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14-12-2024, 20:38
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#1
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Registered User
Join Date: May 2016
Location: Annapolis, MD
Boat: Pearson 39-2
Posts: 24
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Hot water plumbing - expansion tank and mixing valve thoughts
Last year, I had to replace my water heater and recognized it was time to update and replumb the arrangement of my fresh water system this winter. I'm trying to sort this out over the winter so I can do other things in the spring. The boat still has polybutylene pipe so I'll be replacing a lot of that with PEX and the required fittings. Since I've had this boat, I've had issues with the cold water heating up whenever we run the engine, and the hot water is extremely hot. The HW heater is rather close to the engine, so it seems that the hot water gets extra hot, and it has become obvious that backflow into the cold water line has gotten worse over the past few years. There is a check valve on the cold water input to the HW heater but it seems like it is damaged or just not working.
Current equipment:
shurflo Aqua king II pump
Raritan 6 gal water heater (new this year)
NO accumulator or expansion tank
Water filter (currently installed on cold water only)
Engine: Yanmar 3HM35F
Questions:
1 - I'm going to replumb the lines from the pump and water heater until they leave the area around the galley where these are located. I'll replace the check valve etc, but I'm trying to determine if I SHOULD add an expansion tank to the system, either on the inlet or to the outlet of the water heater. I'd rather not do this as the space around the water heater is very small. If I do need to add one, I'd explore T-ing the line and locating it where I have more room, though all the small units I'd purchase are design to be installed in-line.
If you have experience with this problem, do I need to add an expansion tank?
If it is recommended, do I need to add it at the cold water inlet to the hot water tank, or can it go in the hot water outlet, or does it not matter?
2- Mixing valve - I'd like to avoid having scalding hot water coming from the faucet/shower, and it would be great to make the hot water go further. Currently, its hard to get the water temp balanced when taking a shower. So I'm looking to add a mixing valve. As mentioned above, the room around the hot water heater is very tight at the hot water outlet.
Would it be acceptable to use a vinyl braided hose to connect the hot water outlet to the mixing valve? I'm thinking this would make it easier to install the mixing valve a bit further away where there is more space.
What other things should I be considering when installing a mixing valve?
3- Has anybody put a valve on the engine coolant lines going to the hot water heater to limit how much coolant is circulated? It seems like the water in the water heater gets hot fast, so for longer motoring trips, it might be better to keep that heat moving through the engine heat exchanger.
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15-12-2024, 00:06
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#2
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always in motion is the future

Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 20,123
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Re: Hot water plumbing - expansion tank and mixing valve thoughts
Quote:
Originally Posted by SSAsailor
Last year, I had to replace my water heater and recognized it was time to update and replumb the arrangement of my fresh water system this winter. I'm trying to sort this out over the winter so I can do other things in the spring. The boat still has polybutylene pipe so I'll be replacing a lot of that with PEX and the required fittings. Since I've had this boat, I've had issues with the cold water heating up whenever we run the engine, and the hot water is extremely hot. The HW heater is rather close to the engine, so it seems that the hot water gets extra hot, and it has become obvious that backflow into the cold water line has gotten worse over the past few years. There is a check valve on the cold water input to the HW heater but it seems like it is damaged or just not working.
Current equipment:
shurflo Aqua king II pump
Raritan 6 gal water heater (new this year)
NO accumulator or expansion tank
Water filter (currently installed on cold water only)
Engine: Yanmar 3HM35F
Questions:
1 - I'm going to replumb the lines from the pump and water heater until they leave the area around the galley where these are located. I'll replace the check valve etc, but I'm trying to determine if I SHOULD add an expansion tank to the system, either on the inlet or to the outlet of the water heater. I'd rather not do this as the space around the water heater is very small. If I do need to add one, I'd explore T-ing the line and locating it where I have more room, though all the small units I'd purchase are design to be installed in-line.
If you have experience with this problem, do I need to add an expansion tank?
If it is recommended, do I need to add it at the cold water inlet to the hot water tank, or can it go in the hot water outlet, or does it not matter?
2- Mixing valve - I'd like to avoid having scalding hot water coming from the faucet/shower, and it would be great to make the hot water go further. Currently, its hard to get the water temp balanced when taking a shower. So I'm looking to add a mixing valve. As mentioned above, the room around the hot water heater is very tight at the hot water outlet.
