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Hot Water Storage Tanks

Many buildings have hot water storage tanks. While wrapping a standard tank with an insulation blanket does provide some additional protection against heat loss it is far below the efficiency of a highly insulated tank. In addition, care is needed to insure the blanket does not cause tank damage such as condensation build up.

Tankless coil oil boilers were thought to be a good idea, with no tank there are no stand by losses associated with a tank.  The problem is there is an internal tank, about 10 gl, that is kept about 180 degrees 24/7/365.  This is a huge waste of fuel and in the non heating months causes boiler short cycling and lowers operating/fuel efficiency often to 25% or less.   One of the most cost effective upgrades is to add a tank and cold start the boiler. 

Highly insulated tanks normally are made of steel, poly/plastic or stainless steel. In all cases they offer a superior level of insulation, often in the form of foam. Not all are created equal and there are significant differences in heat loss. Large capacity tanks are often used for solar hot water storage. Smaller units supply basic domestic hot water needs.

We always use high efficiency tanks for solar hot water storage and there are a number of excellent choices. Factors to consider include:

  • Weight of tank – heavy tanks are hard to move
  • Size – will it fit though all doorways. Will tank and any plumbing exiting at top fall below ceiling or joists?  Is there enough floor space.  Should it be raised on blocks for protection? 
  • Degree loss per day – Some tanks are as low as 5° loss in 24 hours
  • Glass lined – bumps can cause hairline cracks, then rust & leaks, care is needed in transport, heavy
  • Stainless steel – very durable, expensive, excellent for corrosive well water
  • Poly (Plastic like) – lightweight, excellent for corrosive well water, inexpensive, no dual coil models
  • Warranty period – is warranty transferable?

Some tanks have built in electric elements. We often use these as solar backups, since they do not come on often. Some tanks have dual coils. This allows two different “circuits” or "heat exchagers" to route through the hot water.  For example in a solar thermal hot water system, the lower coil will have the solar heated hot glycol that is used to heat the water in the tank.  The second coil may be for a radiant floor system or a boiler backup.  Some tanks have many coils and multiple inputs/outputs.

If you have a standard water heater, replacing it with a highly insulated unit could dramatically cut your fuel bills and have a payback time of just a few years. Most are energy star rated and are eligible for government tax credits.  As part of a complete solar hot water system you tank would qualify for the solar incentives

marathon heater


We often recommend Marathon tanks.  They run about double the cost of a standard electric tank, but have a lifetime warranty to the original homeowner and a heat loss as low as 5° per day.  They quickly pay for themselves.  They are also eligible for the Efficiency Maine weatherization program and federal energy star tax credit that is set to expire 12/31/13.


Heat pump hot water tanks are gaining in popularity because the price is now reasonable.  A heat pump moves heat it does not create it, so it is very energy efficient.  A heat pump on a hot water tank draws heat from the room and uses it to warm the water.  They vent cold dry air into the room which can be a real bonus in the summer if you have humidity issues in your basement.  Recovery time is a bit slower with these tanks so they add an electric element for times of unusual demand.  We also over size the tank a bit to insure it runs in heat pump mode, not electric backup, the majority of the time.   If the tank will be in a conditioned living space you may not want the cold dry air venting into the room in the winter.  You can simply switch the heat pump feature off for a couple of months.   In some installs we have added a control so you can allow the boiler to provide hot water in the dead of winter, since it is running anyway, and switch off the heat pump tank.  The heat pump will make a bit of noise, similar to a boiler.  We like the AO Smith line.  Their new 50 gl tank has a energy factor of 3.24 which means for every unit of electricity in you get 3.24 unit of heated hot water.  With a regular electric tank it is 1 to 1.  These tanks are 324% efficient and that is why they are so popular.  


Nyle heat pump


Nyle makes a stand alone heat pump that can be attached to most tanks.  This is a great option for when you need a larger storage tank.  It makes a great backup for solar hot water systems.  Like the heat pump hot water tank this also discharges cold dry air.  The units are super energy efficient.


For more information visit Home Power they have a great article on their website.

A very common upgrade is to install air source mini split heat pumps to provide space heating in the main living space and we install a heat pump hot water tank to meet the domestic hot water needs.  Then the boiler is simply switched off the vast majority of the year until such time as the mini split cannot keep up with the homes space heating needs.  

We like the A.O. Smith Voltex which comes in both 50gl and 80gl sizes.  There are a number of makes at the big box stores in 40-50 gl. sizes.   Stiebel Eltron Accelera is a very nice tank and it comes in larger sizes but it is significantly more expensive.

$300 incentive on Heat Pump Tanks from Efficiency Maine


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