Air-to-air vs air-to-water

[JohnMo]

1 hour ago, Beelbeebub said:

best bet would be if we could get a refrigerant that has the low GWP of r290

CO2

[SteamyTea]

1 minute ago, JohnMo said:

CO2

Which is set at 1.

 

CO_2, a decade ago, was the next big thing.  Wonder why it never happened.

[JohnMo]

4 minutes ago, SteamyTea said:

Wonder why it never happened.

Think it needs a big dT, unlike most refrigerants. But you can get them from Mitsubishi I believe.

[Beelbeebub]

Wouldn't the high dT requirement not be an issue with a2a heating. The lower temp can be just above room temp ie about 20C or so.

 

The biggest issue is the high pressure - over 100bar for the higher temps. I think this is at the upper limit of soft copper tubing

[saveasteading]

3 hours ago, Beelbeebub said:

couple of grand each and efficency. CoPs of about 3 tops. 

These are as low as  €300 or so in Spain, with one each side of a wall, so also easy to do.

They are a bit inefficient though, and noisy so  prices rise from there. 

[Beelbeebub]

8 hours ago, saveasteading said:

These are as low as  €300 or so in Spain, with one each side of a wall, so also easy to do.

They are a bit inefficient though, and noisy so  prices rise from there. 

Thats a standard split aystem.

 

The monoblocs are a single unit that fits in the room with no external unit at all, just 2 ducts to move air from outside, through the unit and then exhaust it.

 

All you see on the outside are 2 air vents.

[saveasteading]

12 minutes ago, Beelbeebub said:

The monoblocs are a single unit

 I didnt know of these. What is the advantage?

[RobLe]

I’ve been looking out for R290 based regular split system air conditioners now that regs have relaxed and allow them - there’s one by electriQ, and it seems like more are on the way:

https://naturalrefrigerants.com/the-rise-of-propane-based-residential-ac-in-europe/

[Iceverge]

9 hours ago, saveasteading said:

These are as low as  €300 or so in Spain, with one each side of a wall, so also easy to do.

They are a bit inefficient though, and noisy so  prices rise from there. 

 

Plug that in and set to heat on a damp day in the British Isles and you'll quickly have block of ice on the outdoor unit. 

[saveasteading]

3 hours ago, Iceverge said:

quickly have block of ice on the outdoor unit. 

Why? I thought it was just another ashp.

[joth]

5 hours ago, saveasteading said:

 I didnt know of these. What is the advantage?

No refrigeration pipework makes self install much easier

[Iceverge]

1 hour ago, saveasteading said:

Why? I thought it was just another ashp.

 

They tend to be specced for cooling in a warm climate. Put them in a cool/damp climate and ask them to do some heating and they run into trouble. I think the outdoor units are just far to small to cope without icing up. 

 

A2A for heating is quite popular in Scandinavia. When picking mine I went to the Norwegian websites of the manufacturers and checked their offerings. 

 

I ended up getting the unit from a Italian company through eBay based on the spec from that. 

 

 

 

Edited December 10 by Iceverge

[NIMAN]

7 hours ago, Iceverge said:

 

Put them in a cool/damp climate and ask them to do some heating and they run into trouble.

 

 

Does seem like a fly in the ointment in these parts.

 

Bumped into a Monsieur Steamy thread a bit earlier today that had some good ideas but went cold:

https://forum.buildhub.org.uk/topic/5928-ashp-frosting-cure/

 

So went looking and found an interesting "Frost free" Analysis - including a "non stick" cure but none of those seem available yet. Another one serves up taking images of when frosting starts so could maybe build some of that into a Home Automation routine.

 

7 hours ago, Iceverge said:

 

I think the outdoor units are just far to small to cope without icing up. 

 

 

Not at all sure about that.

 

Having stumbled across a Canadian empirical study that modelled a quadratic COP formula (steady state) to take thermal load (as % of max) and external ambient temperature into account (not yet sure how to reference it yet though but will look into that).

 

I have a few years of hourly weather data available (Visual Crossing gives  900 odd free hours/day).

