Water Heater Installation in a DIY RV Van

Marine Style IsoTemp Spa Van Water Heater Install in our Promaster:

Hot water was a non-negotiable in our DIY RV Van design. You can see from our floor plan that we had designed a wet bath with a shower, and this definitely required a hot water heater installation in the DIY RV. We considered on demand water heaters, both electric and propane. Electric on demand water in the RV would require a large battery bank and an equally large inverter. It would also consume large amounts of Amp-Hours from our system. An on-demand propane water heater would require special venting or even being mounted on a door that could be opened, as the portable ones are not meant for interior use. A traditional propane fired RV water heater is expensive, requires a lot of interior space, and also requires a propane system. We were wanting to do a Promaster van build without a propane system at all.

Let me introduce you to the IsoTherm IsoTemp Spa 15 marine style water heater. This water heater connects to the engine coolant system and heats fresh water with a heat exchanger inside the tank. This is a fantastic option for us. The unit can be operated without any electricity or propane. The water begins to heat as soon as your engine is up to temperature, and the well-insulated tank keeps the water hot for hours. In addition, because the water is heated to the engine coolant temperature, 180 to 200 degrees, the unit includes an adjustable mixing valve to adjust the temperature of the water being provided by the unit. In our case, the 15 liter (about 4 gallon) water tank, heated to a high temperature, and then mixed down to a safe level, will provide much more than 15 liters of hot water. Other vanlife users are reporting hot water 36 hours after the van was last driven.

Tank Connections

The IsoTemp tank has four connections. Two connections are for the engine coolant. It does not matter which connection is in or out. There is a connection for cold fresh water in. The last connection is for hot water out from the mixing valve. An additional drain is on the cold water input side to allow the tank to be drained if freezing is possible. The modifications required at the engine compartment are simply installing a couple of tees into the lines to the heater core, and then routing new hoses back to the water heater. I will go into this process in much more detail below.

IsoTherm IsoTemp Spa 15 Water Heater
Connections on marine style IsoTemp Spa water heater in our DIY RV Promaster van

Engine Compartment Connections

In my case, I cut the short hoses close to the firewall where they run to the heater core. I did not remove any of the factory hose, but simply cut the hose and inserted a 3/4″ x 5/8″ x 3/4″ barbed tee into the old line. The 5/8″ barb connection would direct water to the IsoTemp Spa 15 water heater. Immediately after the 5/8″ barb connection, I added a short length of 5/8″ heater hose, and then I attached a 5/8″ ball valve. This was done in both of the heater lines near the firewall. I used 5/8″ heater hose because this is the minimum size recommend by IsoTherm. Standard stainless steel band hose clamps were used to tighten all hose connections onto the tee and ball valves.

Engine connections for IsoTemp Spa water heater
Tees are inserted in short hoses from firewall

Routing Hoses

From the ball valve, the heater hoses were routed down under the firewall to the bottom of the van. The hoses pass under the driver side foot area and under the battery box. The hoses were encased in foam 1″ pipe insulation and secured against the bottom of the van framing on the driver side. The pipe insulation would help to prevent heat loss as well as protect the hoses from road debris. Galvanized strap and self-tapping screws were used to hold the insulated hoses against the van frame members. The hoses were routed to stay clear of the muffler and kept a reasonable distance away from the exhaust system and the heat it might generate.

Hoses from engine compartment to IsoTemp Spa marine water heater
Hoses route down under driver side, shown encased in pipe insulation

In our DIY RV Promaster van design, the IsoTemp water heater was being located over the rear axle in the center of the floor. This put it between our fresh water and grey tanks, which are under the fixed bed. It was a fairly long run of 5/8″ hose to reach that location, but it has been working fine and heats quickly.

It was easier to mount the necessary fittings on the tank before the tank was bolted to the Promaster van floor. IsoTherm recommends using Loctite PST 592 thread sealant on the hot connections. We did as directed and installed two 1/2″ NPT female to 5/8″ hose barb brass fittings on the IsoTemp Spa 15 tank. We used 592 sealant on the hot water out, but this turned out to not seal well. So, I would recommend teflon tape and plumbing pipe thread sealant on the cold water in and hot water out fittings. On the hot water out, the threads were 1/2″ BSP on our unit, which had the mixing valve. A 1/2″ BSP female to 1/2″ NPT male brass adapter was installed on the hot water out fitting. A 1/2″ backflow preventer was installed on the cold water inlet to prevent hot water from leaking into the cold side of our system. These two fittings would allow a 1/2″ pex swivel adapter to be easily installed on the cold and hot water fresh connections.

Water Heater Location

It is important to carefully locate the water heater with respect to framing members under the van floor. Only certain locations allow holes to be drilled through the floor for access to the bottom side. We located our IsoTemp water heater so that the 5/8″ hoses would pass through the Promaster RV van floor just in front of the rear axle. We used 3/4″ pex metal bend supports to make gentle bends in the heater hose. These supports were used inside the van on the hoses from the tank to make a bend and go through the floor. Two more metal bend supports were used under the floor to gently bend the hoses back to horizontal. In fact, in our install, the metal bend support extended slightly into our holes through the floor, protecting the hoses from the metal edge of the hole. High temperature RTV sealant was used to seal the gap between the hoses and the holes in the floor.

