Octopus Heat Pumps

Spies

So they say they're designed the system to -2/d50, my heatloss is 5.6kw, this doesnt leave much room for error!

Reed_Richards

MCS designed systems don't leave much room for error in underestimating your total heat loss.  But you will need headroom for "time-off" from heating your house whilst defrosting and whilst heating your hot water.  I would not have thought 0.14 kW is enough headroom.

Edit:  I suppose it's possible the curves are average power and take the defrosting into account - but you still need hot water heating time.   

matt_drummer

The bigger problem is what it does at 0c to +5c, when it is cold and damp.

At 50c flow temperature, +3c outside in frosting conditions your heat pump is only likely to be heating your house for 70% of the time and it will spend the other 30% of the time sucking heat from the house.

If your heat loss at that time is 5kW and your heat pump produces 6kW then you will only be getting about 4kW of heat during defrost times. 

That graph takes no account of defrosting, edited as I just saw the question come up.

You won't be able to heat your house enough and if you want hot water as well it will be even worse.

This is more of a problem with this range of heat pumps as they are not fitted with back up heaters like the 9kW range.

Octopus don't understand that the 8kW will perform just the same as the 6kW, you are just not restricted by the software of the `smaller' units. They all modulate down to the same level as they all have the same compressor.

It sounds a bit like scaremongering from me and that is not what I am trying to do. The 6kW may be fine and you'll never have a problem.

But you should be aware of what could happen and why.

It really depends on how close your actual heat loss is to that calculated, how your radiators perform and what flow temperature you need to run.

The graph is nice, quoting a COP of barely over 2.0 at their design temperatures, awesome!

I just checked, my 8kW Daikin was getting a COP of 4.40 at 0c outside, 20c indoors and a flow temperature of 31c. To do this it was using 700W of electricity and generating 3.20kW of heat.

So a 4/6/8kW Daikin heat pump is more than twice as efficient running at 30c than it is at 50c

Spies

Their first response:

I can confirm that the 6kW heat pump will adequately cover the heat loss at your property. The 6kW unit is designed to cover heat loss above 5,800W, and the heat loss at your property is 5,682W, which falls within this range. Please rest assured that you are getting the right heat pump for your property.

I dont like the idea that at temperatures below -2, I likely will be losing heat and not be able to replenish it.

matt_drummer

There aren't many days when it will be -2c all day but that depends where you live of course.

The problem is most likely to come when its a little warmer and you encounter defrosting.

I have said it many times though, they just don't understand that the 8kW Daikin is the same heat pump and there are no downsides to fitting that instead of the 6kW version.

What is most alarming is the efficiency as they have designed it.

The extract posted from OEM of a 6kW Daikin running in design conditions confirms a COP of 2.2, maybe even less if that is not MID monitored, without any defrosts.

Defrosts will make it even worse.

And electricity consumption of around 70kWh per day. On a standard tariff something like £17 a day for heating.

A gas boiler would cost about £10 to produce the same amount of heat.

Spies

I have responded stating that defrost cycles and hot water heating time will eat into the spare headroom of the unit, see what they say, I can't really keep pushing them if they say no again at this point, but I have mentioned that @john-306 was specified an 8kw unit and his heatloss is less than mine at -2

Reed_Richards

 Edit;  Strike that remark about Weather Compensation; WC should give you better economy but here I'm really just talking about COP vs. SCOP here

matt_drummer said:

There aren't many days when it will be -2c all day but that depends where you live of course.

The problem is most likely to come when its a little warmer and you encounter defrosting.

I have said it many times though, they just don't understand that the 8kW Daikin is the same heat pump and there are no downsides to fitting that instead of the 6kW version.

What is most alarming is the efficiency as they have designed it.

The extract posted from OEM of a 6kW Daikin running in design conditions confirms a COP of 2.2, maybe even less if that is not MID monitored, without any defrosts.

Defrosts will make it even worse.

And electricity consumption of around 70kWh per day. On a standard tariff something like £17 a day for heating.

A gas boiler would cost about £10 to produce the same amount of heat.

Yes the performance at -2 C is not great and at that temperature a gas boiler would be much cheaper to run.  But -2 C should be a temperature that is exceeded 99.6% of the time where you live.  For much of that time it will be warmer outside and you will achieve a better COP.  It's the averaged figure, the SCOP, that hits you in the pocket and the figure that should be used when comparing running costs with gas.

That's not to say that if you had bigger radiators and a lower maximum flow temperature you wouldn't achieve a better SCOP; you most certainly would.      

stripling

Reed_Richards said:

stripling said:

Meanwhile, my radiator schedule just has a blank for the bathroom - so I'm leaving the (85w @ dt50º 🤦🏻‍♀️)  towel rail in and trying to get the most powerful smallish (designer) radiator added to another wall and my local plumber will fit it. Every watt counts! But it's more expense. 

Get Octopus (?) to fit a bathroom radiator that can be swapped-out for the designer radiator you really want.

My radiator schedule accounted for every room and corridor in the house.  I had to sign-off against:

1. An unheated corridor, on the basis that radiators in adjoining rooms were a bit oversized.  That has been fine.
2. An underheated bathroom, on the basis that heat would spread from the adjoining room.  That room already had a radiator/towel rail and I couldn't see a way to fit anything more.  That room can be a bit on the cool side in very cold weather and I've now figured out what I should have done, so that was a bit of a mistake.
3. An underheated kitchen.  I should have sacrificed a shelf to fit a taller radiator but we decided against.  In cold weather this room is really chilly first thing in the morning but reaches temperature eventually.  I have now figured out where I could have squeezed in an extra radiator so another mistake.

This was in 2020 so way before Octopus started installing heat pumps.  But my installer was accommodating and we went through 3 iterations of radiator size and placement.  
 
 

@Reed_Richards

Thanks so much for this - it is really helpful advice.
You are right - it is the labour cost for adding a radiator that more than doubles the cost of the rad because of system draining etc., but if Octopus put an ordinary one in - swapping it out will be much cheaper for me because it won't need draining down. 
Plus it is bananas to pay for system draining twice in a few weeks. 

stripling

Spies said:

So they say they're designed the system to -2/d50, my heatloss is 5.6kw, this doesnt leave much room for error!

We're at opposite ends of the country with the same heat pump size problem. Except I'm paying more for it. 🙈

stripling

I just want to say thank you for all the helpful contributions to this thread. 

Imagine - 🐙 started off proposing a 4kw heat pump for my install. Upping it to 6kw took a lot of patience. 
The discussions here are really good for thinking things through.