Geyser Blanket

So following a thread started by Cari Schoombe (can’t for the life of me find it again) which turned into an excellent discussion about geysers and their insulation, I commented that I’m going to buy a geyser blanket, which I did.

My geyser is a 150L horizontal, installed on my roof enclosed between 4 brick walls with a Nutek board covering the top.

So I have wrapped it as best I can (but not completely around it - bottom section is still open) including the pipes with that pool noodle insulation that comes with it. So this still left a cavity of half the size of the box inside which means lots of available air which is excellent for heat transfer/conduction, which is what we don’t want.

The plan is to fill up the remaining space with a roll of roof insulation/isotherm material so cold air cant get inside. Basically staggering enough sections until I reach the lid.

Anyway, doing that this coming weekend.

But during the past week I’ve been monitoring on VRM on how long the geyser takes to heat after one proper use every morning come bath time. Usually 2hours.

The geyser blanket alone shaved off 40 minutes alone! I am suitable impressed as the geyser now only takes 1h 20 min

Hope to still better this with the insulation I’m going to add. :grinning_face_with_smiling_eyes:

Watch this space for an update!

Me thinks someone shortened a shower to give the blanket a fair chance… :slight_smile:

Geysers are already exceptionally well insulated (most of the heat sneaks out using those pool-noodle covered pipes… aka lagging). A blanket will not add an extra 30%. It helps, but if I recall the numbers correctly, it’s single-digit percentages.

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There is only one bath in the morning for the wife. Water level fairly consistent. Son and I bath during the evenings.

I couldn’t believe it myself, but I’ve been monitoring it for a week. 30-40 minutes earlier.

Maybe I just have a very old not so well insulated geyser? Probably not. But there is a definite improvement.

Hey! Wont my VRM stats also show this? How long the geyser used to run vs the past week?

Aaah here we go. I knew I read a paper on this somewhere.

This looks at the percentage savings on the standing loss. You can save about 27% on your standing loss if you put on the blanket and the lagging. That’s just the standing loss, which in this test was 2.3kWh a day. So about 620Wh a day. That should shorten the reheat by about 12 minutes.

Edit: Fixed some numbers that were wrong… Also, 12 minutes is with a 3kW element. With a 2kW element it should be about 18 minutes.

Yes yes I have a 2Kw element.

So that is still a substantial improvement in my eyes! 2,3Kwh per day times 30 days = 69Kwh per month. :slightly_smiling_face:

Edit no wait, hang on… you are saving 620Wh per day, that is 18kwH or so per month, which at present Cape Town rates (lower end) is about R40 a month. At least the blanket will pay for itself in under a year :slight_smile:

Thanks, R40.00 a month down on my current electric bill of R250.00 is excellent!

And yes, for a measly investment of R220.00 lol! Happiness.

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It is good to hear of you success. I am a big believer in Negawatts.
A Negawatt is a watt saved, a watt that you didn’t have to make, because you no longer needed it.
I have spent a bit of time in the last few days considering heat insulation.
What is heat insulation?
Considering your geyser, the laws of entropy try to balance higher temperature energy concentration with the area of lower temperature energy concentration.
Heat insulation is a medium that slows that energy transfer down. It conducts heat poorly, between something of a non-ambient temperature and the prevailing ambient temperature.

The thing is though, the greater the temperature imbalance the faster the transfer happens.
Two identical cups of hot coffee, one I wait 10 mins and then add milk, the other I add milk and wait 10 mins.
The first cup will be cooler, try it and see.
It has been mooted that heating your geyser to a higher than normal temp during the day (using solar), so that it still be warm the next morning without using battery power at night was a hot water solution in one instance.
But as my coffee example shows, it will be quite ineffective given the higher temperature gradient across the insulation. You may heat an extra 25 degrees but only be 4 degrees better off come the next morning.
This has led me consider phase change materials combined with insulation.
I recently discovering that they could be tailored to almost any useable temperature.

Not only that, but I found out that one these chemically inert materials that can be tailored is paraffin. It also seems that if you want designer paraffin, you’d have to be quite lucky where you lived in the world. You would want live somewhere with a SASOL plant.
So a few boxes ticked already, it seems.

