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I've had a search and have not found much discussion of the following (maybe as I'm in warm Australia?):
I need to turn of the cabin heater water flow in warm weather but still keep the flat plate oil heater going, can't work out a simple way to sort this out.
My first WVO project is a summer car with no cabin heater, easy; heater hose from engine just circulates through the oil heater and back.

My current daily drive van will need cabin heat, but our summers are nasty so cabin aircon on full chill also needed.

If I turn of
f the current cabin heat control it stops the water flow to the oil.
If I add some Tee junctions to the heater hose I'm concerned I may not get enough coolant flow to both cabin and oil heaters?

Suggestions please? Huh
My Hiace camper had rear heater just running of a tee in heater circuit so I just fitted a FPHE instead of the heater dead easy and I can turn it of or regulate it cabin heater works fine .
Most modern vehicles have the hot water flowing through the matrix all the time. Flow of heat to cabin is controlled by flaps and mixing with dry air from air con.

You say flow to oil ceases if heater is turned off. This implies flow of hot water to heater matrix is cut off. This means a valve somewhere. Find this valve and add a 'T' piece before this valve, so when water to matrix is stopped, it will continue to flow to oil heater and back to block. It's unlikely there won't be enough heat in the coolant to operate both heater and FPHE. Make sure the pipe runs are as smooth, with gentle bends, as possible and as short as possible. If there is a heat shortage try insulating the 'going' pipes from block to FPHE.
Thanks for the replies, sadly no rear heater.
I’ll need to check out the water flow/valve issue, van is 1989 Daihatsu Delta btw.

Thank you for your replies.
Aye just have the FPHE in parallel with the heater matrix, mine have always been tee'd in like that and always plenty heat from the heater.
Hi again,
Finally got this sorted:
Coolant is controlled by a tap.
I used 2 Y joiners close to the head outlet/inlets.
It has plenty of heat! ?

Thanks for your comments.
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