Pex & hot water question

[Sean]

I would like to re-ask ... about installing the Webasto piping to route hot water thru the cabin heaters, and the engine block,( hot water heat exchanger).
Are there any problems/ downsides to doing this?

Gary,

I recommend separating the systems using a heat exchanger.

In addition to the reason you already identified (more possible points for engine coolant to leak out, possibly causing an engine problem), here are a few more:

(1)  Coolant expense:  Your Webasto (or other brand) diesel boiler and the rest of your hydronic components, such as fan heaters, are very tolerant of a wide range of coolant types.  I use Prestone, but almost any brand or color of coolant, automotive or diesel, will work fine -- use the cheap stuff from Wal-Mart.  As opposed to your main engine, which is very picky about its coolant, requiring special diesel coolant formulation, and regular testing and addition of SCA's.  Much more expensive stuff, and if you have a single system, you'll need to fill the whole thing with this more expensive coolant.

(2)  Coolant changes:  Your hydronic system likely will only need coolant changes every five years or so, possibly longer.  It's a closed-loop, mostly sealed system, whereas the engine coolant is subject to contamination from microscopic migration paths through the head gaskets, and metals migrating into the coolant from high-speed circulation and cavitation issues.  You'll need to change engine coolant on a much more frequent schedule, and, again, by having a single system, you'll be changing roughly twice as much (or more) coolant each time, depending on expansion tank size (and do remember you need an expansion tank on your hydronic system).

(3) Head pressure and flow issues with the engine water pump:  Depending on the size of your hydronic system, it may be asking an awful lot of your engine coolant circulation pump (the "water pump") to try to circulate through the entire system.  You'll likely have to add a boost pump, which will then have to run every time you have the engine running (unless you have a set of valves to bypass the hydronic system).  And then you'll be heating your hydronic system every time you drive, whether you need to or not.

(4) Pump sizing for the hydronic system:  Again, with a single system, you'll need to pump the coolant through the whole engine system every time the system operates, so your electric pump for running the hydronic system while parked will need to be that much bigger (or have that much more back-pressure and wear and tear).  And, don't forget, you'll need to take steps to make sure the engine-cooling thermostat stays closed when the boiler is running (i.e. make sure the 'stat operating temperature is higher than the operating temperature of your hydronic system) -- otherwise, you'll end up paying to heat the outdoors as your precious hydronic fluid flows through the radiators!

(5) Heat-up time for the hydronic system:  When you come home to a cold coach and flip the hydronic system on, it takes a while for all the coolant in the system to come up to operating temperature before your fans will kick on and give you heat.  Having a single system will increase this time, because there is more fluid to heat up and because the coolant is flowing through a cold metal engine block.  You'll have less waste and get heat and hot water much faster by having separate loops.

Almost all the advantages of combining the systems can be achieved with a simple heat exchanger and addition of one small electric pump, to circulate the engine coolant when you want to "pre-heat" the block for cold-weather starts.

We have separate loops, yet still get all the heat and hot water we need while rolling down the road using our heat exchanger.  In fact, we get good heat and hot water for an hour or two after we park without starting the Webasto, just from residual engine block heat and the thermal mass of the system.

FWIW.

-Sean
http://ourodyssey.blogspot.com

[Gary '79 5C]

Sean,
Thanks for the response to me and others, I agree with all that you have presented, and all are tradeoffs as what someone wants to achieve.
Now the disclaimer, I purchased the bus, did not design it.
The coolant costs would be less expensive for the heater loop and longer changes, no doubt.
I do have a solenoid valve which opens when the Webasto loop calls for heat, thus some what isolating the bus loop. Good to know to turn down the T'stat if a leak is discovered within the coach on the road.
You are correct that it takes a long time to warm the coach with the engine. For now this fits our style of travel and use of the coach.
I think I will get the dimensional data from D. Wright and locate a spot for installation. I believe the cost 2 yrs ago was under $500 for a SS unit. My Webasto is located within the engine comp. curb side. I have good access to it, engine and the air compressor etc.
I also am looking at replacing the elect. water heater with a elect./exchanger unit.
Thanks for the input as your's is valued along with others on the board.

Enjoy your sunday.

Gary
P.S. I have several RV'er friends in the office and when they ask about my "Partridge Family Bus" conversion, I email my coach pic's and your site as to just what can be done.

[belfert]

Flat plate heat exchangers are available for way less than $500 even in stainless steel.  Check Ebay for these.

[Don4107]

Sean,

Is your hydronic loop pressurized?

[NewbeeMC9]

Another option that opens up with an Isolated system is the posibilty of using the same loop and fans for cooling, Chill water system using the same distribution system that u have for heating.

be sure to have drains for condensate at the inside HX

Do it your way!!  then post pics!

