September 14, 2015

New Water Heater

We occasionally participate in various boating forums. [See our Some Forums We Read sidebar for links...]

Some of those topics may be relevant here, so once in a while we will repost on our blog for reference.

In this case it relates to one of our spring 2015 commissioning projects, the new 40 liter IsoTemp SPA water heater... [Photo below; Installation Manual]

Original Forum Post: [3-Mar-2015]

Re: Gotta get a new water heater

Originally Posted by Moparots View Post
Hello all new boat owner here, I pulled out the old rusty leaking water heater on my 36Morgan OI. Previous owners had it set up for AC from shore power only. What is every ones opinion on new water heater brands?

The old one was not set up for a engine heat exchanger, how easy is it to install that? Hoses from the water pump? (Westerbeke 44AFour)

Can I get a water heater with the heat exchanger option and not use it? maybe hook it up at a later time.

Looked at the Kuuma, inexpensive. simple 6 gal capacity.

Just looking for every ones thoughts.
I am preparing to do the same thing. After researching I went with an IsoTemp SPA Model [Polypropylene outer jacket vs stainle$$; Mfg# 6P4023SPA0003] for several reasons:
  • Built-in mixing valve [Prevent scalding when engine heats it to 180F.]
  • Super insulated [hot tank purported to last 24+ hours by other owners]
  • SS tank and heat exchanger [no zincs...]
  • 750 watt [1200w available] 115 or 230VAC electric element [half of the usual 1500 watt]
    True, slower to heat from electric, but less load on shore power/generator... And you can set thermostat much higher than usual e.g., 160F to take advantage of that mixing valve and get more usable hot water per charge
  • Mounts Horizontally or vertically

I'll post a blog entry when I complete that project in early May.

Here is a link to Defender's listings FYI.

I hope this is helpful.
SV Denali Rose
Few opinions; only choices based on facts & experience.

Follow-up post: [30-Jul-2015]

Re: Gotta get a new water heater

Originally Posted by Admiral Nelson View Post
I just installed an Isotemp SPA water heater 8 gallons and found that the water does not stay warm for more than 12 hours. Isotemp claim to be 24 hours which has been my experience so far. I was wondering what was everyone's experience with their tank. You have hot water for how long?
Hi Admiral,

We recently completed the installation of the IsoTemp 40 liter Spa water heater I mentioned earlier in this thread. [Mfg# 6P4023SPA0003] My Admiral is very pleased with the results [It replaced a 6 gallon perfectly functional Raritan unit...]

[For those researching new water heaters: We paid US$520 - including USPS shipping to Alaska- for the IsoTemp Spa water heater from Fisheries Supply. This seems to be much more in line with other water heater pricing than I see discussed in this thread...]

You didn't mention the specifics of how hot the water is [or isn't] before and after your 12 hour period of heat retention, nor your usage once the heating source is off. You also didn't mention some of the other environmental factors that will influence your results.

For example, on our boat the engine heats the water to 180° - 190°F and I have the mixing valve [the one that comes installed on the IsoTemp] adjusted to 120°F [the temp of the water you get at the taps.] This mixing of 'superheated' water with cold to achieve the desired temperature at the tap yields 2 to maybe 3 times the tank volume of usable hot water (meaning pleasant showers...)

These are our results, which are likely lower than those of you with warmer 'cold' water per the following:

Note: The ambient water temp of our tanks- which lie against the hull- approach the ambient water temp we boat in- currently mid 40's F or cooler in winter and Mid 50's F in summer... If we're lucky... Mixing warmer cold water will yield even better results... [e.g., 70°F+ cold water would yield 3- possibly 4 times the mixed hot water volume vs. the 2-3 we currently enjoy...]

Regarding heat retention as relates to insulation: Our water and space heaters are installed in the enclosed engine room (which averages 110°F ambient temp while motoring.) When we anchor, we still have hot water 48 hours later if little has been used. If more is used (i.e., if more cold is mixed with the superheated tank of water- say a couple of showers) then of course it doesn't last as long- perhaps 24-36 hours- but we are still gathering data... We find it plenty for 4+ hot showers [2 adults; pausing water flow while bathing; still having to mix in some cold water to get desired temperature...] over a 3 day period, and dishes, etc.

To maximize our hot water retention at anchor, when possible we shower, etc, while motoring so the water heater has enough time to reach maximum temp before shutting down the engine.

One aspect of our install regarding longevity of hot water is our engine room also warms up to 80°F+ when we run the Espar heater, which is also installed there... The engine room rarely cools off below 60°F [even in 20° to 30°F ambient outdoor temperatures] and then only after days at anchor with no or little heat demand [e.g., sunny days in winter- below ~60°N- in our pilothouse reduce the run time for the Espar during those periods...]

This 'warm' engine room environment reduces the burden on the IsoTemp insulation.

If yours is cooling off prematurely without you using any or little hot water, try insulating the hot lines immediately adjacent to the heater, as some conductive heat loss occurs there... You could also further insulate the tank itself if it resides in a cool environment.

You may also want to check the cold water line into the heater after an hour or so of no water being run. If it is warm or hot a small distance from the water heater, you may have a thermal siphon situation and need to install a check-valve or re-route the water line(s).

I also recommend you do some temperature measurements and track hot water usage after the heat source is off so you can estimate the heat loss factors.

An IR thermometer (or IR camera, or low tech version: your bare hand...) will reveal any insulation deficiencies in the IsoTemp insulation layer.

I hope this is helpful.
SV Denali Rose
Few opinions; only choices [good and bad...] based on facts & experience.

This is a work-in-progress photo of the 11 gallon IsoTemp water heater install.
[Horizontal grey cylinder at top of photo. The red pipes are structural support for new water heater; not plumbing.]
The 6 gallon water heater [lower left] it is replacing is still in use in this photo.
The red heater hose on the sole is the rough-in for a new Dickinson Radex 'bus' heater being installed in the pilothouse.
Additional References:


  1. Thanks for the info in the post. I am looking into installing an Isotherm SPA 30, and I am glad to read the positive comments regarding the SPA model line. Also, the discussion on water temperature longevity was very informative.

    1. Thank you Paul. I'm glad you found some useful information in this post. [My reply is delayed because we just returned from a trip which demanded most of our time...]

      I see from your blog [] that your install went well.

      Nicely done.


    1. Paul, Thanks for [scanning and?] publishing the IsoTemp SPA installation manual. I was unsuccessful finding one online.

  3. Paul, how long does it take for the water to get to 160 degrees?

    1. Bolivar, It doesn't take very long.

      Once the engine gets up to temp, the water heater output is hot enough to use within 30 minutes or so [I never timed it, but may do so if I can remember when I start out cold...]

      It likely takes another hour or so to achieve the same temp as the engine coolant, but I haven't measured..


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