Where best to site ASHP

[Iceverge]

 

There's just far too much air being moved. Quick sums say a well built house of about 4kw continuous heating load will need about 32 tons or about 40000m3 of air per day for its ASHP (~ 16 Olympic swimming pools if you work in TV units).

 

Normally the air on both sides of a wall you mentioned will be almost exactly the same temperature. It's the direct radiation from the sun makes your skin warmer. Even if there was a very localised warm pocket of air it's lightly to be in the tens of m3 and would be displaced in under a minute and replaced with the ambient environmental air. 

 

 

Defrosting I gather is a function of taking the ambient air below it's dew point rather than the actual temperature of the air alone. 

 

This can be avoided by having a correctly sized heat pump, more specifically a correctly sized heat pump evaporator fan unit to ensure enough passes thought that none of it is cooled below the freezing point of water vapour. If you're regularly defrosting  your heat pump is overloaded and the battle is largely lost at that stage. There is a trade off to be done with accepting a very small amount of this in the worst conditions VS the cost of upgrading to a larger heat pump. 

 

The increased COP in higher ambient temperatures comes from lower fan energy demand due to reduced need for airflow( to avoid vapour freezing) and a greater delta T in the heat exchanger reducing the pumping requirement for coolant. ( one of the reasons inverter pumps are better as they can match the pumping speeds ( air+water) to whats needed only). 

 

 

All this is top of my head schoolboy physics so I'm happy to be corrected if anyone disagrees. 

 

TLDR. 

 

Heat pump in the sun makes no difference. 

Avoid defrosting by buying a big enough ASHP. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[SteamyTea]

28 minutes ago, epsilonGreedy said:

 

It would help if you and @SteamyTeacould concur on the volume of air drawn through an ASHP. @SteamyTeasays "several tons a day" and you are claiming "up to"  3.7 tons an hour

Not possible to do as it depends on the make and model of ASHP, and the duty cycle.

But if you have some unique inside numbers, post them up.

30 minutes ago, epsilonGreedy said:

Would a southerly aspect help such edge cases?

No. Hardly an 'edge case' if it is constantly frosting up is it. 

[epsilonGreedy]

1 minute ago, SteamyTea said:

Not possible to do as it depends on the make and model of ASHP, and the duty cycle.

But if you have some unique inside numbers, post them up.

 

 

To a scientist of your calibre it should be entirely possible.

1. Start with the heating demand of the house.
2. Next the heat capacity of air.
3. Heat extracted per m3 of air can then be calculated based on the typical drop in temp as air flows through the ASHP.
4. Add a twiddle factor of efficiency loss in the ASHP.

Then you have you have your air volume.

[joe90]

3 hours ago, Iceverge said:

Have you any simple thermometer to keep an eye on temps for us @joe90

As you lot know I don’t do logging temps or anything remotely with tech, all I can say is I have a max min thermometer in the conservatory and if the temp out there gets above 21’ (the temp the house is stable at with mostly no heating on) I open the bifolds into the kitchen diner and into the lounge, when it dips below this I shut them!. I was very interested in this some years ago. AH can’t find it, does anyone remember the Scottish house with a wrap around conservatory with controls that helped heat the house, maybe on the green building forum???

[SteamyTea]

7 minutes ago, epsilonGreedy said:

• Start with the heating demand of the house.
• Next the heat capacity of air.
• Heat extracted per m3 of air can then be calculated based on the typical drop in temp as air flows through the ASHP.
• Add a twiddle factor of efficiency loss in the ASHP.

Off you go then, you know how to do it.  I am not your secretary.

[epsilonGreedy]

Just now, SteamyTea said:

Off you go then, you know how to do it.  I am not your secretary.

 

 

I am a philosopher and historian, numbers are tedious. I was giving you an opportunity to demonstrate your mathematical magnificence.

[SteamyTea]

1 minute ago, epsilonGreedy said:

I am a philosopher and historian

No.

You are a (expletive deleted)ing useless twat with too much time on your hands.

[epsilonGreedy]

20 minutes ago, Iceverge said:

There's just far too much air being moved. Quick sums say a well built house of about 4kw continuous heating load will need about 32 tons or about 40000m3 of air per day for its ASHP (~ 16 Olympic swimming pools if you work in TV units).

 

 

Thanks for the hard numbers. My unit of measure is an average sized house.

 

Consider a house with a 10m long southern wall and eaves 5.5m high. An hour's worth of ASHP air volume is 1600 m3 at 4kW demand.

 

On a sunny crisp still winters day a warmer pocket of air might form measuring measuring 10 x 5 x 5 = 250 m3. In contrast on the north wide the property wall and ground will be colder.

 

It is not implausible that the COP will be a few % higher when heat is extracted from slightly warmed up air. Anyhow combating frosting is the principal point of interest.

[joe90]

Ignoring the above spat, I found it.     http://www.cherrymortgages.com/sustainable_zero_carbon_housing/solar_conservatory_glengariffe_passive_house_viking_ireland.htm

[Iceverge]

1 hour ago, epsilonGreedy said:

It is not implausible that the COP will be a few % higher when heat is extracted from slightly warmed up air. Anyhow combating frosting is the principal point of interest.

 

The reality is that unless you've got the space in front of the house almost entirely boxed in the the air temperature on anything other than a completely still day will be almost exactly the same as ambient. If you do have a sheltered corner the ASHP will certainly immediately stir up and mix the air with the ambient. If you were building a gigafactory and one side was in a shaded valley and the other side to the south of a large hill then maybe it might make a difference. A house is simply too small and the heat pump needs too much air for it to work. 

 

I think the confusion and intuition is to do with how we feel sitting to the north or south of our house. The sun's direct radiation,  and the lack of cooling breezes make all the difference rather than the air temp. Radiation would help an ASHP a tiny bit, and I mean tiny compared to the effect of all the air it's processing. The difference would be much less than 1% I wager. 

 

Other factors such as the fan struggling to draw too much air because the ASHP is too enclosed and a poorly insulated water pipe will have orders of magnitude more significance. 

 

 

[Marvin]

21 hours ago, Iceverge said:

I wager a better ASHP location would be as close to the house as possible to reduce pipe heat losses and in a naturally windy area so the fans don’t need to work so hard. 

Assuming the pipework from the ASHP to the emitters is not too long and very well insulated.