Considerations about water heater installation for safety

General
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There are occasional questions on this forum concerning tankless gas water heater installation on Nonsuch sailboats. Responses to those questions have been answered by Nonsuch owners with experience associated with tankless water heaters on their own boats.

I have read the questions and answers over the years and thought that I should add my perspective, in an attempt to allow those considering installation of and those who have tankless water heaters aboard their boats, to be fully informed of potential risks while providing more prudent installation methods.

My qualifications in this regard are 45 years as a heating and cooling contractor specifically licensed in performing gas appliance installation and service.

The original tankless water heater installed on Nonsuch boats by the builder was the Paloma brand. These water heaters required a vertical gravity vent. Today’s replacement tankless water heater is by Excel and does not require a vertical gravity vent but does require Active Ventilation.

The Paloma water heater installation instructions clearly state that the Paloma water heaters are not to be installed on boats. I believe there are three reasons for this installation restriction.

  1. The enclosed cabin on a boat is a very small space.

  2. The available vent height is about one foot. This height reduces as the boat heels.

  3. There is a continuously burning pilot light.

The one foot of vent height will produce almost no draft under ideal conditions. What that means is vent products, produced by combustion of propane, will spill into the cabin from the draft hood located above the top of the heater. The small cabin space provides little ability to dilute the spilled combustion products before you breathe them.

To test for spillage from the draft hood:

  1. Start hot water flow, the water heater will start.

  2. Hold a lighted match at the edge of the draft hood. While the heater is firing.

  3. Observe the match flame. If the flame is drawn into the hood, there is adequate draft. If the flame blows away from the hood or is extinguished, there is inadequate draft. With inadequate draft, combustion products are entering the cabin.

  4. Perform the test with wind, without wind, with different cabin openings open and closed.

Do not block off the drafthood’s ability to spill into the cabin as that is a sure way to increase carbon monoxide production.

Some have replaced the Paloma with an Excel. Below is from the Excel installation instructions. Take note of IV Warnings 2. and 4. requiring an active ventilation system.

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What is active ventilation?

Active ventilation is a system that uses mechanical fans to provide air circulation in an indoor space. This type of ventilation is particularly important for ventilating spaces prone to moisture and spaces that collect too much heat (such as attics and garages). It also helps the house to be protected from potentially harmful odors and gases, which may accumulate in the storage of chemicals, equipment, and even vehicles.

What is passive ventilation?

Passive ventilation is a system that provides air circulation in an indoor space by using natural methods (for example, thermal buoyancy and airflow). This type mainly uses vents or windows to control and guide air circulation. Passive systems such as these can adjust the air temperature and introduce fresh air while expelling old stale air.

I’ll bet 100% of Excel installations on Nonsuch boats are relying on passive ventilation. Remember, active ventilation is required by the installation instructions.

Why is passive ventilation inadequate? When the wind is not blowing, even with hatches and ports opened, there will be little to no ventilation. Is the water heater capable of operation when there is no wind? As installed on Nonsuches, yes. That means sometimes/most of the time fumes are not removed properly from the cabin of the boat.

Some who have installed Excel water heaters have connected the Excel water heater to the original cabin top vent used for the original Paloma water heater. It appears from forum discussions that most have installed a sheet metal connector relatively tightly enclosing the top of the heater and connecting to the original Paloma through deck vent. I see three likely problems with this approach.

  1. This is recommended nowhere in the Excel installation instructions.

  2. What goes out the vent will be at a higher temperature as compared with the Paloma as there is no dilution air being introduced by the draft hood. This could lead to heat deterioration or combustion of the deck.

  3. The airflow up through the water heater will likely be lower than intended by the manufacturer’s design which will produce increased levels of carbon monoxide.

This is what I would do if I had a Paloma tankless water heater on my Nonsuch; REMOVE IT IMMEDIATELY! Remember, the manufacturer’s instructions indicate to not install it on a boat.

Now the question is, with what to replace it. There appear to be two likely choices. They are; an Excel propane fired tankless water heater or a tank type 115volt / engine heated water heater.

If I were to select the Excel installation, here is how I would install a ventilating system and how I would control operation of the heater to provide the greatest chance for safe operation while satisfying the active ventilation requirement indicated in the installation instructions:

  1. Install the Excel low enough in the boat to allow installation of the following items above it. Also provide 2” of clear space on each side and the front of the Excel to prevent combustion of combustible materials adjacent to the heater. Protect so no combustible materials can fall against the Excel.

