Heat Pump Sizing?

matt_drummer

QrizB said:

I was speaking to a neighbour who's having a heat pump installed right now (OK the crew might've gone home for the night) in his 4-bed semi. He's getting a 12kW Daikin but thinks the surveyor over-egged the heat loss calcs so there's a chance he'll run into the same sort of problems that @matt_drummer had with his.

Should be commissioned by the weekend so I might wander over then and see how he's getting on.

I won't say what he's paying for it, but it's a lot more than any of the Octopus quotes I've seen on this board!

There's no such thing as a 12kW Daikin.

In that range they are 9/11/14/16kW

It will probably be great right now, it's cold enough.

If there is a problem it will show up when it is warmer.

But without some sort of monitoring he may not know there is a problem.

michaels

There seems to be a tendency for installers to make sure heat pumps are 'plenty big enough' - hang over from gas boiler days?  Customers being unhappy with 'just big enough' that is not able to give a heat boost?

michaels

michaels said:

So this would seem to be my baby.  On the underside of the controller circuit board.



Still not easy, it appears to be soldered (via tabs?) top and bottom and it i snot a button battery I am immediately familiar with.  So time for some soldering but I am loathe to remove it in case the whole thing stops working when i don't have a replacement to hand.

All good learning I guess....

Oh well - it appear son close inspection that this is a capacitor so not the missing battery.  There is definitely not a battery on the small controller board but there may of course be one on one of the two main boards inside the heat pump.  Another job for the xmas hols.....

[Aside, the controller came from eBay separately from the heat pump so I guess in theory it might be faulty]

matt_drummer

michaels said:

There seems to be a tendency for installers to make sure heat pumps are 'plenty big enough' - hang over from gas boiler days?  Customers being unhappy with 'just big enough' that is not able to give a heat boost?

Not really.

The problem is the manufacturers.

They produce heat pumps with a badge size and based on that the customer assumes they have a range of operation relative to the badged capacity.

But that is not the reality.

These manufacturers make a heat pump and then hobble it with software  to create a range of heat pumps.

The problem is that they only limit the upper end of heat output, the lower end is the same on all heat pumps in the `range'

That means the `smallest' heat pump in the `range' has the narrowest window of heat outputs.

Now that would be OK if it were a gas or oil boiler, whether it is 80% or 90% efficient is not the end of the world.

But a heat pump that can be 200% efficient or 500%  efficient depending upon how it is installed and operated is a big deal, particularly when you have to buy electricity for it to function.

I think many installers are still oblivious to this.

There is nothing wrong with being too big if the range of heat outputs is big enough, in some ways a heat pump that is too big can be helpful.

Mine is twice as big as it needs to be yet my electricity consumption is low and my SCOP is one of the highest.

These installers just aren't knowledgeable or experienced enough yet, but it will come

FreeBear

michaels said:

There seems to be a tendency for installers to make sure heat pumps are 'plenty big enough' - hang over from gas boiler days?  Customers being unhappy with 'just big enough' that is not able to give a heat boost?

My old Baxi back boiler was rated at 13kW, the replacement combi, at 32kW. Having a heat meter connected reveals that just 5kW of heat is sufficient to raise the temperature at the rate of 1°C per hour. Currently ~5°C outside, so might need a bit more when it is snowing.. Will be interesting to see if the minimum output of 3.2kW is sufficient to maintain a steady temperature with a -3°C outside.

Slowly beginning to think a 6kW heat pump would be large enough to keep this old place warm.

Reed_Richards

It's really not the maximum output of the heat pump that matters, but the minimum.  Or perhaps that should be the minimum amount of power it draws?  Different makes of heat pump can be significantly different whilst, as @mattdrummer points out, a manufacturer can produce the same unit with different nominal power outputs (fixed by software) but exactly the same minimum.

    

FreeBear

Reed_Richards said:

It's really not the maximum output of the heat pump that matters, but the minimum.  Or perhaps that should be the minimum amount of power it draws?

I completely agree. Minimum output was one of the driving factors when selecting the current boiler. Just need some cold days along with some tweaks to thermostat parameters to see if even 3.2kW is too much.

Once I've collected sufficient data, I'll be in a much better position to make an informed decision about a heat pump size.

michaels

Pics show new bits added on Xmas eve, spiro trap and temp probe pocket, not yet insulated; pipe connections outside; view of the unit and my favourite bit, connection to heating circuit teed of radiator tails.

Heat meter now working effectively, Xmas morning we had to reverse the temp probe connections as we had in and out mixed up so should have some cop numbers soon. Just need it to get a bit colder so it stops cycling every half an hour.

michaels

Random question, but how much more energy does it need to raise the temperature as opposed to maintain the temperature? 

WE had an an incident last night when the pump went off because it drew over 20A (first time since we have installed but also first time the outside temp has been -7C) which apparently was the default setting of the 63A wifi breaker (should have been set at 32A)

I know that although we have a 12kw heat pump our rads can not emit more than about 7.5-8kw of heat even with a 55C (max) flow temp and it is taking a while for the temperature to recover even though that output is fine to maintain the temp even with -5 outside temp as we have now.

So it would appear that increasing the temp takes a fair bit more power than maintaining the temp and maybe this needs to be factored in alongside the heat loss calcs?

QrizB

michaels said:

Random question, but how much more energy does it need to raise the temperature as opposed to maintain the temperature? 
...
So it would appear that increasing the temp takes a fair bit more power than maintaining the temp and maybe this needs to be factored in alongside the heat loss calcs?

It depends on the effective heat capacity of your house.

https://en.wikipedia.org/wiki/Heat_capacity

It might be easier to imagine a hot water storage tank (the maths is easier). A tank will have a rated heat loss. Let's assume we've got a 100-litre tank with a rated heat loss of 1.2kWh/day when filled with water at 80C. 1.2kWh/day is equivalent to a continuous power of 50 watts. So it'll only need 50 watts of heat to keep the tank at 80C once it's hot.

Now imagine we've used all the hot water and it's been refilled from the mains, and that cold mains water is only at 10C. We need to heat the water up from cold to 80C, that's an increase of 70C.

Heating a litre of water by 1C takes 4200 joules (ie. watt-seconds), or 1.167 watt-hours. We've got 100 litres to heat by 70C, so that's 70 x 100 x 4200J which is 29.4 million joules or 8.167 kWh. you need to add that much energy in addition to the continuous 50 watts (arguably you could get there with 50 watts but it would take weeks).

So you'll bump your 50 watts up by an extra kilowatt and have a new taankful of hot water after eight hours or so.

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It's not quite the same with houses; their heat loss is higher (mine's around 5kW right now, per my Octopus Mini I'm using less than 6kWh of gas an hour) and the change in temperature is smaller, but also their heat capacity is much higher. Bricks are around 800J per kg C, and a house might need 40 tonnes of bricks. That's 32MJ (almost 9kWh) per degree C, and that's just for the bricks.

Does that help at all?