[Sam 4106]

Hi Rayshound,
You mention using grey PVC electrical conduit for a chase for the pex heating line. Are you aware that PVC will get soft and deform when heated? I called our neighbor, an electrical contractor, to ask at what temperature the deformation will start but he couldn't give a specific temperature. He did say that at 180 degrees it would sag unless it is supported along its entire length and that if it is firmly attached, such as to unistrut, expansion will also cause deformation. Something you may want to consider before you buy it.
I don't know why you want to use two materials, PVC and pex, to accomplish a job that could be done with copper tubing and a short hose or pex at each end. I know that copper is expensive but one length of copper may not cost more than two lengths of plastic.
Good luck with whatever you choose to use. Sam 4106

[Dallas]

Sam,

Copper prices have sky rocketed in the last year or two.
4' of 1" copper is now $29.75 at Plumbing Supply.com and it was only about $8 3 years ago.

100' of Therma PEX is $124.24 at Outdoor Stove Supply.com

Plus, PEX can be run in long lengths without unions or other joints and can make mild bends that copper can't.

I prefer copper too, but with todays prices, it just ain't worth it.

[rayshound]

Thanks Sam, that was the info I was looking for, not happy but thats the way it is. I may be able to run pex in hot pvc pipe. I don't know if just running the hot pvc pipe would be o.k. Thanks Ray

[belfert]

How about FRP pipe instead of PVC pipe for a conduit?  I suspect it would hold up to more heat, but you would have to do more research.  FRP pipe should be available at a good plumbing wholesaler.  No home improvement place will have it.

[Gary '79 5C]

Ray,
Another thing with the PVC Conduit is that even if supported well, the thermal expansion will make long runs "grow" in length. Also causing the appearance of sag. Sam 4106 is right on with his thoughts.

Good Luck,
Gary

[rayshound]

Thanks Gary, I guess another approach might be just run the pex and anchor with some approved "pex" anchor or bracket. I mainly wanted to run a "chase" to run the pex in to get where I won't be able to get to again. Ray

[belfert]

I used some WATTS WaterPEX brand clamps/anchors for the PEX pipe for my domestic water.  I would assume they can be used for radiant heating too.

See http://www.watts.com/pro/_products_sub.asp?catId=70&parCat=530

[rv_safetyman]

Wow Ray, you are getting great input!!

I have the Aqual-Hot with three zones.  I plumbed one zone to the living area and one to the bathroom/bay.    We found that, even in really cold weather, we could do without the third circuit in the bedroom.

We used that zone to plumb to the big Red Dot in the front.  That provides heat and defrost (does a really great job!!).  The Aqua-Hot heat on the road is mostly provided by the engine, so it is like having the engine heat the Red Dot. 

However, it goes it one better.  We got caught in 1 degree weather on our way to Bussin' 2007.  When we woke up, we had a thick sheet of ice on the inside of the windshield.  I turned on the Red Dot and within a few minutes, the windshield was clear (and the engine was warmed a bit).

The Aqua-Hot folks really tried to talk me out of this arrangement.  I think they were worried about liability if something happened to that circuit, or the Aqua-Hot failed and we would be without defrost.  I really am not worried about that event.

For the run from the Aqua-Hot to the Red Dot, I used some very high quality heater hose (industrial quality). 

The heat going down the road is great, as is the defrost.  That may be hard for a guy who lives south of Houston to understand

Jim

[rayshound]

Thanks Jim I appreciate it. Lot's of good info. Ray

[Sojourner]

If you choose hose of any kind that going thorough very hard to access to without conduit.... Then when it sprung a leak, you will have to hope everything is going smoothly pull out old hose while new replacement hose being pulled with-it. However with metal or PCVC (cream colored & for hot water temperature) use for conduit. Worrying about expand & contraction? ....either use rubber sewer connection hose with ss clamps at every long length or leave one end of long straight conduit floating thorough a large home made grommets (3/16" ID fuel line hose with a long single cut slit so it go over metal hole's edge) hole in metal panel. Or use rubber links from muffler shop with 2 metal hooks & ss clamp onto one loose end of conduit to hang. The other end is fasten to chassis or framing or panel.

I would strongly advise to use stiff type of water line such as copper tubing unless it has unavoidable elbow in conduit. Any hose will deteriorate quicker than copper tubing except when frozen. If you use copper tubing... slit 2" long rubber heater hose to slip over tubing & vinyl tape it  to keep it in place while pulling thorough conduit, to avoid vibrating noise inside of conduit.

Another advantage having conduit is it will control water or coolant spray leakage from getting into no no areas.

FWIW

Sojourn for Christ, Jerry