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Author Topic: BTu Question for the Science Guy Types - Please Use Small words  (Read 6755 times)
Nick Badame Refrig/ACC
1989, MCI 102C3, 8V92T, HT740, 06' conversion FMCA# F-27317-S "Wife- 1969 Italian/German Style"
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« Reply #45 on: December 14, 2009, 05:53:21 PM »

Here's  something to think about...

My home needs 100,000 BTU's of heat capacity to heat it and 48,000 BTU's [4 tons] of A/C to cool it!

Nick-
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« Reply #46 on: December 14, 2009, 06:14:29 PM »

I don't think we've mentioned straight out that a big difference between between heating and cooling has to do with the temperature differential.

Cooling the coach to 70 degrees while sitting in 100 degree weather, a 30 degree spread, is a whole different animal than heating the coach to 70 degrees while sitting in 0 degree weather, a 70 degree spread.

That's why the furnace is so often sized so much larger than the AC, the further from the equator you are.

And why campgrounds are more likely to charge for electricity if heating season is involved?

happy coaching!
buswarrior


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« Reply #47 on: December 14, 2009, 11:33:31 PM »

I said much the same thing but my post got stuck in Gmail.

It went:
If you are cooling then the most you might need to change the temp is, 120 degrees minus 74 is a delta of 46 degrees.  For heating you might get a low of -20 and you will need to raise that to 74 and that is a 94 degree delta.  Given that that is even remotely true, it isn't hard to follow that your heating capacity should be approx double the BTU power.  But of course that is dramatically affected, the logic, depending on where you live....what zone.

Still, what Nick says is certainly good logic if we want max flexibility to go anywhere in any season.  You simply cannot live in a bus that is either too hot or too cold.

John's 2 cents( and that may be over valued)

Close, huh?
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« Reply #48 on: December 15, 2009, 07:03:21 AM »

Here's  something to think about...

My home needs 100,000 BTU's of heat capacity to heat it and 48,000 BTU's [4 tons] of A/C to cool it!

Nick-


Is that partly because the Mrs. keeps turning the thermostat up? Wink
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« Reply #49 on: December 16, 2009, 04:45:01 PM »

Bryan

In your last diagram you had two pumps -- one supplying the heater and one supplying the heat exchangers.  In home boiler systems the normal setup is to install the circulation pump pushing the coolant into the heater and allowing the the pressure to return the coolant.

With the two pumps one could act as a restriction for the second even if both pumps are running and are the same make and size.

The last system I installed the diagram called for the pump to be placed pushing the coolant into the distribution manifold. I called the company and asked for clarification because I was not used to seeing it set up that way.  They wanted the expansion tank between the heater and the pump and manifold. I was told that it would balance the pressure better and give a more even heating when all the zones were open.

I am not sure it works any better that way but just something more to throw into the mix here. (By the way I did it just like they wanted)

Melbo
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« Reply #50 on: December 16, 2009, 07:37:22 PM »

Hmmm.  The boilers in buildings that I work on are normally using the circulator on the return, pushing water into the boiler. Also, the zone valves are installed on the return.

The reason for this is to keep all of these parts as cool as possible by having them at the end of the loop. This makes for a longer life for these items. Honeywell Has used a plastic gear in the zone valve motors that gives up when it gets too hot for too long.

I haven't studied any of the bus heating systems enough to find out how they are plumbed.

Tom Caffrey
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« Reply #51 on: December 17, 2009, 06:15:37 AM »

Yes Tom

The zone valves are on the return as is typically the pump

That was my observation was the repositioning of the pump

The diagram has it just before the distribution manifold

Melbo
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« Reply #52 on: December 17, 2009, 07:10:59 AM »

Hmmm.  The boilers in buildings that I work on are normally using the circulator on the return, pushing water into the boiler. Also, the zone valves are installed on the return.

The reason for this is to keep all of these parts as cool as possible by having them at the end of the loop. This makes for a longer life for these items. Honeywell Has used a plastic gear in the zone valve motors that gives up when it gets too hot for too long.

I haven't studied any of the bus heating systems enough to find out how they are plumbed.

Tom Caffrey

I use a outside wood boiler ( OWB ) to heat my shop and have done quite a bit of experimentation on the posistion of the circulation pump.

My final layout uses a primary / secondary piping system.

I also have a oil burner boiler also in the loop.

The circulators are both on the hot side by my choice,  mainly to ensure they are freeze protected.

The system typically runs at 180 degree water more than enough of the cooling effect.

I have also duplicated the shop system in the bus. I took a circulation pump that GM used  ( in the hot line) to send the water to the front of the bus 40 ft away. Now typically the running temp of the bus water is 180* to 200*

I removed the webasto circulator and installed a circulator salvaged from and old GM bus and it pumps  hot water to the webasto then
thru my water heater side heat exchanger then back to the engine.

Bottom line is the placement of the circulator is usually left up to the installer.

Do you recgonize this heat exchanger in the shop Smiley

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Sean
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« Reply #53 on: December 17, 2009, 07:32:22 AM »

Here's  something to think about...