 

So, randomly picking Calendar year 2008, after calculating the estimated hourly fabric / air change loss), matched that with  the hourly A2A COP formula and enabled supplementary resistive heating to manage those hours when the heat pump maximum output was exceeded and turned the heatpump off when the minimum steady state output was exceeded to prevent cycling.   

 

Spreadsheet modelling changes to mini split max output power is straightforward and permits calculation to maximise what, I hope, could be a fairly realistic SEER.

 

The example screenshot is a bit busy but, based on the 8784 hourly records, shows a 4.22 SEER for a 2 zone (2 mini split A2A) structure both set at 18C in the model example (will try to make that clearer in a follow-up later). 

 

There are no smart tarrifs available here yet but could already be natural gas price competive at that all source SEER (resistive / heatpump) heating even without considering off-peak battery sorage.

 

Of course, Home Automation should be possible to acount for smart tarrifs.

 

[Iceverge]

Got ya. Electric heater kicks in when it gets too cold for the heat pump. 

 

Matching a suitable one for our climate isn't too hard though. 

 

 

[NIMAN]

10 hours ago, Iceverge said:

Matching a suitable one for our climate isn't too hard though.

 

Not at all sure about that.

 

For example, all I need do is choose the same Zone 1 / Zone 2 heat pumps (max power at 1700W / 4371 W) and change fabric loss Zone1 W/K from 29 to e.g. 60 and Zone 2 from 149 to e.g. 100 in the hourly quadratic (empirical based) calculator and the combined heat pump /electric heat source SEER drops from 4.22 to 3.77 (image attached).

 

Personally I wouldn't touch any ASHP without a year or 3 of hourly weather data and also having carried out a fabric/airchange Heat Loss Coefficient calculation (W/K) after the refurbishment / extension was built to monitor the actual steady state heat loss over a week or two having heated it to a constant temperature.

 

There are so many horror stories of installer calculated fabric losses / heatpump sizing resulting in SEER of 2 or less to be tempted by another way.

 

The frosting  / home automation supplementary heating / cycle limiting aspect also needs to be more fully investigated & implemeted too imho.

 

[JohnMo]

4 hours ago, NIMAN said:

SEER of 2

EER/SEER is cooling, CoP/SCoP is for heating.

 

Why would you design a system around 18 Deg house heat temp? Way to cold.

4 hours ago, NIMAN said:

Personally I wouldn't touch any ASHP without a year or 3 of hourly weather data and also having carried out a fabric/airchange Heat Loss Coefficient calculation (W/K) after the refurbishment / extension was built to monitor the actual steady state heat loss over a week or two having heated it to a constant temperature.

Suggest you are over thinking the whole thing. Calculation of heat loss is needed, but that isn't difficult.

 

Add 10% for DHW and/or defrosting, which generally occurs between +3 to -3. Design to cover 99.8% of likely outside temperatures.

 

It is unlikely the heat pumps on offer, will exactly match the kW required exactly, so you choose the one one up. Cycling isn't an issue generally and the worst can be fine tuned out.

 

Most issues with A2W revolves around distribution system in house being split into many zones, poorly installed and designed buffers and way to high a design flow temperature. Then home owners ignorant of how to operate and wanting to operate how they did with a gas boiler.  Most these issues are not relevant A2A as the equipment and how it runs is factory designed and set, and the home owner cannot mess with it.

 

Edited December 11 by JohnMo

[Iceverge]

12 hours ago, NIMAN said:

 

Not at all sure about that.

 

For example, all I need do is choose the same Zone 1 / Zone 2 heat pumps (max power at 1700W / 4371 W) and change fabric loss Zone1 W/K from 29 to e.g. 60 and Zone 2 from 149 to e.g. 100 in the hourly quadratic (empirical based) calculator and the combined heat pump /electric heat source SEER drops from 4.22 to 3.77 (image attached).

 

Personally I wouldn't touch any ASHP without a year or 3 of hourly weather data and also having carried out a fabric/airchange Heat Loss Coefficient calculation (W/K) after the refurbishment / extension was built to monitor the actual steady state heat loss over a week or two having heated it to a constant temperature.