Would it be acceptable to use a vinyl braided hose to connect the hot water outlet to the mixing valve? I'm thinking this would make it easier to install the mixing valve a bit further away where there is more space.
What other things should I be considering when installing a mixing valve?
3- Has anybody put a valve on the engine coolant lines going to the hot water heater to limit how much coolant is circulated? It seems like the water in the water heater gets hot fast, so for longer motoring trips, it might be better to keep that heat moving through the engine heat exchanger.
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First thing you need to do is verify that the water heater incl. all its components are in good working order. Most importantly the over pressure valve. Water heater explosions are lethal.
I am not sure if water heaters all have check valves, but I do know the engine can heat cold water just through hoses etc. so it may not be back-flow from the water heater.
I use the over pressure valve instead of an expansion tank. During the heating cycle, a minor amount of water comes out the valve and I just let that evaporate. Now that I think of it, there’s another job for my 3D printer: a little catch & evaporation container for this.
Much also depends on the other plumbing: rubber hoses give more expansion than PEX tubing. I added some water-hammer arrestors.
__________________
“It’s a trap!” - Admiral Ackbar.
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15-12-2024, 07:14
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#3
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Registered User
Join Date: Jun 2019
Location: Rochester, NY
Boat: Chris Craft 381 Catalina
Posts: 6,978
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Re: Hot water plumbing - expansion tank and mixing valve thoughts
I went for an expansion tank on my water heater. It also has a mixing valve (came pre-attached to the Isotemp heater).
For the expansion tank, I have a check valve on the cold input and then a tee fitting after the check valve. One side feeds into the water heater, the other side goes to a 2 gallon pressure tank mounted above the 11 gallon water heater. The pre-charge pressure in the tank is about 55 - 60 psi (depending on engine room temperature), just a little below the pump cut-out pressure of 60 psi. That keeps the tank from taking in much water during normal cycling but lets it take water when the tank is heating and pushes the pressure above the pump cut-out level on the hot side.
That's enough to keep the pressure rise down to about 20 psi when heating from stone cold to full temp which avoids tripping the T/P relief valve (the Isotemp valve is set at 87 psi IIRC). It also avoids having a burst of really high pressure when you first open a hot tap after heating the water heater from cold and is correspondingly easier on the plumbing.
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15-12-2024, 08:13
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#4
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Registered User
Join Date: Jan 2023
Posts: 2,148
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Re: Hot water plumbing - expansion tank and mixing valve thoughts
Having an expansion tank is a much better practice than counting on the pressure relief valve to bleed off excess pressure, for a couple reasons.
Those relief valves are typically set to much higher pressures than the usual boat system is designed for. They are there to prevent the explosion of the tank in the event of a heating element that has failed on, not to protect the various hoses and plastic fittings in a boat’s fresh water system.
Small bits of debris can also block those valves open, then they constantly leak.
This is one of those things that seems like a workable idea, right up until it is not!
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15-12-2024, 08:24
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#5
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Registered User
Join Date: May 2016
Location: Annapolis, MD
Boat: Pearson 39-2
Posts: 24
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Re: Hot water plumbing - expansion tank and mixing valve thoughts
Jedi - Thanks for your thoughts. The Raritan WH is new and it included a overpressure valve, which can be manually activated. I can open it with the manual override and it does release pressure. Indeed, I am not aware that it is releasing pressure so I don't think the HW is getting to that pressure in normal operation.
Good point about stiffer hoses - the Polybutyl piping that I currently have is a little bit soft and PEX seems stiffer so this is something I might need to be concerned about. We had not water hammer issues with the system as it is.
Others - your input on the expansion tank is what I suspected. It sounds like it would be a good idea to include, and Raritan does indicate one in their install guide. If I have to install one, its going to be 1-3 feet away from the T on the intake line.
Will I get similar results if I put the expansion on the outlet?
Install guide:
https://raritaneng.com/uploads/L206v...M-aa9c4c64.pdf
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15-12-2024, 08:53
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#6
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Registered User
Join Date: Aug 2021
Location: PNW
Boat: 35 Ft. cutter, custom
Posts: 2,945
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Re: Hot water plumbing - expansion tank and mixing valve thoughts
Quote:
Originally Posted by SSAsailor
3- Has anybody put a valve on the engine coolant lines going to the hot water heater to limit how much coolant is circulated? It seems like the water in the water heater gets hot fast, so for longer motoring trips, it might be better to keep that heat moving through the engine heat exchanger.
