I'm in the process of selecting an air-to-water ASHP for our upcoming house renovation.  I prefer a split system to remove the risk with a monobloc of the external water supply/return freezing when the unit is shutdown (we often spend long stints away from home in the winter).  Plus our planning permission requires it to be on the wall of the house so I want to minimise size and weight. However I really like one of the Vaillant arotherm models as they use propane as a refrigerant rather than the usual high GWP refrigerants - the only problem (for me) is that it's a monobloc.

 

I was wondering what others' experience is with monoblocs.  I can get around the size/weight issue but it's the freezing I'm wondering about.  Is it a major task adding glycol to the entire heating circuit and does it affect how the system is maintained?  I'd use non-toxic propylene glycol so no problems with leaks/draining etc. 

 

Or are there other solutions? I'm not too keen on the freeze protection valves as I prefer passive solutions (i.e avoiding the problem) rather than secondary protection.

 

 

Plate heat exchanger to separate the circuits.

 

But you are highly unlikely to have the heating off when the weather is so cold that the heating circuit could free.

 

Or do you mean the ASHP frosting up when it is working hard in the right temperature and RH conditions?

9 minutes ago, SteamyTea said:

Plate heat exchanger to separate the circuits.

 

But you are highly unlikely to have the heating off when the weather is so cold that the heating circuit could free.

 

Or do you mean the ASHP frosting up when it is working hard in the right temperature and RH conditions?

Thanks I'll take a look at plate heat exchangers.  I did come across a Panasonic model that uses a intermediate brine circuit which I assume is the same principle, but I havent looked at installing an additional exchanger . The house will be empty for extended periods in the middle of the winter and I'd rather avoid having to leave everything running, plus even if I did leave it on low heat then power trips would shut it down (I'd rather not need to worry about it).

The

43 minutes ago, Fenton H said:

I was wondering what others' experience is with monoblocs.  I can get around the size/weight issue but it's the freezing I'm wondering about.  Is it a major task adding glycol to the entire heating circuit and does it affect how the system is maintained?  I'd use non-toxic propylene glycol so no problems with leaks/draining etc. 

 

Once you have glycol in sufficient concentration, it won't freeze unless you live somewhere crazy cold.

 

We used some special sort of propylene glycol that was less viscous than the standard stuff, but still non-toxic. Can't find it online now unfortunately, but maybe it's what you're already looking at.

 

I wouldn't personally leave an ASHP unused for months at a time. Some of them allow you to do some sort of brief turn-on every day to keep things moving. I'm pretty sure ours (Panasonic Aquarea from 2015) has a setting along those lines.

I went with an inhibited ethylene glycol with reduced toxicity, Thermox DTX.

3 hours ago, dpmiller said:

I went with an inhibited ethylene glycol with reduced toxicity, Thermox DTX.

 

Actually, that rings a loud bell. 

@Fenton H - Vaillant have a plate heat exchanger for the Arotherm Plus, so you just have glycol in the primary circuit between the HP, heat exchanger, and DHW cylinder coil. It provides protection to -18°C at the required concentration. The HP also has a frost protection feature so it will run anyway even if no call for heat.

I've seen anti freeze valves installed instead of using glycol. Apparently glycol reduces the performance of the ashp 

23 hours ago, J1mbo said:

@Fenton H - Vaillant have a plate heat exchanger for the Arotherm Plus, so you just have glycol in the primary circuit between the HP, heat exchanger, and DHW cylinder coil. It provides protection to -18°C at the required concentration. The HP also has a frost protection feature so it will run anyway even if no call for heat.

Thanks for this, I actually found this solution a few days ago, just what I'm looking for and having researched the market Vaillant seem one of the best for providing whole systems that are readily available in the UK.

 

20 hours ago, willbish said:

Apparently glycol reduces the performance of the ashp 

not heard that before!!!!, we used a minimal amount of antifreeze anyway as we are in a warmish climate relatively.

4 hours ago, joe90 said:

not heard that before!!!!, we used a minimal amount of antifreeze anyway as we are in a warmish climate relatively.

 

Think it's something Daikin teach on their install courses. 

Probably to try and sell more of their seriously expensive valves 

 

https://www.climatecontrolsandspares.co.uk/glycol/daikin-altherma-afvalve1-anti-freeze-valve.html