Mini Split AC A2W conversion Design-Q

Don_Karlos · 03-30-22, 10:08 AM

Hi,
I'm currently working on a conversion design to convert a Mini Split AC Cooling/Heating Unit to an A2W System for heating purposes. The first Question I have is, what is the best place to place the Water Heat exchanger. Most commercial HPs have it in the outdoor unit. Most of the diy conversion I found in the web are different and they have the WWHX close to the heater System in the house. What is the better Solution?
Thanks in advance
Karl

Robl · 03-30-22, 10:48 AM

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.

Don_Karlos · 03-30-22, 02:29 PM

Ok, thanks for the explanation. That means, that if I can get my unit as close as possible to the basement, where my heating system is, I could build the WWHX into the outdoor unit. I would have less than 1m WW Line outside my building hull. I will use a simple anti freeze cord system and put alot of insulation around the WW and CW Line to the WWHX. That will simplify the plumbing for the WWHX. Thx.

03-30-22, 10:08 AM	#1
Don_Karlos Lurking Renovator Join Date: Mar 2022 Location: Magdeburg, Germany Posts: 5 Thanks: 0 Thanked 1 Time in 1 Post	Mini Split AC A2W conversion Design-Q Hi, I'm currently working on a conversion design to convert a Mini Split AC Cooling/Heating Unit to an A2W System for heating purposes. The first Question I have is, what is the best place to place the Water Heat exchanger. Most commercial HPs have it in the outdoor unit. Most of the diy conversion I found in the web are different and they have the WWHX close to the heater System in the house. What is the better Solution? Thanks in advance Karl

03-30-22, 10:48 AM	#2
Robl Helper EcoRenovator Join Date: Oct 2020 Location: Cambridge, UK Posts: 31 Thanks: 3 Thanked 27 Times in 14 Posts	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.

03-30-22, 02:29 PM	#3
Don_Karlos Lurking Renovator Join Date: Mar 2022 Location: Magdeburg, Germany Posts: 5 Thanks: 0 Thanked 1 Time in 1 Post	Ok, thanks for the explanation. That means, that if I can get my unit as close as possible to the basement, where my heating system is, I could build the WWHX into the outdoor unit. I would have less than 1m WW Line outside my building hull. I will use a simple anti freeze cord system and put alot of insulation around the WW and CW Line to the WWHX. That will simplify the plumbing for the WWHX. Thx.