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You can use a valve, on engines without a connected water heater the ports are usually just closed off with a couple of pipe plugs.
However, that doesn't really change the temp, what it does do is increase the recovery time needed to bring the colder water up to temp.
Over extended running times the engine will always try to get the water up to ~ the same temp as the engine's thermostat range.
PS, a tank installed with a check valve as an expansion tank will do nothing to operate as an accumulator, ie, no action to prevent rapid pump cycling when only cold water is used.
__________________
Beginning to Prepare to Commence
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15-12-2024, 20:54
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#7
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Registered User
Join Date: Jul 2022
Posts: 81
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Re: Hot water plumbing - expansion tank and mixing valve thoughts
I just went through some of this. 2009 boat, originally came with PEX plumbing and no expansion tank (or accumulator). The TPV relief valve went through a marine grade hose over to a thru hull up high on the side of the boat. I believe this is a requirement now - must go overboard, not the bilge? It also had a mixing valve.
I am second owner, and replaced the 11-gal Kuuma aluminum water tank once already and this time installed an 11-gal IsoTemp.
Anyway -
#1. I put in a check valve with the IsoTemp. It just seemed to make sense to avoid any lost heat going back out the cold input line. I also installed a 2-gallon expansion tank after the check valve, and before the water tank. This was the convenient location for me, it could go anywhere after the check valve I guess. No clue how much pressure built up in the system before other than I would get some weeping from faucets here and there once in a while which has seemingly gone away now. As others have noted, the PEX piping does not expand as much as the older hoses so that pressure needs to go somewhere. I have the expansion tank set to 5psi higher than the water pump - which is a variable speed pump and no accumulator tank. I have never had any 'hammering' or anything like that in the water system ever.
#2. Yes on the mixing valve - the IsoTemp came with one, and for the older tanks I had to replace it once. Treat this as a safety item, not a convenience item, especially if there are kids on board. 180F from engine coolant is far too hot for hot water.
#3. I have valves on the coolant lines between the engine and the water tank. They are there for service/maintenance and as an emergency disconnect. With the water tank, once it is warmed up and unless people are taking a lot of showers while underway, I don't think it would be siphoning much heat from the engine anyway? Having it go to a bunch vent-style heat exchanger to blow hot air around the cabin would be a different kind of deal for concerns there?
EDIT - for the TPV discharge going outside the boat and not the bilge, that may be required only if the boat also has a 'shore water' hookup system? I can't recall where I stumbled across it appearing to be a requirement nowadays.
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16-12-2024, 00:47
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#8
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always in motion is the future

Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 20,123
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Re: Hot water plumbing - expansion tank and mixing valve thoughts
I am not a fan of expansion tanks because of their tendency to fail. If you want to eliminate them, you do need to address some issues in a different way:
- water pressure pump short cycling. This is traditionally addressed with an expansion tank. You need to convert to a variable speed pump to prevent short cycling without an expansion tank.
Of course these pumps can fail too and instead of a pressure switch there’s an electronic variable speed controller that is the primary point of failure. The remedy is the same: carry a spare. A tip: buy two Marco UP3/E pumps and install them in parallel but each a separate breaker/switch. You normally run one or the other but when you need a lot of water volume when using showers, multiple faucets etc. you can run both.
- high pressure. Upgrade to PEX tubing which is designed to handle high pressure up to 90 psi and much more for cold water.
- water hammer. When you use PEX instead of flexible hose and don’t have an expansion tank, there is little room for the water to expand or absorb shocks. You get those when a flow of water is abruptly interrupted by something like a solenoid valve in a washing machine or an electric toilet. For this you can use special arrestors that are like miniature expansion tanks but they don’t have a membrane. They simply have an air bubble that can compress.
All the above probably only makes sense when one is redoing the plumbing anyway
__________________
“It’s a trap!” - Admiral Ackbar.
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16-12-2024, 02:35
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#9
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Registered User
Join Date: Jul 2022
Posts: 81
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Re: Hot water plumbing - expansion tank and mixing valve thoughts
Quote:
Originally Posted by s/v Jedi
...This is traditionally addressed with an expansion tank. You need to convert to a variable speed pump to prevent short cycling without an expansion tank.
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I think you are referring to an accumulator here?
Physically, they are the same thing, but have two different purposes.
An accumulator does what you describe, and if there is a check valve in a system with a hot water tank, would be installed before the check valve.
