ASHP v air conditioning for cooling

[Ommm]

I'm speccing out an ASHP system with radiators at the moment.  One of my requirements is cooling in the summer for a couple of rooms (ideally want temps <20C which no amount of passive measures are going to help with).  While it's not advertised or supported by the installer I think I know how to put the ASHPs being offered into cooling mode, so I seem to have two options:

 

1. Fan coil radiators on a separate ASHP zone, insulated pipework, condensate drains.  Going to cost about £300-400 for each radiator plus associated electrics/plumbing works
2. Separate multisplit air conditioning unit with refrigerant pipework to each room, wall mounted fan evaporator unit. Maybe £800-1200 for the A/C unit plus installation (perhaps some DIY but need F-gas for the refrigerant side), let's say another £600.

 

ASHP is cheaper but not enormously so.  The ASHP is easier to expand to other rooms by fitting more FCUs, although insulating all the internal pipework and running condensate drains would be annoying it wouldn't be impossible. ASHP is potentially quieter than a typical A/C wall unit.

 

But I'm wondering about the relative effectiveness of each approach.  With an ASHP in cooling mode you're pushing around cold water at 7C, and your temperature change is dependent on how much air you can blow past the heat exchanger in the FCU.  With a refrigerant system the heat is being absorbed by the refrigerant evaporating.  I can't help but think that the phase change of evaporation is going to be more effective than just shipping around cold water.

 

So I'm wondering if anyone who uses an ASHP in cooling mode has any advice as to how it compares to 'proper' A/C?

Edited August 15, 2021 by Ommm

[JamesP]

Have a look at this link

I have tried using the ASHP in cooling mode but had minimal effect here, we have no shading at all so can overheat on the very warm days. More effective is to close all the windows for us.

Also plumbed and wired for separate AC unit to cover the upstairs bedrooms and main living space to install later.

For us, getting a good nights sleep was the priority.

[Ommm]

Thanks, I've been posting to that thread   I thought I'd start a new one as that thread has rather got into the specifics of sourcing and installing fan coils, whereas I'm more interested in performance of ASHPs against air conditioning.

 

What are you using for emitters for your ASHP in cooling mode?

 

(We're east-facing with no shading which has lots of solar gain problems, but when you want to cool below night time air temp no amount of shading will help)

[Andehh]

Just a bump from me, very keen to know more about this as well! Thanks

[Dave Jones]

could you mount the indoor part of a split system into your incoming side of the MVHR so it cools it right down, a decent KW split system ?

[Andehh]

Even more interested in this thread now, as going for ASHP & MVHR as well....!!

[Thorfun]

I can't comment on the effectiveness of either systems but we are going through this decision at the moment and we have decided to use a separate A/C system (ducted and split unit mixture) around the house. it is a much more expensive solution but less complicated as the ASHP isn't performing multiple functions. Plus we can use the A/C on the first floor as supplementary heating if required as we're not having UFH in the bedrooms.

 

obviously, this is a personal choice for each project and we have, at least, decided to simply first fix all the pipework we could require for the A/C system and then if budget allows fit straight away otherwise we will live in the house for a couple of years and see how it performs and if we need cooling we can add the A/C units at a later date.

 

it all comes down to your modelled cooling requirements and whether the ASHP can provide that level of cooling in the end and even a well calculated model can't guarantee the results as far as I can see and so simply preparing for the worst seems the most prudent solution for us.

[Marvin]

Of course using he ASHP for cooling requires a lot less energy than heating because:

 

Heating:

Outside air temp 0 degrees

Inside required temp 23 degrees

Difference 23 degrees

 

Cooling

Outside temp 33 degrees

Inside required temp 21 degrees

Difference 12 degrees

 

[smart51]

I'm considering passive cooling for my new build.  Since we're digging foundations anyway, it is no hardship to dig a trench for an underground air pipe.  And since we're having MHRV, all the incoming air comes through a pipe anyway, why not an underground pipe?

 

In the UK, the temperature below ground is 12°C all year.  Even if you only go a couple of metres down, it is 14°C in the summer and 10°C in winter.  Winter air is pre heated by passing underground and summer air is pre cooled.  If your incoming fresh air is, say, 17°C, do you need powered cooling in Britain?

[IanR]

10 minutes ago, smart51 said:

I'm considering passive cooling for my new build.  Since we're digging foundations anyway, it is no hardship to dig a trench for an underground air pipe.

There was lots of chat on this 10 years ago or so. iirc the consensus then was to use silver-lined pipes to avoid health issues related to mould growth. Not sure what the current thinking is, but do your research, it could be a breeding ground for mould.

