How long to heat water

[Hayden Watson]

Already thought about that but the hot water outlet is at the top of the tank with the safety release just below that with the ports much lower. I burped the safety valve just in case.
https://www.westmarine.com/buy/west...allon-front-rear-mounts--13915103?recordNum=1

 

[jibes138]

the cross section area of a 3/4 inch hose is 0.442 sq. inches. The cross section area of a 3/8 hose is 0.110 sq. inches. You only have 25% of the flow area going to your HX versus say a Raritan that takes 3/4 inch fittings. It sounds like you have several variables working against you.

 

[Ron20324]

... the cross section area of a 3/4 inch hose is 0.442 sq. inches. The cross section area of a 3/8 hose is 0.110 sq. inches. ....

+1 to Jibes. As in tank vents, double the hose I.D. gives you FOUR times the area.

 

[Hayden Watson]

But the 3/8" is a function of the engine and there is nothing that can be done about that. In the videos you can see that there is sufficient flow and the temperature drop across the heat exchanger in the water heater is about 10º. As the flow reduces this deference will increase so the flow seems to be sufficient. The problem appears to be with the efficiency of the heat transfer in the water heater's exchanger.

 

[jssailem]

problem appears to be with the efficiency of the heat transfer in the water heater's exchanger.

You’ve accomplished great detective work.
Now what.

1. Seek new better performance from a new water heater?
2. Or know the limits of the equipment on board and plan accordingly ( no hot showers unless having motored for 5 or more hours)?

 

[jibes138]

But the 3/8" is a function of the engine and there is nothing that can be done about that. In the videos you can see that there is sufficient flow and the temperature drop across the heat exchanger in the water heater is about 10º. As the flow reduces this deference will increase so the flow seems to be sufficient. The problem appears to be with the efficiency of the heat transfer in the water heater's exchanger.

In the videos it looked like minimal flow at idle and just a little better at elevated RPM's, a heck of a lot less than a pump with 3/4 inch fittings. How do you know that a 10 degree delta T is enough considering that there are losses like heat transfer through the walls of the water heater to the outside world? My M25 was plumbed in series with the engine coolant and you just have a newer version of that engine. Why not take the flow from the top of the thermostat that goes back to the coolant reservoir and run that hose to the water heater and then from the water heater back to the coolant reservoir? You'll think you are in the hot springs at yellowstone park with all the hot water you'll be basking in. Then just use the bypass loop for the cabin heater instead of trying to run both off that minimal flow? Lastly, ask the Head Mistress for advice. She used to work for Raritan and is pretty knowledgeable about water heaters. Maybe she can describe their heat exchangers for comparison.

 

[Hayden Watson]

jibes: I estimated the flow at idle to be about ¼ gal/minute and at cruise between ½ - 1 gal/minute. With a water / water heat exchanger the efficiency of transfer is almost 100% so based on the low assumption of ½ gal/min x 10º gives 5º @ 1.0gal/min. If I assume 100% efficiency 5º-1gal/min / 6-gallon tank I get 0.83º/min. This should yield about 30º / hour gain and I am only getting 30º. I suspect that my temperature drop measurement of the inlet / outlet hoses may be inaccurate. I can only measure the hoses and they are of two different types. If there was a large temperature drop across the inlet / outlet than I would agree that increasing the flow would increase the heat gain. In my case I still have high outlet temperature so the problem is that the heat exchange is not happening which is a thermos/mechanical problem with the water heater itself.

I think the only two options as stated by jssailem are to replace heater or live with it. The current efforts have improved the situation to where 2 hours will get me to about 120º so until the old unit starts leaking I will probably live with it.

 

[jviss]

I noticed an enormous difference when I got a new water heater, to very hot water, very fast, with the new one; 20 or 30 minutes to 'scalding' in a 6 gallon tank. Was well worth it!

 

[jibes138]

30 degrees per hour? If you start at 50 degree water in an hour it will be almost warm enough to brush your teeth :>) . Might be faster and easier to heat water on the stove, fill a sunshower bag to take a shower.

 

[RoyS]

I have a modified coolant loop in a 1979 Yanmar 2QM15 connected to a 10 year old, Force 10, 6 gallon water heater. My modified coolant loop by-passes the normal engine heat exchanger until the coolant temperature reaches the remote thermostat setting of 160F. The water heater is always in the loop. Operating the engine at the dock (unloaded) and monitoring the engine and water heater temperature yields results as follows: It takes ten minutes for the engine to reach 145F. Unloaded, the engine will not go above 145F regardless of engine speed. It takes 1 hour in these conditions to heat the domestic water from 60F to 100F. These small diesel engines unloaded simply do not provide that much heat. The inexpensive water heaters probably do not have great heat absorption capabilities either.