  2. Install a fabricated exhaust hood above the Excel. The sides of the hood should taper to the central round opening at the top. Space the bottom hood opening 1” above the top of the Excel. The hood should be centered on the Excel and the hood dimensions should be 2” wider and 2 “ deeper than the top dimensions of the Excel. Do not connect this hood to the Excel. Connect with round pipe to the fan in 3. below and then with round pipe from the fan in 3. below to an opening through the deck. Adequate provision must be taken to prevent overheating at the deck penetration.

  3. Install a fan designed for installation in a gas vent system. This fan needs a proving switch.

  4. Install a manual switch that you turn on to start the fan in 3 above.

None of the items in 1 - 4 above are available from Excel. You will need to figure suitable devices and sources yourself.

Sequence of operation: Turn on the new manual switch when hot water is desired. The closed manual switch will start the fan in the vent system which will close its proving switch. The closed fan proving switch will enable the water heater through wiring that you will need to devise. upon opening a water tap, the flow sensor in the Excel heater will start the heating function.

The above is somewhat complicated, adds devices subject to failure and will require thought and effort. It is the only way I know to positively satisfy the Excel installation instructions’ requirement for active ventilation and to satisfy a desire to remove the combustion produced water vapor from the boat.

The other water heating option is the tank type heated by either/both 115 volt electricity (shore power) / diesel propulsion engine heat. These water heaters are available from more than one manufacturer, are used on virtually all boats other than Nonsuch boats, come with complete installation instructions, are well known by marine service technicians, provide fewer leak points for propane and by their type don’t pose any insurance related questions.

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I’ve noted that the heating vent pipes on my boats have always been wrapped with the same fiberglass insulation tape used on engine exhausts. The deck penetrations have to be cut to a size that accommodates the wrap.

I think that this is what Ward’s referring to.

– Bob

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I’m not about to contest your expertise Ward but as one of those who have been safely living with an Excel water heater for several years I might be able to add some real world observations and precautions.

The biggest issue I can see with your well thought out criticisms is that in the real world, on my boat anyway, the Excel never operates continuously for more than a minute or two (and I actually use my shower).
I have checked the temperature of my deck and vent piping and there is no issue for these short use cycles.

I have a hard wired carbon monoxide detector just outside the cabinet where the heater is situated and check it regularly. I’ve never had an alarm.

Also Excel heaters claim to have oxygen sensors and will shut down if there is an issue with low levels.

It’s always good to have informed advice and your post will make me recheck my installation for any shortfalls.

Thanks for the heads up.

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Paul,

Your point about intermittant and limited duration of use is valid. It also appears that in over 45 years of on demand water heater use on Nonsuch boats there apparently has not been injury .

Thanks for your input.

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Appreciate the detailed explanation, Ward. Just to add to the knowledge bank, our 1987 30U came with a Rinnai tankless water heater. Not sure why we had that brand when it appears everyone else originally had a Paloma.

We replaced the heater in 2014 with a new Rinnai (shipped it in from England in the misguided belief it would fit the same) and it came with a passive vent/hood that allows for mixing of cabin air with the exhaust. I will test it this spring to make sure it is venting as you suggest, but other than the pilot light, the unit is never on for more than a minute or so when it is in use and if we are not using it - for showering or washing dishes - the relay is off/no pilot light.

We too have a CO detector and whenever we are using the unit, close the small hatch next to the exhaust and have other hatches open. I may add a temperature check since my son gave me an infrared temp gun and this gives me another excuse to play with the tool.

But still, food for thought.

Thanks.

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Thanks Ward, and others for sharing your knowledge and experience with the tankless propane water heaters. I scared the c**p out of myself about 10 years ago when experiencing issues with the stubborn pilot lighter, and manually frigging with it, when it suddenly started with a small explosion and singed my beard and eyebrows. Stopped using it then. I had considered replacing with an Excel, and replumbing the 3 joined propane lines with a manifold in the propane locker and 3 separate lines. But I am trying to simplify and make the boat safer for ‘old man’ sailing in her current refit. So I’ve disconnected the propane, decided hot water is not a priority, and have used a countertop butane cook stove. I did see a C&C 33 recently with an Excel tankless heater mounted externally on the pushpit railing, which seems safe but not efficient. I am presently getting her ready to relaunch after a 5 year hiatus ashore and if I reconnect propane at all I would install a new single propane line for the original cookstove (with an updated regulator, auto shut-off connected to propane sensor, etc., and separate thru-coaming manifold for a pushpit bbq line). Maybe a small electric/engine effluent hot water tank if I feel the need for running hot water. I’ll forego the original catalyctic propane cabin heater and use my trusty Origo alcohol space heater at anchor, or electric heater dockside with shore power. A few other improvements on the go presently for relaunch this summer.