@mikeatthezoo , you said you’ve had some estimates. Did you layout all these queries to the suppliers who quoted you. Did they provide any advice?

If you have enough space, you might consider opting for a larger hot water cylinder.  That would reduce the risk of running out of hot water.  However, it would also take more energy, and therefore heating time, to warm the whole tank.

Posted by: @mikeatthezoo

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it appears that the Daikin and Mitsubishi 8kw (not the 8.5) are physically very similar to the 6kw

Mitsi 8kW has a 6kW variable and 2kW fixed compressor so presumably modulates down to the same as the 6kW.

All Daikin models <= 8kW are same hardware sofaik, limited in the firmware. 

Posted by: @mikeatthezoo

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it is correct, where is the 8kw Mitsubishi R290 going?  

No idea.  It's currently an interesting unicorn with two compressors.  I assume destiny will depend on sales.

Posted by: @mikeatthezoo

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I am worried that as we drop cylinder temperature to 50 or even 45 ideally we will start to become more dependent on heating water quickly after it is used.  6kw doesn’t go very far in that context.

Suggest to work out your use pattern if this is a concern. There is no substitute for doing the arithmetic

Posted by: @terry1812

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@mikeatthezoo , you said you’ve had some estimates. Did you layout all these queries to the suppliers who quoted you. Did they provide any advice?

I did; Octopus won’t budge on their design flow temp (predictable issue) but also won’t relent on the 6kw; it’s also their R32 and I think at this point my preference is R290 as not being dependent on the immersion for legionella cycle has cost benefits.  Both others are quite happy to go up a size in terms of heat pump but aren’t specifically clear on how likely I am to encounter short cycling, both are “happy” to design around that.

Posted by: @grahamf

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If you have enough space, you might consider opting for a larger hot water cylinder.  That would reduce the risk of running out of hot water.  However, it would also take more energy, and therefore heating time, to warm the whole tank.

We’re going to 300 litres, but as you allude to every time we increase in size we increase the time it takes to heat because of the amount of energy it takes to achieve that.  That said, going for the R290 model does allow me to (for the time where demand might be a challenge) increase cylinder temperature to address some of my concern.

Posted by: @jamespa

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Posted by: @mikeatthezoo

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it appears that the Daikin and Mitsubishi 8kw (not the 8.5) are physically very similar to the 6kw

Mitsi 8kW has a 6kW variable and 2kW fixed compressor so presumably modulates down to the same as the 6kW.

All Daikin models <= 8kW are same hardware sofaik, limited in the firmware. 

Posted by: @mikeatthezoo

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it is correct, where is the 8kw Mitsubishi R290 going?  

No idea.  It's currently an interesting unicorn with two compressors.  I assume destiny will depend on sales.

Posted by: @mikeatthezoo

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I am worried that as we drop cylinder temperature to 50 or even 45 ideally we will start to become more dependent on heating water quickly after it is used.  6kw doesn’t go very far in that context.

Suggest to work out your use pattern if this is a concern. There is no substitute for doing the arithmetic

 

Use pattern is what concerns me - most days we’ll drop a 143 litre tank from 60-65 degrees to 30-35 degrees a minimum of twice, as the wife showers both ends unsympathetically to the energy bill and son does every other day as well, usually opposing me but sometimes same day.

Having the heat pump reheat the cylinder for 60-90 minutes three times a day feels like we might end up risking climate comfort, but trying to modulate the Mitsubishi 8.5 down to the 2.8kw (50% of heat loss calcs) is a razor thin margin.

Posted by: @mikeatthezoo

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Having the heat pump reheat the cylinder for 60-90 minutes three times a day feels like we might end up risking climate comfort, but trying to modulate the Mitsubishi 8.5 down to the 2.8kw (50% of heat loss calcs) is a razor thin margin.

Lets look at this the other way round.  If your loss is 5kW and your heat pump can do 6.5 (absolute worst case with the Vaillant so far as can be told) you have 1.5*24kWh to spare each day even on the worst day.  If push came to shove you could add another 3*24kWh by turning the immersion on.

Assuming you shower at 40C and the input water is 10C then each litre of shower water consumes 126kJ.  So each kWh will heat 28l of water.  Thus 36kWh will heat 1cu m of water.  Unless I have made a mistake you have plenty to spare.  A bit of thought with the scheduling should sort it.  I think you may be worrying unnecessarily.

Posted by: @jamespa

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Lets look at this the other way round.  If your loss is 5kW and your heat pump can do 6.5 (absolute worst case with the Vaillant so far as can be told) you have 1.5*24kWh to spare each day even on the worst day.  If push came to shove you could add another 3*24kWh by turning the immersion on.

Assuming you shower at 40C and the input water is 10C then each litre of shower water consumes 126kJ.  So each kWh will heat 28l of water.  Thus 36kWh will heat 1cu m of water.  Unless I have made a mistake you have plenty to spare.  A bit of thought with the scheduling should sort it.  I think you may be worrying unnecessarily.

That is fair - my main goal is to ensure that I don’t end up in the position of having to “watch” cylinder temps and I appreciate that is something I’m going to have to manage my way out of after the initial installation through scheduling and temperature changes.

i appreciate there’s a whole heap of mentality changes in moving to a heat pump, but I do draw the line at getting yelled at from a cold shower!

If you get an R290 pump you should be able to heat the tank to 65C which will give you 550l of water at 40C from a 300l tank.  You probably don't want to stay with that setting as cop will be poor, but it could get you started whilst you are working things out!

Cylinder temp sensors are often about half way down, worth bearing in mind.  With decent stratification you might have quite a lot left even with your current setup before the shower goes cold.

Got the results back from the one we’re proceeding with; rads will all have manual TRVs but the only ones likely to be set are the two larger bedroom ones; all in one zone, will have a 100l volumiser on the flow side of the heating loop and the cylinder will be a 300 litre cylinder.

Heat pump will be the 8.5kw R290 Ecodan; bigger than necessary but hopefully with the design accommodating for that it deals with our desires around the water cylinder and manages the space heating in a way that the pump is happy with.  Expected SCOP from the design is 4.49.