 

There are so many horror stories of installer calculated fabric losses / heatpump sizing resulting in SEER of 2 or less to be tempted by another way.

 

The frosting  / home automation supplementary heating / cycle limiting aspect also needs to be more fully investigated & implemeted too imho.

 

 

 

Heat pumps can be done badly. There's no doubt about it. No different than any wet heating system.  An A2A generally is much less vunerable to a crappy install than an A2W. 

 

Running your house on direct electric and monitoring the usage patterns is no bad idea. We did it for a few years and had a really good idea of the house's usage. 

 

Calculations tend to run into a big issue once a real house is considered. There's too many variables, or children as I like to call them. 

 

 

 

 

[SteamyTea]

18 hours ago, NIMAN said:

Personally I wouldn't touch any ASHP without a year or 3 of hourly weather data and also having carried out a fabric/airchange Heat Loss Coefficient calculation (W/K) after the refurbishment / extension was built to monitor the actual steady state heat loss over a week or two having heated it to a constant temperature.

"All models are wrong, some models are useful"

 

Easy enough to get the weather data.

[NIMAN]

20 hours ago, JohnMo said:

EER/SEER is cooling, CoP/SCoP is for heating.

 

Oops, newbie here - sorry about that!!

 

20 hours ago, JohnMo said:

Why would you design a system around 18 Deg house heat temp? Way to cold.

 

You must be younger and/or richer than me but that wouldn't be difficult. We used to have large icicles on the inside of the windows at times.

 

The hourly spreadsheet heat loss / cop calculator (based on the emprical study quadratic) is setup as 2 zones with a dividing concrete block wall currently set at 65W/K. Intenal heat transfer between the zones is also accounted for if wishing to investigate different target temperatures in each - I am contemplating internal wall insulation on the internal divider to reduce it to 13W/K in order to maintain 21C / 10C zones with a log burner in the cold one to make it comfy on the odd party night. It'd save a fortune in running costs but simultaneusly massively undersize / oversize the 2 heatpumps compared to 21C single zone throughout.

 

I've attached, as an example, my current 1958 external wall buildup calculation. Without the hole ridden Urea Formaldehyde cavity insulation U is 1.665 and. Before the insulation crumbled it seems like it could have been 0.574.  If you have any idea if that could now be somewhere in between the 2 figures or, perhaps, worse now than when it was a clear ventilated cavity it'd be nice to know.

 

Millions live in leaky houses with opaque fabric constuction - in my opinion, anything other than a new build is a calculation guessing game:

 

 

 

20 hours ago, JohnMo said:

Add 10% for DHW and/or defrosting

 

The empirical study was for A2A steady state i.e. flat out at top end and maximum modulation at the low end but zero cycling / zero DHW / no frosting.

 

20 hours ago, JohnMo said:

Add 10% ...[]... for defrosting, which generally occurs between +3 to -3.

 

The frosting study I encountered began with reference to a previous study which reported frosting conditions between -5C & 7C with RH between 60% ad 100%. It then went on to divide and categorise useful regions (e.g. slight frosting increasing COP due to the larger evaporator surface caused by the frosting) - from the not so nice  ... in other words, its complicated.

 

I haven't yet seen a document describing the manufacturers algorithmic why/when they decide to start pumping de-ice energy out of our houses rather than, for example, simply turning the unit off before the stuff is likely to appear (or spot it on a camera).

 

3190 hours during 2008 here met the conditions described (oops, on re-reading this noticed I had forgot to filter those between heating / non heating hours ).

 

Anyway, it seems strange, to me, not to investigate this further.

 

21 hours ago, JohnMo said:

Design to cover 99.8% of likely outside temperatures.

 

Heat pump COP falls with both temperature and to either side of, around, 50%, of thermal load as a percentage of maximum capacity - a single quadratic covers it all.

 

You might get lucky with your design ethos but why bother.

 

21 hours ago, JohnMo said:

It is unlikely the heat pumps on offer, will exactly match the kW required exactly, so you choose the one one up.

