My thoughts. I wouldn't be game to install a Paralleling Switch for House Batteries & Engine Start Battery. The only time I can think something like this may be required is when the engine start battery is flat and you need power to start the engine.
If the engine start battery was only just short of enough grunt to start the engine then its terminal voltage may still be reasonable and if the house batteries were connected to it, the inrush current would not be too scary. But if the engine start battery was flat due to an internal fault/short, then connecting a good set of say 800A house batteries to it has the potential to unload a lot of current in a short time into the fault. The result could be quite scary and a loud noise, molten lead and acid could appear from nowhere!!.
I have 5 x 160A batteries and each battery is capable of unloading 1,600A for 5 seconds. That's a combined total of 8,000A at you finger tips. If you have a paralleling switch on your boat, just make sure you know what you are connecting this beast to when you rotate that little paralleling switch (probably rated about 400A)
On my boat, there is no connecting between the house batteries and engine start battery positive terminals, but the negative terminals have been commoned up. I have a change over switch installed which allows the engine starter motor (and engine circuits) to EITHER receive power from the engine start battery OR the house batteries but never both at the same time.
Also my engine battery is sealed lead acid and the house batteries are AGM which have different charging requirements.
I saw in another forum topic where the guy requesting information on the electrics for a winch was advised to connect the positive terminals of his house and engine start batteries. I hope the information I have just provided my enable a more informed decision be made before doing this connection.
My thoughts anyway 
I fitted one of these blue sea
www.bluesea.com/products/7610/SI-ACR_Automatic_Charging_Relay_-_12_24V_DC_120A
www.bluesea.com/products/8690/Dual_Battery_Bank_Management_Panel
So far not a problem works well with my solar panel keeps both batteries topped up 120 amp each deep cycle cat batteries.As Im parked up for a while yet do not need any more house batteries. keeps every thing going in the boat.
Ordered two 'hundred meter rolls of Tyab tinned wire last week should be a lot of spaghetti running around the boat in the coming ,months
Ill be wiring in a fridge freezer unit next month it will be interesting to see how much power it uses.
wow, you have lot of Amps, serious power station, but
do you have any wiring which can carry those Amps, probably Not.
well, they are safe and stored in the batteries.
guess the only starter and winch motors are hungry for extra Amps.
Did you have ever need to charge house bat. from engine ?
Probably not, why bother to install it now, if you engine bat. dies,
disconnect eng. bat. , reconnect to the house until you get replacement.
Sure, I have switch between house and engine bat. but they are the same capacity
and the same way engine charging them.
Each system, boat wiring is different, people mostly referring to own boat.
In most cases works well, also can be improved.
God, it's all beyond me. I really must read Nigel Calder's Mechanical and Electrical Manual that is sitting on my coffee table. 
Select to expand quoteDrRog said..
God, it's all beyond me. I really must read Nigel Calder's Mechanical and Electrical Manual that is sitting on my coffee table.
Definitely read Calder if you have it. He explains the benefits of having a paralleling switch and, even better, paralleling relays. The latter are best, as they usually have an "auto" mode which only put the banks in parallel when there is excess charge, by means of voltage sensing. I leave mine in "auto" mode and my starter battery will never go flat, even if I drain my house batteries.
Crusoe is quite correct in that if one battery bank is heavily discharged and the other is well charged, there may be a spike of current when paralleling first occurs, so the relay and the wiring must be rated to handle the load. That said, you can usually avoid dead-battery scenarios with proactive battery monitoring. Treat yourself to a dual battery monitor so you can monitor the health of both battery banks.
BTW, paralleling or isolator switch settings should never be changed done under load.
DrRog..reading might by more confusing, typicaly explain basic principles with simple examples.
just went through above mentioned ACR, one schematic shows a fuse for the house,
next schem. and the same page completly omitted.
Which one is correct, even profesional product is full of mistakes.
HG ..nice panel,, like your bat. switch with "combine batterie position " that's
what you try achive.
ACR sound like a good idea but proper function on the boat for extended
period of time would be doubtful.
