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Old 09-10-2020, 19:31   #1
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Join Date: Oct 2020
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Hydronic pressure system questions

Hi all,

I am designing a hydronic system for a small camper of 12m3 and have a Thermo Top Evo 5kw. I have seen references to ditching the header tank and going for a pressure system but have had trouble finding details on circuit design. I am still at the paper design stage of the whole build so change is still easy

The camper will be built from fibreglass and foam with 30mm walls/50mm floor and roof with double glazed acrylic windows so well insulated. It will be used year round, our temps fluctuate between 30 deg C to about -7, rarely lower at ski areas or inland during big southerly storms.

Hot water will be via a plate heat exchanger with tempering valve,, such as done by TwoAtSea, with a 400w DeLonghi ladder towel rail in bathroom which will be in use all year round. Winter circuit will be likely a 975w alloy radiator, possibly 1300 w finrad, +/- blowers for extra/boost heating if I find I need these. My build is weight sensitive so this is subject to being changed if the ~14kg radiator is too much weight compared to a larger kw blower.

I am aware I need sufficient load to prevent short cycling and will likely include a buffer tank of 6-10l for this. My planned load for the passive heating only when hot water exchange is not running is 400 + 1300 + 975 = 2675w, about the low output rate of the Evo.. Blower and/or hot water will increase this.

I would like to have a pressure system and haven't been able to find much detail on how to do this. Webasto suggests system liquid volume of min 4l, ideally 8l for the 5kw heaters and 2bar pressure for a closed system. They also suggest 1l expansion per kw which would make for a large expansion tank of about 5l.

Questions:
If I include a alloy or stainless buffer tank with a 2bar radiator cap, is a separate PRV still required?

Will 6-10l be big enough for a buffer?

With a total system volume of approx 8l what size expansion tank do I need?

Where in the circuit should the PRV/gauge, plate heat exchanger and buffer tank be situated?

For such a small system, is a parallel circuit still recommended?

Do I need a temperature release valve as well?

Should I bring off a side branch to a level higher than the highest installed element with an automatic air release valve?

What is the best way to fill with a coolant mix - with a secondary cheap pump from a bucket with coolant mix?

Any advice on design from the hydronic gurus would be much appreciated. I am in NZ (Chch) so if there are any other Kiwis on here (DaveFromNZ, TwoAtSea? ) would love to chat and pick your brains sometime, particularly on where to source parts locally.

Also, should we create a sticky to the various threads about hydronic systems?
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Old 09-10-2020, 22:15   #2
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Join Date: Oct 2019
Location: Kennebunk ME
Boat: Owner built 60’ Aluminum Expedition Yacht.
Posts: 1,854
Re: Hydronic pressure system questions

Hi. Is this a boat or a camper. There is a lot of information from the narrowboat folks in England. You don’t need to over plan the system and the more complex it gets, the more things can go wrong. My expansion tank has a dial temp gauge, over pressure / temp. I’ll post more later.
I bought a wimpy marine heater box then a military 3 speed exchanger which made a huge difference. I’ll see if I can find more references for you.
Happy trails to you.
Mark and his toasty manatees
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