I am just spitballing, but lets just say your geyser tank was in a larger tank of paraffin and then that in turn was insulated. I have selected my paraffin to melt at 56 degrees C. I use solar power to heat my geyser, but instead of overheating the water, I put that energy into the latent heat of melting my paraffin.
Along comes the night, now I don’t have that big temperature delta anymore so the heat exchange process is slower. But at 56 degrees my paraffin starts to solidify (freeze).
To do that it must lose/transfer over 200kJ/kg+ of energy until it is completely solid.
It also maintains 56 degrees until it is solid.
Now the water that we want to keep warm has to lose 4.2kJ/kg to drop 1 degree.
Where does this latent heat energy go?
Well it escapes via the insulation and it transfers back into the water.
But it only escapes via the insulation at the rate dictated by the now lower temperature gradient. It cheats the system by effectively being a massive amount of energy with a small insulated surface area.
Only at 56 degrees, mind you, but it seems to me that that water is going to stay at 56 degrees for a long,long time.


Another method would involve a vacuum? How effective will a vacuum be?

Very interesting Phil.

Very effective, but very difficult to maintain and expensive to build.

I am talking in relative terms to retrofitting a geyser inside a 500L Jojo tank, a 210L drum of paraffin and a few rolls of Rockwool and chicken mesh to wrap the Jojo on the outside.

Edit: For a small geyser just use the 210l drum and wrap that. Cheap as chips.

… and hope it never needs maintenance or repair :slight_smile:

There is also a little trick with pipes as they enter and leave a geyser called a “heat trap” worth reading up on.
Every little helps.
If you look after the nilliwatts, the Negawatts look after themselves. That’s what I always say.

What about simply building a wooden box around the geyser and line it with the insulation materials you have in the roof? That might achieve most of the results for less cost and more maintenance friendly. I’ve no scientific reason to suspect this other than intuition.

The heat trap is very interesting. Basically here you want it. In a chimney you don’t want it (cold plug), so you can get it out by first starting a small fire which burns hot and clean (otherwise you might smoke up your house before the plug is out.

More insulation is obviously better.
We are both talking about storing energy more efficiently, but using two different mechanisms.
Let’s do some sums, with a few assumptions.

The ambient temperature and the cold water is 15 deg C.
The geyser heats a volume of 100L to 60 deg C.
(Fact: It takes about 4.2kJ/kg (ltr) to heat water/degree).
Energy needed (60-15) 45 * 4.2kJ* 100 + losses = 18900kJ
1J/sec = 1 watt. 3600J = 1 watt/hour, therefore 18900kJ = 5.25kWh
I deal with losses later.
With me so far?
OK, @plonkster’s paper talked about 2kWh of leat loss/ day. (I’ll stick with that number).
So our 60-degree water if left alone, will have lost 7200kJ.
(Note 1: This is not strictly true because the study wouldn’t have left it alone but replenished the losses through the thermostat function, but let’s work with this number because it is what we have).
That 7200kJ loss is an indication of the quality of the insulation. A geyser blanket will improve the quality of the insulation and make those losses less in a 24hr period.
There is no way around supplying the energy needed to heat the material. But in my special case, I don’t care about that because it comes from the sun. What I care about is needing to replace that 2kWh of energy from my battery, and the cost and inefficiencies in that process because I want hot water the next morning and the sun hasn’t been shining for the last 12 hours of the night.
I am getting my energy very cheaply and efficiently, but I only have 18900-7200 = 11700kJ left of what I stored as thermal energy.
My thermal energy storage (11700/18900) is only 62% efficient.

Right, now I am going to strip the insulation of a similar geyser and fit it in a Jojo tank with 210 L of my phase change 56 C degree paraffin.
I am going to assume the same quality of insulation, but there is a bigger area so let’s say the losses are now double 4kWh/day. ( guessing here, but you’ll see they could still be a lot higher and this would work).
I come along with my free solar energy in the day and I have to expend the same amount of energy to heat the water 5.25kW/h.
So far so good.
But now I need to heat up that paraffin as well. (Fact: It takes about 2.5kJ/kg (ltr) to heat paraffin/degree).
So 210 litres * 45deg * 2.5kJ/kg = 23625 kJ or 6.56kWh of sensible heat.
Why do I call it “sensible” heat?
Sensible heat is actually a real term that is used to mean the type of heat that changes the temperature of a substance. It is the opposite of latent heat, which does not change temperature only the state of the substance.
So we have to factor in the latent heat we had to supply to the solid paraffin to melt it at 56 degrees.
(Fact: It takes about 236kJ/kg (ltr) to melt paraffin 26 at 56.3 degrees).
That is 236 * 210 = 49560kJ = 13.77 kWh.