 

There are online databases with details of tens of thousand of them - i'd be surprised that, if you really want one, it could not be a very close match.

 

21 hours ago, JohnMo said:

Cycling isn't an issue generally and the worst can be fine tuned out.

 

 

I'm currently looking at 282 hourly records (from entire 2008) in the calculator with choice of heatpump (2 no' 4.7kW max units specified at 7C) and both zones set at 21C for the entire heating period in which they were unable to modulate low enough without cycling (external temps ranging from 17.6C to 20.9C).

 

How is it known that switching the units off at greater than 17.6C (and using resistive heating) might not be less costly to run and with less wear & tear.

 

21 hours ago, JohnMo said:

Most these issues are not relevant A2A as the equipment and how it runs is factory designed and set, and the home owner cannot mess with it.

 

I like the "seeming" simplicity of A2A - it seems to concentrate discussions around what really matters (energy rather than plumbing).

 

I'm not necessarily talking about messing with anything - simply read the supplied remote IR codes and, with some relevant sensors, set some meaningful Home Automation system routines up as investigation using the manufactureres existing interface.

 

22 hours ago, JohnMo said:

Suggest you are over thinking the whole thing.

 

Think you are under thinking the whole thing ... but would much appreciate being further debunked where necessary since she says i'm rarely correct.

 

As I say, mostly new to this stuff and with no practical experience of it 😉

[NIMAN]

14 hours ago, Iceverge said:

 There's too many variables, or children as I like to call them. 

 

 

True, but even they are, mostly, somewhat calculated - even if it is only by one of the couple 😄

[NIMAN]

For the record, related to this comment:

 

 

The smallish sample of mini-splits involved in the empirical investigation demonstrated lowest output (at lowest external temprature) at around 10% of maximum output (at the highest external temperature at which that highest output was acheieved) i.e. effectively a fixed amount.

 

My spreadsheet model also, currently, uses this fixed 10% to calculate lowest steady state modulated output.

[Beelbeebub]

The point to note about output modulation is that it can be misleading to just think of a fixed % of the. Maximum.

 

This is because the point at which you are likely to need the lowest output is usually when the outside temp is at it's highest

 

Your HP mauly be able to modulate down to 25% of max output but that max output grows as the outside temp increaces

 

So 25% of 5kw at - 5C is 1.25kw

 

But if your output is 8kw at 15C it's 2kw.

 

And your demand at 15C is lower than your demand at -5C.

 

So for the true picture of modulation you need to look at your highest power output at the lowest OAT and your lowest power output at the highest OAT.

[SteamyTea]

5 hours ago, Beelbeebub said:

So for the true picture of modulation you need to look at your highest power output at the lowest OAT and your lowest power output at the highest OAT.

Yes, I think that is right.

And then you need to cheat the CoP values at the extremes.

It all boils down to the energy/power.

8 hours ago, NIMAN said:

%

As mentioned, percentages are very misleading. Get the absolute numbers.

 

In statistics, which energy transferred in the biosphere is all about, always ask is it a big, or small number, then ask if it is relevant.

[NIMAN]

1 hour ago, SteamyTea said:

 

As mentioned, percentages are very misleading. Get the absolute numbers.

 

The quadratic developed from the empirical study was based on normalised figures - the machines used had different absolute figures.

 

%'s are then, of course, the opposite of misleading (since the Physics appered to be very similar across the sample).  

 

7 hours ago, Beelbeebub said:

So for the true picture of modulation you need to look at your highest power output at the lowest OAT and your lowest power output at the highest OAT.

 

Based on my observation of the normalised experimental figures, I chose, for now, to limit the lowest outputs to 10% and use a relatively blunt 3 band external temperature lookup table for the maximum outputs.

 

This is an example graph of my spreadsheet quadratic developed from the empirical study of (COP v THERMAL LOAD AS A PERCENTAGE OF MAXIMUM CAPACITY) at 4 external temperatures:

 

(The empirical study was carried out at an internal temperature of 21C - I asked the Author for permission to share the link some time ago but have received no reply)