Hi all,

I was looking for something on Gumtree and came across this.

http://www.gumtree.co.za/a-other-home-garden-items/george/hot-water-wood-fired-donkey-100litre-open-system-hot-dip-galvanised/1001051854190910002988709

Well build and this guy recons he can heat up 70lt water, 77 deg C in 55 min with 1.5kg wood.

Another company in South Africa that also builds them.

http://water-heaters.co.za/

I hope something like this might help the guys out on the farms and lodges (assuming that you have access to enough fire wood), and its cheaper than good quality solar geysers.  I also realize that most  guys out there on the farms already saw this or have something similar, but i just liked the idea and decided to share. I also grew up with the old "donkeys", but it was all home made and now someone realized how well they use to work and are building them a bit stronger than the ones I grew up with.

They look quite strong and well build and the prices seems fair.

I did a quick calculation for using wood in my case, We buy 9kg wood for R12 up here. So if I should heat up a geyser once a day using 1.5kg wood, it will cost me R2 per day.

I tried to make the comparison, for city use (!), but I had to work out how much energy there is in a piece of wood and try and figure out a cost per kwh, and I came to the conclusion that it MIGHT work if you can get the wood cheap enough, but that it's not that easy to calculate. I didn't quite give up, I sort of ran out of time and went to bed and forgot to look into it again.

Some stuff I found on the internet suggest that wood has about a third of the energy content of LPG per kilogram. Since a gas geyser is much less efficient than an electrical geyser, and LPG costs close to electricity, I already know that electricity (fron the grid) is cheaper than LPG. If you have natural gas from a pipeline (I have family up North who has that), then sometimes it is slightly cheaper to use gas. But it remains pretty close for the moment. So if wood is cheaper than LPG, it might be cheaper than electricity. Determining either solution would prove what I would like to know.

So looking at the internet again, dry log wood has 4kwh per kg, so 1.5kg will contain 6kwh of energy. 

If it heats 70 liters of water to 70 degrees (and you started at 15, for argument's sake), that's a delta of 55 degrees centigrade. 70 * 55 * 1.16 ~= 4.5kwh. So then this "donkie" (a rocket stove by the looks of it) is 75% efficient. That doesn't sound completely ridiculous.

4.5kwh costs R10.26 in Cape Town now (I use more than 600kwh a month, so all savings are reckoned at the higher rate of R2.28). So if I can buy 1.5kg of wood at less than R10, then we're in business.

I can get 100 pieces for R85, so 85c a piece. So all I need to do is weigh one and see how many it takes to make 1.5kg :-)

It sounds very plausible to me...

3 hours ago, plonkster said:

Since a gas geyser is much less efficient than an electrical geyser, and LPG costs close to electricity, I already know that electricity (fron the grid) is cheaper than LPG. If you have natural gas from a pipeline (I have family up North who has that), then sometimes it is slightly cheaper to use gas. But it remains pretty close for the moment.

@plonkster I just need to ask a question about this statement, I have heard it so many times in the past, was your calculation made on heating 150L of water with Electricity vs heating 150lt of water with gas? And did you consider the different ways in witch they will be used?

In my opinion people tend to miss the bigger picture. Although the cost of LPG is the same or slightly more than Grid power, there is still a huge saving in using gas. With a electric Geyser, you heat 150lt of water, use for example 40lt of that, replace that 40lt with cold water and leave to cool down till tomorrow to be re-heated for your next 40lt. With the new age geysers, you will lose and additional 2.25kw over a 24hour standing period that you will also need to heat up.

With a Gas Geyser, you would only have to heat up your 40lt plus a bit extra to heat up the pipe to the shower ext. I have seldom came across someone that uses 150lt of hot water. So in my opinion, gas is a great solution for anybody that don't need a lot of hot water.

I had 2 Geysers in my house when I bought it, because of how the house is build one was only feeding the kitchen basin and the other one the two bathrooms. The domestic worker did dishes twice a day, Roughly 30lt day. I decided to rip out the one 150lt 3kw geyser and replace with a 6lt/min gas geyser. The LPG consumption on this geyser is 1kg  per running hour. It takes 2.5min to fill the basin, twice a day. that is 5 min per day, so 12 days for 1kg of gas,,, compared to 2.25kw standing loss per 24hour plus the usage.

IMO, even if the gas is more expensive, it will still be worth it.

If my thoughts concerning this is wrong, please correct me, I saw that you've got the quickest calculator around here and I must admit, I hate doing in dept calculations.

17 minutes ago, Jaco de Jongh said:

was your calculation made on heating 150L of water with Electricity vs heating 150lt of water with gas?

Yes, it's based on apples of apples. You are right, to be fair, I should compare to using a "instant hot water" heater, or what the Americans calls a "tankless" system. The difference is not that bad however.

18 minutes ago, Jaco de Jongh said:

use for example 40lt of that, replace that 40lt with cold water and leave to cool down till tomorrow to be re-heated for your next 40lt. With the new age geysers, you will lose and additional 2.25kw over a 24hour standing period that you will also need to heat up.