My home needs 100,000 BTU's of heat capacity to heat it and 48,000 BTU's [4 tons] of A/C to cool it!


Buses, however, and RV's in general, have a different set of issues.  With a well-sealed and well-insulated coach, solar heat gain through the glass and even the metal, combined with the various heat loads inside the coach from occupants and appliances, make cooling the coach more of a challenge than heating it.

We have real-world data:  Our coach has 45,000 BTU/hr of hydronic heating, and ~55,000 BTU/hr of air conditioning.  We've been more than comfortable in temperatures well below freezing for days on end, and even then, the furnace does not run full-time but, rather, cycles on and off.  By contrast, on a 115°+ day, the A/C's struggle to keep up, running full-tilt.  Admittedly, we have a lot of glass, but we do have dual-cell thermal shades along with silvered mylar foamed window inserts, 20' awnings on both sides, and a polar white roof.

We have found, in fact, that in temperatures down even into the 40's, we can often avoid using the heat simply by parking with one side facing south, and opening the window coverings on that side on a sunny day.  Of course, we also have five mammals aboard, and our inverter is in the living space.

Since I have not yet weighed in on the earlier question about hydronic runs, let me add that I strongly recommend separating engine and household systems.  You will have the added expense of a heat exchanger and another small pump (for engine preheating), but there are many advantages:
  • Engine coolant changes, which are required more frequently than hydronic changes, will involve far less coolant.
  • The hydronic system can use inexpensive antifreeze, as opposed to the more expensive SCA-laden antifreeze required for diesel engines.
  • A leak that develops in the household system will not threaten the more critical engine cooling.
  • During normal parked heating operations, heat will not be wasted on the engine block, which otherwise accounts for a large loss (the exchanger itself represents a much, much smaller loss).
  • The main engine water pump is not being asked to circulate through the much larger hydronic system, possibly overtaxing the pump.
  • Likewise, the electric hyrdonic pump is not being asked to circulate through the main engine loop all the time; the smaller pump used for engine preheat will only be run for short periods on days where preheat is called for.

I will be covering hydronic systems during the BYOB workshop in Arcadia in January, following Bussin' 2010.

-Sean
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« Last Edit: December 17, 2009, 07:36:26 AM by Sean » Logged

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« Reply #54 on: December 17, 2009, 08:33:08 AM »

Dallas,
This doesn't answer your question as to the formula, but it answers it in a way in what works for me.  I knew nothing about how to heat my coach when I converted so I deferred to Dick Wright from Wrico .  He has done hundreds, so he knew what works.  He sold me the 40k Webasto with 6 7k fan forced 12v heaters.  I don't know how they rated them at 7k, but that is what the stock number was.
I also bought the 45k heat exchanger to hook to the engine coolant to scavenge heat from the engine and preheat the engine.

All my heaters are in series, 3 in the lounge/galley, 1 in bathroom, 1 in bedroom, 1 in mechanical bay.  The front of the bus gets the water first then head back toward lounge, then back to galley, then back to bathroom, then back to the bedroom, then to the mechanical bay, then back to furnace.   So, as you can see the water will be cooler in the bedroom than the lounge.  That's ok because, as Sean said, lots of windows up front means more cold air thus needing more heat. 

Now, after all this was installed and we took some trips to the cold country, we found we were still cold in the lounge and plenty warm in the bed and bath.  Hmmm, what to do???  I began searching for more heat exchangers.  I didn't want any more 12v fans to draw battery juice, so I opted for finned radiators.  I bought a 1000btu tube for behind the couch, a 500btu tube for the wall adjacent the lounge chairs, and a 500 btu tube under the dinette against the wall.    This greatly improved the comfort of the lounge/galley. 

Now, there is one problem with my setup, though I've overcome that to my satisfaction.  Even when the front zone t-stat reaches its temperature and the back zone t-stat is calling for heat in the bed/bath, there is still heat coming off those 3 finned radiators.  That is okay because its not enough to make the front uncomfortable and it is rare, because the bed/bath needs so little heat for comfort because of so few windows. 

Now, when I am underway and heat is not needed I still need to heat my hot water supply, so I have a 3 way valve easily accessable in the bay I turn to bypass all heat exchangers and only make the loop through the water heater, thus no heat in the coach living area. 

I guess to sum up my system--I have a few more heat exchangers (btus needed) than a furnace (btus made) can put out.  If I run everything and the t-stat settings are never reached then obviously the webasto will never turn off.  I've been in some minus 0 places where that has happened, but we were comfortable in the coach. 

In your friend's application where it was burning hot at the first heater and cooler down the line, I would just damper the air output to send more btus down the line.   I think this is easier than designing a multiple parallel system. 

David


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« Reply #55 on: December 17, 2009, 08:37:40 AM »

One thing I forgot to mention.  My driver's bus heat and defroster are still in the coach and separate from the Webasto.  When driving in freezing cold and snow at 60mph it takes the driver's heat, defroster and the whole house system to stay warm in the coach.   It gets cold up front near those windows.

David
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