Air Source Heat Pumps

richardc1983

stevehead wrote: »

I've just booked in a service for my 2 units. This will be done tomorrow and will set me back 60 quid.

Mine are being done also today/tmrw after they have installed the 3rd unit..

He said he wasnt going to take the covers off but I said u need to take the covers off to get to the coils! Think he was trying to cut corners!

Cardew

stevehead wrote: »

In reply to myself, I can work out the COP like this:

First, calculate the kW Sensible Heat Gain with the formula
H = (v x d) x (Cp x (ts-tr))
Definitions
v = Volume Flow Rate (m3/s)
d = Density of Air (Kg/m3)
H = Sensible Heat Gain (kW)
Cp = Specific Heat Capacity of Air (KJ/Kg K)
ts = Supply Temperature (C)
tr = Room Temperature (C)

Current Conditions:
Current Outdoor Temp = 14.5C
Temp Indoor Unit Air Input = 25.0C
Temp Indoor Unit Air Output = 41.3C
Unit Power Consumption = 445W (0.445kW)

H = (0.11667 X 1.2) X (1.012 X (41.3 - 25)
H = 2.31kW

Then divide this by the Power Consumption of the unit to get the COP
2.31 / .445kW = COP of 5.19

So it doesn't make the 5.45 COP claimed*, even when the outdoor temp is well above the test temp of 7C.**

I am bothered by my measuring methods, and have ordered a thermometer to better capture the off-compressor temperatures.

Sources:
Specific Heat Capacity of Air = 1.012 J/(g.K)
(194m above sea level / 23C / 40.85% RH) - Wikipedia typical room conditions
Density of Air = 1.2Kg/m3 @ 23C - engineeringtoolbox.com
Air Volume Flow Rate at Lo setting = 420m3/h (0.11667 m3/s) - Mitsubishi spec for unit

*Eurovent Certified
** EN14511 Outdoor Test Temp

Steve,
Interesting post, but I suggest there is little point in trying to work out COP to that accuracy on those assumptions. Some quick thoughts:

1. The Air volume Flow rate could be wildly out for your installation.

2. Should not the 'density of air' be for the outside ambient conditions, not room temperature?

3. Room temperature would need to be an average of the whole house.

stevehead

Cardew wrote: »

Steve,
Interesting post, but I suggest there is little point in trying to work out COP to that accuracy on those assumptions. Some quick thoughts:

1. The Air volume Flow rate could be wildly out for your installation.

2. Should not the 'density of air' be for the outside ambient conditions, not room temperature?

3. Room temperature would need to be an average of the whole house.

Thanks for looking Cardew, in response....

1. Air Flow Rate. Don't see how this can be much off the stated flow unless the indoor exchanger is choked with crud. The fan speed on the indoor unit don't vary, it has 3 fixed speeds.

2. The 'density of air' relates to the air actually moving thru the the indoor heat exchanger, and it's the heat gain of this air that's being measured.

3. Room Temperature is measured at the exchanger input, as that's the air that's about to gain some heat. If I averaged the whole house temp that would give me a better COP, but would be a sleight of hand.

Yes, I know I can't repreduce lab conditions, and only have access to consumer grade measuring kit. It is a method I can repeat as the winter comes on to watch how the COP falls as it gets colder. Obviously it won't take into account defrost cycles.

stevehead

richardc1983 wrote: »

Mine are being done also today/tmrw after they have installed the 3rd unit..

He said he wasnt going to take the covers off but I said u need to take the covers off to get to the coils! Think he was trying to cut corners!

Gotta take the covers off! How else is he going to clean it?

mech_2

stevehead wrote: »

Actually your beloved combi will only have a DHW efficiency of 75% - the average for current combi's. It's only the primary heating circuit that passes thru the additional condensing HE.

"Additional condensing HE"? There is one burner and one primary heat exchanger that condenses water vapour from the flue gases. I run DHW at a lower boiler temperature than the central heating. Easily low enough for condensation to form.

You do know that the UK SEDBUK ratings only apply to the boilers’ space heating performance and completely ignores hot water efficiency.
Next year that'll change due to new regs, but for now your water heater could be terrible and you'd never know.

All I'm saying is that my combi does condense when producing hot water.

mech_2

stevehead wrote: »

In reply to myself, I can work out the COP like this:

Actually I like this. At least it's something to go on.

It would be interesting to see what you arrive at with cooler temperatures.

Swipe

stevehead wrote: »

I've just booked in a service for my 2 units. This will be done tomorrow and will set me back 60 quid.

How often do you get yours serviced Steve? Every 12 months?

richardc1983

stevehead wrote: »

Gotta take the covers off! How else is he going to clean it?

I know I dont know if he was joking but he was on about just spraying through the air outlet to the coil!

stevehead

Two guys from the service company were here for 2 1/2 hours this morning. Other than drinking my coffee and yapping, they followed a system similar to a car service check off list.
Mainly, checking everything works / any damage or wear.
Cleaning consisted of exposing the exchangers and spraying solvent & flush.
They didn't carry out a pressure test directly. They measured the air in/out temps to see if they were within manufacturers guidelines, and if not then a pressure test would have been done.
At the end, I received a Service Report detailing the checks and work carried out, and that it was completed to manufacturers specifications (for the warranty).

This is my first service on the 2 units. One is 13 months old, and the other is 18 months old. Other than a cleanup, nothing untoward found.

richardc1983

I will see how my guy gets on today. hes gonna strip them down and clean them with coil cleaner etc.

Apparently it doesnt need rinsing he left the bottle here from yesterday and it says that on the instructions.