There is no "additional" 2kwh over 24 hours. The amount that it cools down by overnight IS the standing loss. So to continue the example, you use 40 liters of water (lets say at 55 degrees) and replace it with cold water (lets say at 15 degrees), so you need to reheat 40 liters by a delta of 40 degrees, so you use 40*40*1.16 = 1.85kwh. You will then lose an additional 1.85kwh (for a 150 liter geyser, I think that is the SABS mandated number), so that in this use case (where you only use 25% of the water in 24 hours) you can save by switching to gas.

For my use case, where we use the whole 150 liters and sometimes more (5 people), the standing loss isn't such a big concern.

You do make a very good point though. Even in winter, where I clock a good 12kwh on the geyser, 16% of that is standing loss. Very very good point. I doubt it makes up for the cost of buying LPG in bottles, carting it to your house, and dealing with the lower efficiency (around 70% I think), but it is definitely something to take into account.

Of course, an electrical "tankless" heater will still beat the pants of LPG in these cases, at least for the moment :-)

@plonkster. Sorry this boertjie's English is not that good, and I am sure everybody saw that already. What I meant was that in addition to the 40lt of cold water, there will also be the standing loss.

So in my case 2.25kw will be losses and 1.85kw is what I really need to heat up my 40lt.. You are better off with your 16‰ loss..

Sent from my SM-N910F using Tapatalk

13 minutes ago, Jaco de Jongh said:

Sorry this boertjie's English is not that good

I wondered about that... :-) But I do deal with people on a local facebook group who seems to think that the geyser practically runs completely cold by the next morning. If that was the case, their gut feeling that "turning it off will save me money" is correct! If it ran cold that fast, only turning it on for an hour before bath time would literally save you hundreds of rands a month. The only problem is... it takes 8kwh to heat 150 liters of water from 15 to 55 degrees. If the standing loss is around 2kwh it means a quarter is lost. The relationship between temperature and energy is linear, so three quarters of the energy is still in there, or 30 of the 40 degrees delta. So in 24 hours, the geyser is still at 45 degrees :-)

Turning it on for an hour? Well, it only needs to put back 2kwh, and with a 3kw element that's going to take 40 minutes. The "switch it off and turn it on for an hour before shower time" recipe literally saves you no money. You end up putting back the standing loss in 40 minutes and after that all costs are the same.

Well... not completely. Newton's law of cooling says it cools down at a rate directly proportional to the difference in temperature. Or T = ce^(kt), where you can calculate c and k for your geyser by plugging in the initial temperature (T=55), the starting time (t=0), and then doing it again using the standing loss (T=45 and t=24 hours). So leaving it off during the day DOES mean it cools down slower... BUT... how much does that save you? Well, a small fraction of 2kwh... maybe 50 cents :-)

But you still make a good point about the tankless type of heater. One I will definitely keep in mind in future.

Edit: Well c=55 for my example, because when t=0, e^kt = 1, so c must equal the starting temperature. Working out k is a bit more problematic, need to use natural logs and all that :-)

You see what I say, I don't even have that symbols on my calculator😯

Sent from my SM-N910F using Tapatalk

6 minutes ago, Jaco de Jongh said:

You see what I say, I don't even have that symbols on my calculator

I put my HP 20S calculator on the roof of my car one day and forgot about it. I lost it somewhere in the street. I am SO upset with myself about that. Even now, years later, I am STILL upset about that. Thankfully, a similar calculator is only a download away with today's smart phones, and I don't have any problems with "you cannot use a smart phone in an exam" at this point in my life :-)

People post memes on Facebook about how another day passed where they didn't use calculus... well fair enough, but I find myself using Trig fairly often. When I made the frames for my solar panels for example... others would probably have used a protractor, not me, I calculated how long that piece of steel should be :-)

As a sideline. There used to be a chap here in Pinetown who made "donkeys" like this with an option for any fuel source and limited loss. He had designed them for the rural areas, an originally to run with a parafin heat (low heat) and with some thermocouples on the outlet for a small bit of electrical power (enough to charge a cellphone/small battery)... I was very interested, but then was out of touch for a while and his business disappeared/no longer around, but the concept was brilliant and when I tested back in 2009 it seemed like an awesome idea... I think he was trying to promote them to government, but the solar geyser option might have had a few extra "advantages" to the powers that be...

What about paraffin water heaters where you only heat the water you want to use. 

It takes 2 minutes to heat the initial water and thereafter you have a flow of hot water for as long as you have a flame.

On 2016/08/07 at 2:59 PM, Jaco de Jongh said:

Hi all,

I was looking for something on Gumtree and came across this.

http://www.gumtree.co.za/a-other-home-garden-items/george/hot-water-wood-fired-donkey-100litre-open-system-hot-dip-galvanised/1001051854190910002988709

Well build and this guy recons he can heat up 70lt water, 77 deg C in 55 min with 1.5kg wood.

Another company in South Africa that also builds them.

http://water-heaters.co.za/

I hope something like this might help the guys out on the farms and lodges (assuming that you have access to enough fire wood), and its cheaper than good quality solar geysers.  I also realize that most  guys out there on the farms already saw this or have something similar, but i just liked the idea and decided to share. I also grew up with the old "donkeys", but it was all home made and now someone realized how well they use to work and are building them a bit stronger than the ones I grew up with.

They look quite strong and well build and the prices seems fair.

I did a quick calculation for using wood in my case, We buy 9kg wood for R12 up here. So if I should heat up a geyser once a day using 1.5kg wood, it will cost me R2 per day.