An expansion tank is in a hot water system only, and serves to contain the expansion of the water when it heats up and avoid over-pressuring the hot water portion of the system.
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16-12-2024, 03:00
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#10
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Senior Cruiser

Join Date: Mar 2003
Location: Thunder Bay, Ontario - 48-29N x 89-20W
Boat: (Cruiser Living On Dirt)
Posts: 51,809
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Re: Hot water plumbing - expansion tank and mixing valve thoughts
Cross-Linked Polyethylene “PEX” tubing typically has standard hydrostatic pressure ratings of 160 psi at 73°F [1105 kPa at 23°C], or 100 psi at 180°F [690 kPa at 82°C], or 80 psi at 200°F, per ASTM F876, & F877, and CSA B137.5.
For PEX pipe, the pressure rating depends on the wall thickness, and dimension ratio [SDR-9, in NA].
Consult the specific PEX manufacturer's literature, for appropriate pressure ratings.
__________________
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"
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16-12-2024, 09:13
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#11
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Registered User
Join Date: Sep 2019
Posts: 651
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Re: Hot water plumbing - expansion tank and mixing valve thoughts
I added a small expansion tank and a Cash Acme mixing valve and it's been working well so far. I plumbed the valve directly to the water heater outlet. It was a tight space but fitted with 90 deg adapters on the cold in and hot out.
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17-12-2024, 00:36
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#12
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always in motion is the future

Join Date: Feb 2009
Location: in paradise
Boat: Sundeer 64
Posts: 20,123
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Re: Hot water plumbing - expansion tank and mixing valve thoughts
Quote:
Originally Posted by pnw2022
I think you are referring to an accumulator here?
Physically, they are the same thing, but have two different purposes.
An accumulator does what you describe, and if there is a check valve in a system with a hot water tank, would be installed before the check valve.
An expansion tank is in a hot water system only, and serves to contain the expansion of the water when it heats up and avoid over-pressuring the hot water portion of the system.
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Different words and countless differences over the many languages used around the world but all for the exact same device so which words are used isn’t important to argue over here.
My point is that flexible hoses act like an expansion tank and may be enough to eliminate the tank, but the best method is to change to a good variable speed pump and for plumbing using PEX, ABS or PVC pipes rather than flexible hose this is a must.
__________________
“It’s a trap!” - Admiral Ackbar.
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17-12-2024, 02:02
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#13
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Registered User
Join Date: May 2013
Location: Oregon to Alaska
Boat: Wheeler Shipyard 83' ex USCG
Posts: 3,660
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Re: Hot water plumbing - expansion tank and mixing valve thoughts
In another life, I installed film processors that had to maintain a water temp within a couple degrees. You have to have a check valve so water doesn't back up the cold supply line. An expansion tank saves a lot of strain on valves and seals. People say water doesn't expand and compress, but it does, just a lot less than air. An expansion tank is more important on a small system than a big one.
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17-12-2024, 10:27
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#14
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Registered User
Join Date: Aug 2021
Location: PNW
Boat: 35 Ft. cutter, custom
Posts: 2,945
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Re: Hot water plumbing - expansion tank and mixing valve thoughts
No matter what some engineers say, swatting at flies with a sledgehammer in the quest of perfection can have its limits.
After 2>3 replacements of the check valve I got rid of it.
With my small heater, (~5 gal,) so what if a quart or so of hot water pushes back into the supply line, the accumulator tank provides the needed volume.
Heat loss? no big deal, the first time you open a hot water faucet some cold water goes into the inlet anyway, and the engine is providing far in excess of any heat required to heat the water.
The TPV/faucets/lines have never leaked a single drop in,,,, it's been too many years to remember.
Problem faucets? get rid of the "chromed plastic" RV stuff, put in some forged brass stuff from American Standard or Chicago.
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Beginning to Prepare to Commence
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17-12-2024, 10:33
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#15
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Registered User
Join Date: Jul 2022
Posts: 81
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Re: Hot water plumbing - expansion tank and mixing valve thoughts
Quote:
Originally Posted by Bowdrie
No matter what some engineers say, swatting at flies with a sledgehammer in the quest of perfection can have its limits.
...SNIP...
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I agree. Keep it simple always means fewer things to break or maintain.
I dithered about the KISS theory quite a bit before I ended up installing the expansion tank with my hot water tank upgrade.
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