 

[jibes138]

The heat exchanger in the water heater needs to be a coil with enough surface area to be effective. A simple half circle pipe is only heating water in one spot in the tank and relying on convection to circulate fresh water. Very inefficient.

 

[Stu Jackson]

Hayden,

Here is an article about adding a bus heater to the coolant loop on a C34.

http://c34.org/wikiwp/?rdp_we_resource=http://c34.org/wiki/index.php?title=Cabin_Heater

Please note that Bob’s diagram shows ½” hose from the 3/8” lines at the top of the coolant pump and back to the thermostat housing. The text says 5/8”. My water heater takes 5/8” hose from that reducer bushing. Most Kuuma and Seaward heaters take 5/8”, too. Comments on hose size of ¾” should make no difference.

But note that Bob’s system has the bus heater in series with the water heater and it works just fine.

I replaced my Seaward water heater with another Seaward in 2013, 15 years after we bought our boat when the OP replaced his 12 year old heater. The heaters have a reported life of between 10-16 years (I know, a wide variation).

My M25 has always heated our water in less than 45 minutes to scalding hot. I moved from San Francisco (57 water temp) to British Columbia (47 water temp). In a year of living here I found no appreciable change in how long it takes to heat the domestic water under power. I have found that the water in the tank cools off more quickly once at anchor, ‘cuz it’s colder “down there” where our heater is under the galley sink.

I can’t conceive of how you were running your M25XP at 140 degrees. I don’t know of any thermostats that would fit an M25XP other than 160F and 180F thermostats. But you seem to have that behind you.

The hose size you see is 3/8" and is only for the first couple of feet and then the rest of the circuit is 5/8". On the M25XP, the bypass ports are 3/8" so it needs to start and end with that if I use the bypass. I prefer to keep it in the bypass as a safety measure. much shorter run of the critical hose and if I get a leak in the bypass loop I can cut out 6" of good hose and return to straight bypass hose eliminating any problems in the heaters.

So you got the hose sizes right. I couldn’t view the videos because my plug in wasn’t working. Just to confirm: your engine has the “regular” connection to the heater as shown in Bob’s diagram: from the top of the coolant pump to the heater and then back to the thermostat housing. Is this short 3/8” hose what you mean by “bypass?”

For the first 30 minutes I ran in gear at 1900 rpm which is full cruise speed on my M25XP.

1900 sounds awfully low for cruising speed on an M25XP. I run at 2450-2800 on our M25 engine. WOT is 3000. My thermostat opens and gets up to 180F engine operating temperature when I run in gear in about 5 minutes. I get scalding hot water in 30-45 minutes.

It has never heated water well from the engine but gets fully hot on 120v in about 30 minutes.

Unless it’s because of your 140 operating temperature. But now that you got your 180F thermostat, you should get scalding hot water in less than an hour. You may well have a dud of a heater.

Seaward still makes heaters, they are step above the Kuuma. http://c34.org/bbs/index.php/topic,7893.0.html

My M25 was plumbed in series with the engine coolant and you just have a newer version of that engine. Why not take the flow from the top of the thermostat that goes back to the coolant reservoir and run that hose to the water heater and then from the water heater back to the coolant reservoir? You'll think you are in the hot springs at yellowstone park with all the hot water you'll be basking in. Then just use the bypass loop for the cabin heater instead of trying to run both off that minimal flow?

I don’t understand this at all. What bypass loop? If you follow Bob’s sketch and Dave’s first sentence here, it’s the same thing. Coolant pump>>3/8” hose>>3/8” to 5/8” bushing>>water heater and like that back to the thermostat housing.


Good luck.

 

[jibes138]

Stu, early in the thread Hayden described a bypass loop that flows to the water heater before the thermostat opens. My water heater was plumbed in Series to the main engine coolant flow that went through the engine HX. Not sure if yours is the same? It took forever to heat water at idle at anchor as when the thermostat opens the heat sink of the water heater knocks the temp down and closes it again. Steaming under load I'd get scalding hot water very fast, maybe 15 to 20 minutes. That was with a Raritan 6 gal water heater and I had a similar Superstor for a while that worked the same.