Hoses from IsoTemp to underneath van
Hoses and bend supports exiting van living space in front of rear axle
Water heater hoses in pipe insulation
5/8 hoses in pipe insulation secured to Promaster bottom frame members

Filling the System with Coolant

Before connecting the long 5/8″ hoses to the ball valves, a funnel was used to prefill the hoses and water tank, as much as possible, with Mopar Coolant. This helped to remove most of the air in the new system before it was connected to the engine coolant lines. After all connections were secure, the engine was run up to temperature, then allowed to cool. After this process, additional coolant was added to the reservoir under the hood. The bleeder valve screw on one of the heater hoses from the firewall was also used to release some of the air in the system. This process of running the van to heat up the coolant system, letting it cool, then filling the coolant tank back to max was done several times. I also found a couple of slow leaks at the barb connections that required the hose clamps to be tightened slightly. I would recommend that you check these connections very carefully the first few trips that are taken with the new water heater system installed. Our installation needed less than a gallon of additional coolant to fill the hoses and IsoTemp heat exchanger to bring the system back to full.

Ball valves for IsoTemp water heater connection
Brass ball valves to allow disconnecting the water heater system

The IsoTherm IsoTemp Spa water heaters also can be plugged into 120 volt shore power to heat water if needed. We will rarely use this option, but it is nice to have a backup. However, based on other vanlife comments, the heater may trip a reset button when water is heated from the van engine. This reset button may have to be depressed before the 120 volt AC system will work. I drilled a finger sized hole in the cover at the location of the reset button in order to make it easier to access. The green switch mechanism in the photo below has a small red push button switch that will sometimes need to be accessed. The photo below shows the water heater access cover removed.

IsoTemp water heater fully connected
IsoTemp Spa fully connected, reset is small red button on blue-green switch assembly

We are very happy with the IsoTemp Spa van water heater install for our DIY RV van. The water heats quickly, and the output temperature can be easily adjusted. We are able to generate hot water while driving, and then hot water is available to wash dishes or shower once we stop for the day. We can avoid installing a propane system, and our battery bank does not have to be abnormally large. This heating process is quiet and efficient. If hot water is desired in your DIY RV van build, I would certainly recommend a marine style water heater such as the IsoTherm IsoTemp Spa series.

Products and Tools used in this project:

IsoTemp Spa 15 Water Heater 15 liter (~4 gallon) marine style water heater with an adjustable mixing valve to control hot water output temperature
BSP to NPT AdapterConverts output of mixing valve to standard 1/2″ NPT male for attached a pex swivel adapter.
Backflow Preventer Prevents hot water from leaking into cold water plumbing system
5/8 Barb to 1/2 NPT1/2″ NPT female to 5/8″ hose barb to connect heater hoses to water heater, 2 needed
Loctite PST 592High temperature thread sealant for engine coolant fittings on water heater
Teflon TapeTeflon plumbing pipe tape for BSP adapter and backflow preventer on water heater, use with plumbing thread sealant
Thread SealantPlumbing pipe thread compound sealant for BSP adapter and backflow preventer, used with teflon tape
5/8 inch Heater HoseHeater hose for runs between water heater and engine bay, this is a 50 foot roll, and I need about 40 feet
Stainless Steel ClampsClamps to secure 5/8″ heater hose to 5/8 barbs on tees, ball valves, and water heater fittings; I used 12
Pex 3/4 Bend Support Metal bend supports (designed for 3/4″ pex pipe) to gently curve 5/8″ heater hose from the water heater through holes in the van floor and then back to horizontal, I used 4
High Temp RTV SealantHigh temperature sealant to seal gaps on holes through the Promaster van floor around the 5/8″ heater hoses
Foam Pipe Insulation 1″ pipe insulation to insulate and protect the 5/8 inch heater hoses that run under the Promaster van RV floor
3/4 x 5/8 x 3/4 Tee3/4″ in and out, with a 5/8″ tee off at 90 degress, installed in short hoses from firewall, turned to point down toward the drive side of the engine bay, need 2
5/8 Barb Ball Valve5/8″ full flow ball valves installed right after the tees, allow the hoses under the Promaster RV van and water heater to be disconnected if needed, need 2
Mopar CoolantOne gallon of additional coolant to fill the new heater hoses and water heater heat exchanger, I used just over 3/4 of the gallon
Cable Zip TiesZip cable ties to secure pipe insulation to hoses in between galvanized straps and to metal bend supports
Galvanized StrapMetal galvanized strap material to secure both hoses in pipe insulation to the metal framing members under the Promaster DIY RV van

Self-Tapping Screws
1/2″ (3/4″ would also work) self-tapping screws to attach galvanized metal straps to van frame members to hold hoses snug against the van

We also have produced a YouTube video detailing the Promaster RV marine style water heater install that provides more detail. Click the image below to view the video.

YouTube video of IsoTemp water heater install

Happy Trails…

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