12v Heating

Hi,

We are currently heating part of a greenhouse with a 500w mains fan heater which in the recent cold UK weather is taking about 5kw per day or 200w per hour.

Looking to provide a backup source for up to 4 hours that can be automatic, so that effectivlely means it must be derived from 12v caravan batteries.
Bottled gas and paraffin ruled out because of the fumes etc.

The problem is we cannot find any air heating elements that run from 12v.

Had thought about an inverter but to drive the 500w main heater would probably need an inverter rated at 1000w which are not cheap.

So far the only possibilities seem to be using one or two of the ceramic fan heaters sold in car shops which do not seem built to last for 5 minutes never mind 4 hours, or possibly the use of four standard 12v 50W halogen bulbs with a pc fan behind them.
Power resistors are another option but work out very expensive.

Would be interested to know if anyone has tried any of these methods or can suggest a more robust 12v heater without breaking the bank.

thanks

I guess you mean 5 kWh per day or an average power of 200 W

I would just use the 50W halogen bulbs. One of these would be more robust MULTICOMP|MC14649|RESISTOR, POWER CERAMIC, 300W | Farnell United Kingdom and would generate 144 W or so. You could use two of those, and maybe other values to get the power that you want.

Agreed with Diver, light bulbs are efficient as heaters, not for its light.

Why not simply use something that has a large thermal mass and heat that with mains voltage?

The problem with heating anything is that you need to get the energy from some place. Running any amount of heaters off of batteries takes massive amounts of battery capacity to get any usable levels of heat. 1 kilowatt hour equals 3414 BTU's and one typical car battery can only supply around 300 - 400 watts without damage.
Plus that energy in the battery had to come from some place so taking it out of a battery means you have to put it back some how which can mean charging it off a vehicle which burns more fuel than what the energy in the battery is worth or charging it off of the grid which is a waste of time and money since the grid power could have just been used to run the heater directly in the first place without having spent money on chargers, batteries and some form of heating devices.

The only sources of heat cheaper than electric is burning a fuel of one form or another or using thermal mass storage that takes day time solar heat energy and absorbs it then releases it later.

I agree that trying to do this with 12v batts is already breaking the bank. 2KWh out of cold lead-acid batts is extremely expensive!

You can get a propane RV furnace which is a heat exchanger, which means it separates combustion air from the inside air. It requires 12v for the fan. They have automatic electronic gas valves and starters, too. They use a thermostat input- an unpowered mechanical thermostat connects 2 wires when a bimetallic spring cools down and bends. Heater turns on gas, waits, ignites, waits for thermocouple to show flame. No flame, retry. Repeated retries, no flame, kills the gas to keep from blowing something up. Just like any other home furnace.

But you don't necessarily NEED a heat exchanger. The catalytic radiant heaters are much cheaper, although they lack the automatic electronics of course.

Plants won't mind the exhaust, as long as the combustion is complete. In fact they love it! CO2 is plant food!
However, if the space to too well enclosed and the heater runs for a long time, it may use up so much oxygen that the mixture becomes too rich and fails to burn completely. In this case CO could be formed. Plants actually don't mind CO, but it'll poison humans if the level's high enough. It wouldn't take a lot of ventilation to keep this from happening, though.

Honestly? I don't know about the UK, but in the USA, natural gas heat is MUCH cheaper than electricity. You might forget the electric heater and just put in a small gas heater and turn it on for the entire winter. And again, plants thrive on elevated CO2 levels.

Unless you are projecting the light outside of the space to be heated then just about any incandescant 12 v light bulb will do. I do agree that the RV furnace makes a lot of sense though your application may not accomodate that well.

You didn't clarify whether you're looking for backup power in case of an outage, or you're trying to save on the electric bill.

As backup power, caravan batteries into lamps will do a passable job. Your battery must be massive. Place the lamps so they won't confuse the plants. Place the battery, charger, and whatever else inside the environment to be heated, so the power 'wasted' in charging is put to use.

As energy savings, it simply doesn't work. Period.

I would have to disagree with not using propane or natural gas. Propane only gives off CO2 and water when it burns. The CO2 is a very welcome addition in a greenhouse, so I don't see any reason not to.

Hi All,

Thanks for the many ideas and suggestions.

I'm just looking for something to provide 200w of heat per hour for a max of 4 hours at a time, which is the longest power outage we have ever had for many, many years.

I already have a 85ah battery , so going with the a bank of bulbs to produce the 200w seems the simplest and cheapest way.

It also means it can switch in automatically, which is not possible with the very small gas heaters we have here.

Wp100 said:

I'm just looking for something to provide 200w of heat per hour for a max of 4 hours at a time, which is the longest power outage we have ever had for many, many years.
I already have a 85ah battery , so going with the a bank of bulbs to produce the 200w seems the simplest and cheapest way..

Click to expand...

Won't be anywhere close.
By nameplate value, an 85 AH batt is about 1020W-h at 25C. But many derating factors apply.
1. The nameplate is for a 20hr discharge, which means a 4.25A rate. You're looking for the rating at 16.7A, which is less. Note that the available capacity is much better when powering a 200W load all the time as opposed to cycling a 500W load 2/5th the time.
2. Your usable capacity will decrease at colder temps, esp the high-rate capacity.
3. You typically don't want to discharge the batt all the way down. This degrades the battery significantly faster.
4. The overall battery age will reduce capacity.

And 200W isn't enough to heat anything bigger than a... well, a fairly small box.. I have a hard time seeing how this is gonna do the job if this is a room.

I'm saying DON'T switch the gas at all. Just light it and leave it lit all winter for your heating. The cost of a BTU of gas is much lower than a BTU of electricity, and the plants will use the CO2, and a power outage won't affect it at all. If "gas outages" occur, well, I've never heard of them.