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Adding Insulation to a Hot Water Tank

By Bruce Barbour - June 2020

I have recently added an extra layer of insulation to the hot water tank of my Sanden heat pump system.

Please note that the hot water service has a separate tank to the compressor. The method would be unsuitable for a combined unit where the heating system and the tank is combined in the one unit - be that a heat pump compressor or a gas heater.

I already had most of the material necessary for the project. I only had to buy a roll of tape and a tube of silicon. The availability of the materials determined some of the choices in the method of insulation. If different materials had been available I might have used those in preference to buying new materials. There are many ways of doing this. This is just one.

Original Sanden Hot Water System
The original hot water system.

Note that I had added additional insulation to the pipe work some time ago, soon after the unit was installed.


HWS
                      with top cap
The first step was to cut the polystyrene top hat. It is 150mm polystyrene as that is what I had available from the house building. Diameter was the diameter of the tank plus twice the thickness of the insulation, allowing an additional bulge for the section of tank that had the pipes sticking out of it.
Tank covered with insulation
Stuck the insulation on with a combination of silicon sealant (used as a glue) and clothe tape. Originally I was going to use R2.5 wall insulation (as that is what I had) but that was not flexible enough to shape around the tank so I split the batts down the middle making them R1.25 batts.
Tank covered without top cover
Wrapped silver foil around insulated tank. Used an upper layer and a lower layer. Attached the upper layer to top hat with long screws with washers. Used further tape to hold in position.

Used four pieces of foil for ease of handling.

(Only used foil because that was what I had. It would not be worth buying a roll to do this. Could use other plastic sheeting.)
Final arrangement
Cut a round piece of foil and covered the top, securing to the side of the top hat with long screws.

Note that I also cut a slit in the insulation and coverings so the Pressure Relief Valve (PRV) was not prevented from operating and could be accessed.

Effectiveness

From the data on the Enphase system it appears that the insulation will save about one quarter to one third of a kilo Watt hour per day. (I had hoped for a half kWh per day saving.) If I was using normal grid electricity this would save about $32 per year. However as I have a photo-voltaic system my electricity cost for at least some of the electricity used for heating the water is less than grid electricity. Consequently I estimate my cost saving as $16 per year.

My extra material costs were for some tape and a tube of silicon sealant. This will be paid for in the first year. If I had to buy all the materials - at a guess - this would cost approx. $100. This would be paid for in about six year. However if I costed my labour the equation would be different. I spent at least 8 hours over 2 days doing this. If I costed my labour at a very cheap $30 per hour the total cost would be $340. It would pay for itself in 21 years - so not financially viable. Luckily my labour is free - to me.

Other Comments

A real cheap and dirty way of getting some extra insulation on the tank would be to use an old doona quilt. Wrap over the top and around the tank. Tape. Water proof with a large heavy duty plastic bag such as a bag that would come off a single bed mattress. Tape in position. It will probably only be partial tank insulation. Ensure PRV is not interfered with. Not very pretty nor as good as what I did but very quick and cheap.

Couple of issues to be aware of. If you were considering insulating your tank, do this at your own risk. The information on this page is provided for general interest as a description of what I did and some other musings, not a recommendation to do it. I don't know whether the insulation would impact on tank warranty. If I had to make a claim I would probably remove the insulation prior to making the claim. I also realised after installing the insulation that I had potentially made 5 star mouse accommodation (a lovely heated area). I will have to monitor this and adjust if it is actually an issue.

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