So to heat up my geyser takes 5.25 + 6.56 + 13.77 = 25.58kW/h.

Ah, but I am now having to make 25.58kW, not 5.25kW everybody cries!
-No you’re not, you are just storing that amount, you don’t have to make it every day, just build up to it, you only have to replace what you use. It’s just like having more batteries, doesn’t mean you use more power.

Ah, but I am losing 4kWh, not 2kWh everybody cries!
-True, but consider it further:
I have 21.58kWh stored energy the next day. My storage efficiency is now (21.58/25.58)= 84%.
What’s more in the first example my water temperature would have theoretically have dropped 17 degrees,( see Note 1 above), whereas using PCM it will still be 56 degrees.
In fact, the maths would show that cold water could be added the next day and without any solar the next day and it would still heat up to 56 degrees. No insulation in the world could do that.

Ah, but paraffin is a nasty chemical and flammable, I don’t want it around my water supply or electrics, everybody cries!
True, I am calling it paraffin because that is what it is. It is flammable, but not explosive, you may be more familiar with its use in your load-shedding candles. It is a chemical yes, it’s that red coating on your cheese. It’s wax and it is also an excellent electrical insulator. Please don’t stop eating gouda or candles on birthday cakes.

It is a bit difficult to get your head around, a liquid having so much more energy than a solid at the same temperature.
Maybe this pic will help, just think of that flat line at 56 degrees.
There is the same amount of energy transfer that happens in ice>water as is needed to heat that glass of water to 80 deg C.

It’s quite mad actually, and it works in both directions. For example, I could use typical night-time temperatures to freeze a substance at say 15C, and then run a fan through it, instead of an AC in the daytime for cooling.


I would like to be convinced of this. I have a Kwikot 200l horizontal geyser (with blanket) and a Geyserwise controller.
I switch the geyser off at bed time and then on again in the morning. The temperature drop on a chilly highveld night is substantial according to the Geyserwise readings.
I concede that measuring the overall geyser temperature what with layering of the hot water confuses this issue but I am yet to be convinced…

Thanks, I understand the explanation and the reason you want to use a PCM and appreciate the maths, will definitely have a more detailed look!

Phase change cooling is something I’m reasonably familiar with, not just the sweating kind (humid environment are horrible because it just doesn’t work and my wife - girlfriend back then - lived in Richards Bay), but in computer cooling system to get sub ambient temperatures.

What I’m suggesting is that there might be ways to insulate better (vacuum/box lined with insulation material) that might have practical benefits while not too much downside. Encapsulating your geyser in a liquid/solid could make changing the termostat, for example, quite a challenge.

I see it as a bit more than evaporative cooling, as you point out that has a lot to do with equalizing humidity as much as anything else.

Don’t get me wrong I am a proponent of insulation, by all means, use it where it is fitting.

Enclosing the geyser in whatever material will make reaching the thermostat challenging. As long as it is on ground level I will manage.
These things are relative.
I see nothing more challenging than rafter skipping in a 60 deg C ceiling as a 60-year-old even to an uninsulated geyser. That is a place I will no longer go.

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Check out the newer B+ rated geysers from like Kwikot and others.

I have one and on a rainy ice cold night in Cpt, the geyser is ice cold on the outside, inside like +60Deg as per Geyserwise temp probe.

Methinks, geyser pops, get a B+ rated one, THEN see if the rest makes a difference.

Yes I get the gist of what Phil is saying, even though the maths is a little over my head though but that doesn’t matter. Seeing as the paraffin concept is akin to Phil not wanting to crawl in hot dusty ceilings at awkward positions and angles, this is something only serious enthusiasts would do. If my setup was different, I would’ve definitely considered it.

So leaving us with the insulated box/capsule idea, though obviously not as effective.

I have now gone one step further yesterday as said above. My geyser’s box is now completely filled with 10mm roof insulation (the kind with tinfoil on the one side of the spongy material) leaving almost no space. So I trust that the cavity is now even better insulated.

I will report back here on my rudimentary, non scientific and crude (but non placeboeous) observations. :grinning_face_with_smiling_eyes:

Maybe, just maybe I reduce the warming up time by another 5-10 minutes.