 

[Stu Jackson]

Stu, early in the thread Hayden described a bypass loop that flows to the water heater before the thermostat opens. My water heater was plumbed in Series to the main engine coolant flow that went through the engine HX. Not sure if yours is the same?

That's why I asked about the "bypass," Dave. I don't understand it unless it's a tempering bypass, in which case I'd close it.

Going through the engine HX is NOT what we have, as described in my last post - it's series through the coolant pump. I never did understand using the HX for water heating. Still don't.

 

[Hayden Watson]

Hayden,

Here is an article about adding a bus heater to the coolant loop on a C34.

http://c34.org/wikiwp/?rdp_we_resource=http://c34.org/wiki/index.php?title=Cabin_Heater
...
comment #1 I replaced my Seaward water heater with another Seaward in 2013, 15 years after we bought our boat when the OP replaced his 12 year old heater. The heaters have a reported life of between 10-16 years (I know, a wide variation).

comment #2 I can’t conceive of how you were running your M25XP at 140 degrees. I don’t know of any thermostats that would fit an M25XP other than 160F and 180F thermostats. But you seem to have that behind you.

comment #3 So you got the hose sizes right. I couldn’t view the videos because my plug in wasn’t working. Just to confirm: your engine has the “regular” connection to the heater as shown in Bob’s diagram: from the top of the coolant pump to the heater and then back to the thermostat housing. Is this short 3/8” hose what you mean by “bypass?”

comment #4 1900 sounds awfully low for cruising speed on an M25XP. I run at 2450-2800 on our M25 engine. WOT is 3000. ...
Good luck.

Thanks for the comments Stu. I numerated some of your comments above in red and will respond below.
comment #1: My original WH was replaced in 2005 and lasted 17 years. the "new" Kuuma is now 12 years old so hopefuly it will spring a leak soon. ;>)
comment #2: I tested to see what the temp was with no thermostat and it stabilized at 120º. I am sure that the 140º temperature was do to a faulty thermostat that had a small lead that would allow some water to get by in the closed position. With the new thermostat the engine heats up to full operating temp of 180º faster than I used to get to 140º.
comment #3: My system is plumbed just as shown in Bob's diagram. One other interesting point is that I get about 7º drop across the bus heater core with the fan not running and it increases to about 20º drop when the fan is on. There is no way that a water to air heat exchanger should be extracting more heat than a water to water does.
comment #4: On my M25XP WOT is 2700 in neutral. 80% - 90% of no-load WOT is the recommended max sustained RPM so that would be 2160 - 2430. I have an AutoProp and it has "happy spots" that run very smooth and outside of those RPM's it produces a lot of vibration. Idle = bad. Bump above idle = good. 1400 good. 1925 = good. 2400 = good. everything other than those four RPM's rattles the pots and loosens the fillings. I only gain about .5 knts going from 1925 to 2400 and it is a lot noisier so 1925 it is.

 

[Sovt]

This thread is wandering a bit.

First thing I do when presented with a unusual mechanical issue is to ensure that the installation was done exactly the way the manufacturer specified.

You didn't show the location of the tank but if the coolant loop in the heater is above the coolant level of the engine there a specified way the system is to be plumbed. See the attached file from Universal.

 

[Hayden Watson]

Thanks for the diagram.
I described my installation earlier in this thread but will restate. It is installed exactly as described in the directions you provided from Universal. No part of the system rises above the top of the expansion tank. The system has been purged by re-circulating coolant at high volume until no bubbles came out of the pump discharge and then I left the pump running for an additional minute to make sure no more would come out. I have significant flow through the water heater loop but very little drop in temperature (in my opinion) across the coil in the water heater. It appears to be a defect in the heat transfer within the heat exchanger of the heater and cannot be corrected other than by replacing the water heater.

 

[Sovt]

I missed your previous post about not having any coolant levels above the engine coolant level. In the hydronic world most installations look for a 20deg. Delta T. It appears that you are not getting that so as you stated, a new heater may be your only option.
Best of luck

 

[Ron20324]

Engine water pumps only push so much water so far. Maybe you need an aux pump to move all that water that distance?

 

[Hayden Watson]

If the flow was non-existent I would not get 160º temperatures at the water heater. If the flow was inadequate, I would get much higher temperature difference across the inlet / outlet hose. I have 150º coolant coming out of an exchanger in 60º water. I have plenty of heat, I just can't get it out of the coolant and into the water. The problem is a manufacturing error in the heat exchanger and this can only be corrected by replacement. If I had come to this realization when I bought it I might have been able to get a warrantee replacement but now I am stuck with it.