Terminals A & B are interchangeble, means if one of batteries drops under
10 volts, ACR would work.
If control line /name Start Insolation / would not get sufficient voltage drop,
,they specify current 15mA, ACR would not work.
in FAQ..they mention ..it might happen even when batteries are not fully charge
because of voltage drop is not sufficient .!.
ACR is design to carry charging current only...why 2 fuses and wiring 150 times overrated.
......it only get some charging current for one battery only, never starting or house current ....
The way they describe the unit, you excused, they have mistakes.
If you not sure ask, avoid future troubles.
Select to expand quoteDrRog said..
God, it's all beyond me. I really must read Nigel Calder's Mechanical and Electrical Manual that is sitting on my coffee table.
If you go to my profile and go to page 8 of my gallery you will find a number of scanned pages of an article called "Through the Power Maze" that was published in Cruising Helmsman some time ago. It is worth downloading them.
It is in layman's terms and easy to understand. It is a very up to date electrical system plan.
Select to expand quoteCharriot said..
DrRog..reading might by more confusing, typicaly explain basic principles with simple examples.
just went through above mentioned ACR, one schematic shows a fuse for the house,
next schem. and the same page completly omitted.
Which one is correct, even profesional product is full of mistakes.
HG ..nice panel,, like your bat. switch with "combine batterie position " that's
what you try achive.
ACR sound like a good idea but proper function on the boat for extended
period of time would be doubtful.
Terminals A & B are interchangeble, means if one of batteries drops under
10 volts, ACR would work.
If control line /name Start Insolation / would not get sufficient voltage drop,
,they specify current 15mA, ACR would not work.
in FAQ..they mention ..it might happen even when batteries are not fully charge
because of voltage drop is not sufficient .!.
ACR is design to carry charging current only...why 2 fuses and wiring 150 times overrated.
......it only get some charging current for one battery only, never starting or house current ....
The way they describe the unit, you excused, they have mistakes.
If you not sure ask, avoid future troubles.
When you get battery cable for free why not, One of works contractors gave it to me
My solar panel and the ARC appear to work fine all Ive wanted was to keep my batteries fully charged and they have done that.
The double fuse was fitted as I fond out later on thats there maybe a problem with the ARC unit I still have to check the serial number see if its in the range of the possible fault
Ive just ordered last week, two , one hundred meter rolls of Tyab twin core double insulated tinned wire will be here next week I have not started with the electrical system yet every thing electrical has been temp since Ive had the boat. Give it another year
That photo is over 6 months old theres a lot more temp wiring in there now and the configuration would not be the same as the above photo is showing and will change completely over the next twelve months.
This is the diagram that Ive followed
Guess, you have that under control. if you have any discrepancies ..ask.
Proper function of ACR....first set up monitoring using LED
second using solar cut of switch and the right connection of control line of ACR should work ok
third..before you finalise wiring and ACR playing up, before you leave the boat, connect
both pluses with jumper, solar cut off takes care of it, both bat. will be fully charge all the time.
when you get on boart, remove the jumper and monitor proper function of ACR
Before you star any new wiring would be good idea to draw all required circuitry.
Select to expand quoteCharriot said..
Guess, you have that under control. if you have any discrepancies ..ask.
Proper function of ACR....first set up monitoring using LED
second using solar cut of switch and the right connection of control line of ACR should work ok
third..before you finalise wiring and ACR playing up, before you leave the boat, connect
both pluses with jumper, solar cut off takes care of it, both bat. will be fully charge all the time.
when you get on boart, remove the jumper and monitor proper function of ACR
Before you star any new wiring would be good idea to draw all required circuitry.
One of the reasons Ive just done temp stuff is im not sure what Instruments to use yet and still sorting out where Im placing things . The two batteries on board at the moment are one engine and the other will be my anchor windlass and spare I have not bought any house batteries yet as I'm not sure what power will be used on board. Once I get my fridge installed in the next month or so I should have a vague idea and that will also tell me what size solar panel I need. The one I have takes up the cabin roof . But I wont alter that till I have my masts back on and sails on as I'm not sure what shade problems Ill have and how much power Ill loose
