If vacuum is the best insulator, why don't water heaters use it?
Vacuum sounds like the best way to insulate. In a vacuum there is no air, and therefore no heat loss.
But when it comes to water heaters, things are not that simple. While vacuum insulation works wonders in a Thermos flask, it is rarely used in water heaters.
The way we insulate water heaters needs to balance performance, durability, safety, and cost. And that is where foam wins.
1. A vacuum can be a safety risk
To insulate with a vacuum, you need a double-wall, strong enough to hold negative pressure without collapsing.
You can make a thin vacuum wall for small items like flasks, as your coffee isn't under high-pressure. But if it involves a large, high-pressure water heater tank, vacuum walls have a high chance of exploding.
Foam is more stable than vacuum. It has an added benefit of being able to reinforce the tank, making the heater safer.
2. Vacuums are hard to maintain
Vacuum flasks can explode too, if the pressure build up inside is too high. It can happen if gases are produced inside by bacteria. Because the vacuum-insulate is so sensitive to pressure, the defect rate and cost to maintain goes up.
If a vacuum seal is even 0.1% damaged, it would fail completely and leave the heater with zero insulation. On top of that, it is impractical to repair on the spot. Chances are, you will need to buy a new heater.
If a normal heater costs about $100/year to maintain, a vacuum heater will cost you around $500/year to maintain. You will be paying a premium for highly skilled technicians to do delicate work with the high pressure equipment.
I have chosen AOSBath Digital due to its thicker thermos insulation and the instant water function. It has been 6 months of use now. Some times I forget to switch on the heater but it still delivers hot water. We are glad to have found AOS, and I am happy with the decision.
Cheryl Law
Google review
The water heater is amazing. Hot water that never fails. It can keep hot water for long periods of time. Saving me some money of not needing to switch it on again.
said ishak
Google review
3. Vacuums are too expensive
Imagine you have a water heater that insulates with vacuum, but it costs $5000. Will you still pay for that? For me, it won't make sense. The performance gain is not worth the price jump. Foam is a far more cost-effective and easier material to work with.
It's not any ordinary foam we are talking about. See how it is injected into the tank to fill every cavity, creating a tight, consistent seal? This foam doesn't need maintenance and lasts for years.
I have chosen AOSBath Digital due to its thicker thermos insulation and the instant water function. It has been 6 months of use now. Some times I forget to switch on the heater but it still delivers hot water. We are glad to have found AOS, and I am happy with the decision.
Cheryl Law
Google review
The water heater is amazing. Hot water that never fails. It can keep hot water for long periods of time. Saving me some money of not needing to switch it on again.
said ishak
Google review
Bonus: what about other materials?
A vacuum insulates well because it traps air. Foam traps air too. What other materials insulate by trapping air? We look at styrofoam and aerogel.
• Styrofoam is not ideal for water heaters, because the air pockets in Styrofoam are larger and less tightly sealed. Air can move out of the styrofoam.
• Aerogel is 99% air but insanely expensive and brittle.
At the end, you want a good balance between cost and performance. The high-density foam AOS uses is still is the best material to insulate.
Been using another brand of heater for over 5 years and always felt that the insulator is not working well, the heated water always cools off by morning time. After using AOS, there is definitely difference in insulation and the water remains warm even 24hours later!
Katherine Lim
Google review
What this means for you
Good insulation is money-saving. It locks in heat at no cost to you.
The water in your heater should stay hot, even if it is is switched off.
You want it to retain heat for hours, or even over a day!
This post was inspired by a bunch of fascinating questions, each one pushing us to think harder and explain better. If you have ever wondered how something works, do ask us! Good questions lead to better answers and that's how our community grows!
Insulation FAQ
What are R-values and why they matter?
R-values measure how good insulation really is. It is like a scorecard for insulation. The higher the R-value per inch, the better the material resists heat flow.
• Styrofoam: about R-3 to R-4
• High-density foam: around R-6 to R-7
• Aerogel: around R-10
Fragile, expensive, and not practical for water heaters.
What is Closed-cell vs Open-cell foam?
Not all foams are created equal. The difference between a mediocre insulator and a top performer comes down to the cell structure. Is it open-cell or closed-cell?
Open-cell foam is like a sponge. The bubbles inside are connected so air can move around inside. It is light and cheap, but lets heat escape easily.
Closed-cell foam is completely sealed. Each bubble is isolated and packed with special gas, not just air. Because they can't move, they are great at retaining heat.
AOS water heaters use high-density, closed-cell polyurethane foam:
✔️ Heat stays in
✔️ Moisture stays out
✔️ Performance is good for years
How to tell if insulation is good before buying?
Insulation is often overlooked, but it plays a huge role in saving energy.
Unfortunately, there is no standard or rating system for insulation in water heaters. Some manufacturers may cut corners, and you would never know just by looking.
what can you do:
• Ask questions
• Read reviews
• Look for trusted brands
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