves on the returns.

[Roger Mills (aka Set Square)]

No - Danfoss 3-port valves are spring return, and the motor continuously runs stalled in the mid and CH positions.

[CampinGazz]

You might find it simpler to use 2 x 12 V dc centrifugal pumps with non-return valves on the returns.

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you know what, you might have cracked it,

the eberspacher has one pump already, i can get another one pretty easily, i just have to check where the flow controll thing is in the heater, i dont think it's the pump motor, afaik it's just 2 wires, + and - 12 v, powered whne the heaters operating, and also powered when the heater goes into stand by mode.... i.e. main power on, but the temp has reached and stayed at or above 82 degrees for more than 5 minutes,

There is an eberspacher timer unit for this heater, it gives 3 operating times a day, and what i was thinking....

Have a thermostat straped to the calorifier just like a domestic hot water tank does, it's just a basic switch after all isnt it.

use the eberspacher digital timer unit, and have it set for 3 x 25 minute operating times throughout the day, have the power to the heater start up circuit go through the tank stat (via a relay if needed etc) so if at one of the 25 mintue cycles the tank stat if on, the eberspacher fires up and heats the tank of water up, shutting down after it's reached 82 degrees C for 5 minutes as above, or when the time peroiod expires,

If the tank stat is open when a time period is active, nowt happens, the tank's still hot (apparantly a small calorifier can keep it's heat for 24 hours, and only needs 20 minutes to bring the water upto temp from cold.. that's based on much larger tanks than i'll be using, so i'd get faster heat up temps)

there'd be 2 pumps as suggested with non return valves etc, and a relay to select which pump operates, normaly closed contacts allow the 1st pump to operate when the calorifier wants heating.. i.e. pump selector relay is not energised.

Then for the heating side, i dont really need times for that, as when it's cold i'll put it on, there'll be thermostatic rad valves to regulate the temp in the different areas of the van (living area, bathroom etc)

So when i want heating, i turn a switch on, which is connected through a room stat, if the van's cold enough according to the room stat, power is sent down the heater start up wire bypassing the timer (may need to use a relay to avoid backfeeding the timer, not sure if it'll throw a wobbely and bring up a fault code.. unauthorized heater usage or something :) and it'll also switch the relay on the pumps to power pump 2 when the heater calls for the water to circulate.

So the eberspacher heats the water up, pump 2 pumps it around the radiators untill the room stat shuts it off, the water going through the heater is at

82*C for 5 mins, or i turn it off manualy.

I could think of a way to make pump 1 operate at the same time if needed by the tank stat and timer being on i guess. thus doing the same as a 3 way mid position valve does, but with 12 volts DC... and a lot more complications.

does the above sound feesable? any input welcome, as i don't know that much about centeral heating systems.

on boats where these heaters are usually used, they all seem happy to have a manual valve they select hot water or heating with, but i like things to be automatic, the stand alone water heater i had was, as was the gas fired blown air heating system, i'm just combining 2 units into one, and heating with hot water now, but i want a better system than i had before, ie. one fuel source, one unit, and hopefully easier use, tho that may be pushing it.

[Aidan]

Yes, they are. So some of the Sunvic 2-port actuators are mo-mo.

I meant the 2-port valves. The spring return to closed on power failure is a safety feature in some installations, so the OP would have to devise a safe installation if he went for the motorized valves.

[Aidan]

Now I come to think of it, you can get actuators for small valves with 'wax motor' or 'thermo-hydraulic' actuators, which work on DC voltages but they're generally 24V AC/DC. I think they're similar to thermostatic rad valves with an added electric resistance heater to achieve the electric control.

Danfoss Randall, Siemens, etc., do them in the UK. They're usually used on radiator valves but will operate a small valve for DHWS just as well. I don't know how the electric consumption compares with motorized valves, as this might be an issue.