gltrimble
2017 170 4x4
I currently have an Isotemp Spa 4 gallon marine water heater plumbed to my engine coolant system. I use the engine heat to provide for most of my hot water needs occasionally topping it off using the built-in 750 watt Isotemp electric heating element. I also have the H12 option which includes the Espar D5 hydronic heater. This diesel fueled heater is useful for providing hot water when camped or stationary for multiple days without significant sun to charge my batteries. The drawback to using the Espar D5 is that the heater has to heat the entire engine block which limits the heat transfer to the water heater.
Running the D5 for a 40 minute cycle typically raises my domestic hot water temperature approximately 20 degrees F. Multiple 40 minute cycles of the D5 are required in order to get hot water. The van ECU logic also limits operation of the D5 to three 40 minute cycles before I have to start the engine. This is to prevent excessive drain on the starter battery.
To improve the hot water heating efficiency of my Espar D5 I made some simple plumbing modifications to improve the heat transfer. The first mod was to put the Isotemp water heater and the HVAC heater core in series rather than in a parallel flow path. This placed the Isotemp upstream of the heater core and also doubled the flow of hot coolant thru the Isotemp. This resulted in a +10F improvement in heating the hot water. I was now seeing a 30F rise in the hot water temperature for the initial 40 minute run time of the D5. I tested this by simply pinching the hot coolant flow to the heater core closed, forcing all the coolant flow thru the Isotemp before returning to the engine and D5 Espar.
The second modification was simple but more involved than the first. It involved adding a bypass circuit that would allow some of the coolant to bypass the cold engine and return directly to the D5 heater. This was achieved by some minor plumbing reconfigurations and the addition of a heater flow valve. Here are the before and after flow paths for the heater system.
The heater flow valve is controlled by a 5 ft choke cable mounted below the driver’s side dash. The bypass plumbing line previously existed as the rear heater, now Isotemp, (H88) return line. It previously intercepted the heater plumbing coolant return downstream of the heater core. I shifted the return line over a few inches to an adjacent hot coolant line which was just upstream of the coolant electric recirculating pump thus bypassing the cold engine entirely.
My hope was that enough coolant would take the path of least resistance, the bypass, and return directly to the D5 and Isotemp. This would promote faster heating. My concern was if too much flow utilized the bypass line the limited coolant volume, no reservoir, could quickly overheat, tripping the D5. If this occurred I reasoned that the heater flow valve could be used to throttle the bypass flow. I was counting on enough coolant to pass thru the cold engine to limit the temperature rise.
Testing of the reconfigured system revealed no need to throttle the bypass flow. The result was another +10F gain in hot water temperature. Total heat gain by the Isotemp was now +40F compared with the initial +20F for a 40 minute cycle time.
Heater flow valve and reconfigured plumbing
Below is before and after at firewall on connection.
For this to work you will need at a minimum the H12 Espar D5 hydronic heater or a DIY equivalent. The H88 rear heater option makes the bypass and return line a simple mod. Lacking the H88 just requires a bit more heater hose. You will also need to override your ambient temperature sensor to operate your D5 above the 39F limit. This is detailed in this forum using a simple DPDT switch and a resistor (I use 15k=18F).
This modification also comes with minimal risk. Leaving the bypass open while operating the engine will just decrease the hot coolant flow to your domestic water heater. The engine coolant loop is not affected. I managed to make the modifications and only lose about 8-12 ounces of coolant using hose line clamps and some left over bourbon corks.
For the next few weeks I will be carefully inspecting the underside of my van for any coolant puddles.
Materials:
3 feet of 5/8” heater hose
Heater flow valve
3/4” Elbow in place of factory Tee
5/8” Tee to split heater core from bypass
Reuse factory Tee at firewall
Hose clamps
Beer optional
Total cost about $25.
I will try and post this on my build thread also.
Sent from my iPad using Tapatalk
Running the D5 for a 40 minute cycle typically raises my domestic hot water temperature approximately 20 degrees F. Multiple 40 minute cycles of the D5 are required in order to get hot water. The van ECU logic also limits operation of the D5 to three 40 minute cycles before I have to start the engine. This is to prevent excessive drain on the starter battery.
To improve the hot water heating efficiency of my Espar D5 I made some simple plumbing modifications to improve the heat transfer. The first mod was to put the Isotemp water heater and the HVAC heater core in series rather than in a parallel flow path. This placed the Isotemp upstream of the heater core and also doubled the flow of hot coolant thru the Isotemp. This resulted in a +10F improvement in heating the hot water. I was now seeing a 30F rise in the hot water temperature for the initial 40 minute run time of the D5. I tested this by simply pinching the hot coolant flow to the heater core closed, forcing all the coolant flow thru the Isotemp before returning to the engine and D5 Espar.
The second modification was simple but more involved than the first. It involved adding a bypass circuit that would allow some of the coolant to bypass the cold engine and return directly to the D5 heater. This was achieved by some minor plumbing reconfigurations and the addition of a heater flow valve. Here are the before and after flow paths for the heater system.
The heater flow valve is controlled by a 5 ft choke cable mounted below the driver’s side dash. The bypass plumbing line previously existed as the rear heater, now Isotemp, (H88) return line. It previously intercepted the heater plumbing coolant return downstream of the heater core. I shifted the return line over a few inches to an adjacent hot coolant line which was just upstream of the coolant electric recirculating pump thus bypassing the cold engine entirely.
My hope was that enough coolant would take the path of least resistance, the bypass, and return directly to the D5 and Isotemp. This would promote faster heating. My concern was if too much flow utilized the bypass line the limited coolant volume, no reservoir, could quickly overheat, tripping the D5. If this occurred I reasoned that the heater flow valve could be used to throttle the bypass flow. I was counting on enough coolant to pass thru the cold engine to limit the temperature rise.
Testing of the reconfigured system revealed no need to throttle the bypass flow. The result was another +10F gain in hot water temperature. Total heat gain by the Isotemp was now +40F compared with the initial +20F for a 40 minute cycle time.
Heater flow valve and reconfigured plumbing
Below is before and after at firewall on connection.
For this to work you will need at a minimum the H12 Espar D5 hydronic heater or a DIY equivalent. The H88 rear heater option makes the bypass and return line a simple mod. Lacking the H88 just requires a bit more heater hose. You will also need to override your ambient temperature sensor to operate your D5 above the 39F limit. This is detailed in this forum using a simple DPDT switch and a resistor (I use 15k=18F).
This modification also comes with minimal risk. Leaving the bypass open while operating the engine will just decrease the hot coolant flow to your domestic water heater. The engine coolant loop is not affected. I managed to make the modifications and only lose about 8-12 ounces of coolant using hose line clamps and some left over bourbon corks.
For the next few weeks I will be carefully inspecting the underside of my van for any coolant puddles.
Materials:
3 feet of 5/8” heater hose
Heater flow valve
3/4” Elbow in place of factory Tee
5/8” Tee to split heater core from bypass
Reuse factory Tee at firewall
Hose clamps
Beer optional
Total cost about $25.
I will try and post this on my build thread also.
Sent from my iPad using Tapatalk
Last edited:
