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Old 03-30-22, 09:48 AM   #2
Robl
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Join Date: Oct 2020
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If the HX (usually a Brazed Plate Heat Exchanger, BPHE) is inside the outdoor unit, then it is simpler to make gastight. Most domestic A2W systems in the UK are monobloc like this; They are cheaper, don't need an f-gas engineer to fit, and the monobloc is factory sealed for life. However this will mean water circulating to the outdoor unit, and in most climates there is a freezing risk, and a way must be found to prevent the damage that would eventually occur due to freezing of water in pipes and the heatexchanger.
Possible freezing solutions I've come across are (in addition to insulating outdoor water pipes & the HX):
Glycol in the water system; it's expensive for a big system, so sometimes there is an additional heatexchanger and pump in the house, then the bulk of the water circuit can be plain water.
Electronic protection - rely of activating the system to protect it from freezing. Not so good if you get power cuts.
Anti freeze valves - positioned at the bottom of the system, automatically opening to let out a bit of water, so warming the system up a bit, without need for electricity. Likely needs a vented system with auto fill.

In contrast, HX inside the house is conceptually simpler, but will need longer heatexchanger fluid runs, more brazed joints, some of which are indoors. R290 is a popular gas on this forum, there are limits to how much of it can be used indoors due to an explosion risk.

I guess to answer your question - if an A2A unit is being modded, the non F-gas option is ruled out already. Maybe that's why split systems are more popular here.
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