Air Source Heat Pumps

stevehead

Thought I'd bump this thread with a ref to some interesting reading: Here's a link to a chapter in David JC MacKay's book 'Sustainable Energy - without the hot air'.
http://www.inference.phy.cam.ac.uk/withouthotair/c21/page_146.shtml
David's take on ASHP's starts at the bottom of that page.
If you like his style the rest of the book is a good read too.

eg
Condensing GAS = 90% efficient
New Gas Power Station + ASHP = 175% efficient
GSHP's no good in UK urban environment

richardc1983

Very interesting read... thanks.

How are your heat recovery units doing on a night when outside temps drop to about 7c at present?

stevehead

richardc1983 wrote: »

Very interesting read... thanks.

How are your heat recovery units doing on a night when outside temps drop to about 7c at present?

Difficult to quantify due to varying conditions Rich, all I can offer is an impression.
It goes without saying that they keep the house fresh and dry as they certainly ventilate.
I switch the ASHP's on for less hours than I would normally, and I put it down to keeping the windows closed.

Firstly - cooking for a family of 4 in an open plan downstairs generates a lot of heat & wet. We'd usually open the door / windows to get rid of the smells & condensation. I just let the Mitsi HRV deal with air exchange so the downstairs stays warm. There is no evidence that we have cooked in around an hour, but the temp remains 4C higher than before cooking, so the ASHP remains off. I even leave the washing up bowl to cool down before emptying it - I'll have that heat back please!

Secondly - no open window in the upstairs bathroom. A hot shower causes the bathroom air temp to rise. The Kair removes the humidity, but leave the bathroom 2C warmer than pre-shower.
There's also no air moving from downstairs to upstairs and out the open window.

7C is the temp at which the Kair claims 86% heat recovery.

Andy_WSM

stevehead wrote: »

I even leave the washing up bowl to cool down before emptying it - I'll have that heat back please!

:T Superb!

Sounds like a great unit Steve.

richardc1983

Its great Steve. I want one! I hate cooking smells especially if it involves a frying pan and oil or grill.

I have the kitchen window open but the smell still lingers.

IN fact all the windows in my house now are on the ventilation catches and ive got the ASHP on as its only 9C outside today.

Have you had to use the heating yet?

stevehead

Just a tad - you can see my leccy creeping up this month.



This graph is from iMeasure. All the spikey bits are where I've entered the details a bit late. iMeasure dips one reading, then adds it to the next so it looks very uneven.

richardc1983

Mines been on all day with the windows on the catch etc, shud be off for till 10pm now though as the central heating for the whole building has come on till 10!

Im actually getting a wall mounted unit fitted on friday for the back room as my flat mates room is currently at 12C during the day, far too cold and the radiator just doesnt heat the room up as the room is so cold, despite the house having been refurbished fully 5 yrs ago!

mech_2

Seems to me that MHRVs are at odds with an ASHP. One technology is trying to warm the air from outside by cooling air from indoors and the other is trying to cool outside air to warm the air indoors. What you want is to apply the heat pump to the stream of air from indoors, thus rather than losing 15% of the heat from ventilation you use it to improve the heat pump's COP.

(PS: ASHP 175% as efficient as gas heating? I think not).

richardc1983

Thats a good idea mech but actually having the exhaust air duct located behind the air intake of the outdoor unit to recapature the little bit of heat that is lost from the house would probably mean steve relocating his outdoor units which would cost money.

It would prob cost more money to do so than the ammount of heat that is lost from the house I would say.

Steve im sure will reply soon. The idea is workable though but not with steves setup.

A lot of buildings use heat recovery technology where some rooms being cooled the heat removed from that room is sent to a differnt part of the building for either heating rooms requiring heating or to heat hot water.

These systems cost a lot of money.

stevehead

mech wrote: »

Seems to me that MHRVs are at odds with an ASHP. One technology is trying to warm the air from outside by cooling air from indoors and the other is trying to cool outside air to warm the air indoors. What you want is to apply the heat pump to the stream of air from indoors, thus rather than losing 15% of the heat from ventilation you use it to improve the heat pump's COP.

(PS: ASHP 175% as efficient as gas heating? I think not).

Hi Mech,
I see how you concluded this - I could duct the outflowing air over the ASHP compressor unit and grab the lost heat back. Neat. The outdoor ASHP unit is only about 4m from my HRV Exhaust.
I won't do it though. The ASHP outdoor unit moves a massive amount of air through it, whereas the HRV just gently exhales. As the HRV exhaust is only ~3C higher than ambient, I feel the benefit would be negligible.

All ventilation is 'at odds' with all forms of heat. Just that all ventilation is 100% loss except MHRV, so it's the best of a bad job. Now if I didn't actually have to live in my house......

I've got a brand new - as in opened this month - gas generator 4 miles away. My leccy comes from the most efficient gas turbines in the UK just down the road. Add that to my ASHP's COP of 5.45 and 175% is on the low side.