[Marvin]

26 minutes ago, smart51 said:

I'm considering passive cooling for my new build.  Since we're digging foundations anyway, it is no hardship to dig a trench for an underground air pipe.  And since we're having MHRV, all the incoming air comes through a pipe anyway, why not an underground pipe?

 

In the UK, the temperature below ground is 12°C all year.  Even if you only go a couple of metres down, it is 14°C in the summer and 10°C in winter.  Winter air is pre heated by passing underground and summer air is pre cooled.  If your incoming fresh air is, say, 17°C, do you need powered cooling in Britain?

The mathematics on this one is important. How much pipe surface is required to change the temprature of the air entering the pipe to exiting the pipe to signifcantly effect the building. It's a bit like a GSHP in that way. I think it's quite a length of pipe! 

 

 

 

 

[joth]

2 hours ago, smart51 said:

If your incoming fresh air is, say, 17°C, do you need powered cooling in Britain?

Depends entirely on what solar gains and other heat sources are in the building. If it's a dark cave with one inhabitant, probably not, if it's a greenhouse containing bitcoin server farm and 5 people then some active cooling will be necessary.

Also, how happy are the inhabitants to sleep with doors and windows open. I don't believe MVHR ventilation rates would ever shift enough air to keep even a single bedroom cool indefinitely. The heat output of one bed-bound human is greater than that which one mvhr outlet alone can offset . 

 

The most effective cooling in our house is automated stack venting via skylights.

The ashp+FCU is very useful, mainly held back by poor sizing fcu for the ashp, resulting in a lot of short cycling. I will add a larger buffer tank at some point (and probably another fcu) to balance it out.

 

A dedicated A/C would definitely have been more effective, primarily because it would have required me to pay for professional design and installation of a fully specified system. As it was I just slung in an fcu and pipes on the offchance it was useful, and not finding it is, I'm on long haul project to upgrade it to get it to full potential.

If UK ashp installers were used to design + install of cooling systems it would be very different matter. 

Edited September 26, 2021 by joth

[SteamyTea]

3 hours ago, Marvin said:

How much pipe surface is required to change the temprature of the air entering the pipe to exiting the pipe to signifcantly effect the building.

You can work it out.

Surface area of pipe, thermal conductivity of pipe material, thickness of pipe material and the temperature differences.

Just the same as working out the thermal losses through a wall or window.

You may have to use a multiplier for the ground temperature depending on the soil type and moisture content.  Use an indices if you want to get posh. 0 makes the number become 1, 1 keeps the number the same and anything greater than 1 makes it grow.

 

I am not sure, after thinking about it for a decade or so, if mould is a problem. Decent filters (HEPA and activated charcoal) should be fitted, as well as insect and rodent barriers.

Then just an annual sweep with a chimney sweep brush. So design cleaning in from the onset.

Some sort of water drainage may also be needed.

 

[Gone West]

6 hours ago, smart51 said:

I'm considering passive cooling for my new build.  Since we're digging foundations anyway, it is no hardship to dig a trench for an underground air pipe.  And since we're having MHRV, all the incoming air comes through a pipe anyway, why not an underground pipe?

 

In the UK, the temperature below ground is 12°C all year.  Even if you only go a couple of metres down, it is 14°C in the summer and 10°C in winter.  Winter air is pre heated by passing underground and summer air is pre cooled.  If your incoming fresh air is, say, 17°C, do you need powered cooling in Britain?

I looked at this around twelve years ago when I was designing my PH and decided it wasn't worth the cost and effort.

https://www.rehau.com/uk-en/ground-heat-exchanger

 

[smart51]

3 minutes ago, Gone West said:

I looked at this around twelve years ago when I was designing my PH and decided it wasn't worth the cost and effort.

https://www.rehau.com/uk-en/ground-heat-exchanger

 

What kind of cost is a ground heat exchanger on a new build?  (We're planning for about 200m2, possibly single storey if that is a big factor).  It's not the sort of thing I've seen prices for online.

[Gone West]

1 minute ago, smart51 said:

What kind of cost is a ground heat exchanger on a new build?  (We're planning for about 200m2, possibly single storey if that is a big factor).  It's not the sort of thing I've seen prices for online.

I'm sorry I don't still have the costings from my research. IIRC I contacted Rehau and they gave me the name of a design company who gave me the info I needed.

[smart51]

Prices from £2300 to £5300 depending on size.  That's probably a 25 year payback.  Something that would need a bit of thought before committing to.

https://www.phstore.co.uk/awadukt-thermo

Edited September 27, 2021 by smart51