Heat Pump Sizing?

matt_drummer

smallblueplanet said:

What are the downsides to having a heat pump that is 'perhaps' either under or over specced? I presume under spec can't provide enough heat? Or is it in both cases to do with efficiency/scop of the pump?  

The downside of going too small is that you won't get enough heat.

The downside of going too big is loss of efficiency and difficulty of operation when it's not so cold.

But you need to know your heat pumps.

The 4,6 and 8 Daikins are the same heat pump, they all modulate down to the same output as they have the same compressor, by getting the 6 or 8 when you may only need the 4 won't harm the efficiency in any way as they all go as low as each other. All you get with the `bigger' ones is a higher maximum output which you get with no penalty other than a higher cost of purchase.

matt_drummer

saveallmymoney said:

smallblueplanet said:

What are the downsides to having a heat pump that is 'perhaps' either under or over specced? I presume under spec can't provide enough heat? Or is it in both cases to do with efficiency/scop of the pump?  

Undersized is actually likely to be more efficient at higher outside temperatures, as it won't cycle, but in very cold weather additional heat source might be needed.

Oversized will cycle and lose efficiency, although the way it heats the house will be more familiar to people who have a typical fossil fuel boiler setup so i can understand most installers going that direction.

This is generally true.

But a warning!

The quoted output is no guarantee of what you are actually getting.

Everybody would think that a 16kW Daikin Altherma 3 is almost twice the size as the 9kW Daikin Altherma 3 but it isn't, they are both 16kW heat pumps, it is just the 9kW is restricted at the top end.

For this reason, you will get no more short cycling with the 16kW than you would with the 9kW in the same installation.

What you do get with the 16kW is far less chance of requiring additional heat sources when it's really cold.


Just to add, Daikin are not the only manufacturer to do this, just beware and do your research before comittining
to something you may regret.

Please also don't think I am anti Daikin, I am not and I really think that the right Daikin in the right house and you have a winner.

My 9kW was lovely and worked really well, just a shame my heat loss is around 4kW at -3c!

michaels

Based on gas usage, it appears my gas boiler happily modulates down to 2kw and of course adjusts the flow temp down into the 20s C to reflect the outdoor temp and thus heat demand.  It would seem a step back to move to a heat pump that can't do this and has to pulse hot and cold rads to maintain an indoor temp when I thought the whole point of efficient heat pump operation was to have a constant flow and flow temp....

matt_drummer

I think most of us with close to the correctly sized heat pump have no issues and are very happy.

My heat pump runs at around 31c all year (when needed) and the heat is very gentle.

The house feels like a pleasant summers day rather than noticing the heating on.

It also costs me less than gas which is a bonus but was not my motivation.

If you get it right a heat pump is a massive step forwards over a gas or oil boiler.

I thought you weren't getting an Octopus install in any case?

Just get a nice Vaillant, at your heat loss something like the 12kW Vaillant would be good by the sounds of it.

The bigger Vaillants are apparently even more efficient than the smaller ones.

Your nearest friendly Heat Geek will be able to advise you properly though.

saveallmymoney

michaels said:

Based on gas usage, it appears my gas boiler happily modulates down to 2kw and of course adjusts the flow temp down into the 20s C to reflect the outdoor temp and thus heat demand.  It would seem a step back to move to a heat pump that can't do this and has to pulse hot and cold rads to maintain an indoor temp when I thought the whole point of efficient heat pump operation was to have a constant flow and flow temp....

Annoying for us geeks who are watching it, but given the low amount of energy needed for heating in these conditions it won't make much of a practical difference to efficiency.

I think higher modulation ranges will come as the UK market for air to water ASHP's matures.

matt_drummer

saveallmymoney said:

Annoying for us geeks who are watching it, but given the low amount of energy needed for heating in these conditions it won't make much of a practical difference to efficiency.

.

And this was the problem with the Daikin range.

My 9kW could go a little lower than 4kW of output.

The issue is more the minimum power input, none of them in the range go below 900W input.

That means at low outputs your COP is limited to multiples of 900W

My 8kW Daikin goes down to 300W input so I get 2kW of heat at a COP of over 6

If I got my 9kW down to 3.5kW of heat the COP is around 4

It was using two to three times the power and putting out to much heat.

A small version of a big heat pump is actually a massive problem when you want to turn it down because you can't turn down the electrical input past a certain point. 

In relative terms for me, 900W was a huge amount of power at low heat requirement and the efficiency was poor.

One other issue with these Daikins, they take a while to settle down into a heating cycle.

Thinking that you run them for 30 minutes every couple of hours will end up with poor efficiency. The first 30 to 60 minutes are the least efficient.

saveallmymoney

@matt_drummer It was reading your posts on OpenEnergyMonitor forums that encouraged me to fit the smallest Daikin i think i can get away with. I have the 7kw and have seen it go as low at 300w input although summer has come and ruined my testing :-)

matt_drummer

It is actually much easier to add some heat on the coldest of days than it is to live with a grossly oversized heat pump.

It is probably quite sensible to design a system that works well on most days and is a little undersized for the coldest days, but that is not how most of the installations are done here at the moment. 

Summer isn't going so well where I live, 6c last night, so we actually had the heating on a bit.

koru

matt_drummer said:

I am pretty sure that Octopus would fit the 16kW, in fact I know they have because installers have told me so.

I have seen someone else post about Octopus only being willing to go up to the 11kW Daikin. (Might be on a different forum, I can't remember.) 

Perhaps this is because they don't understand the point you make. Perhaps they are just put off by the higher cost of buying the bigger models.

DougMLancs

It also needs to get MCS sign off for the grant though and they won’t do that if it looks way oversized on paper.