Our DHW this year (Jan & Feb)  has given a COP of 3.6. That is 5 days a week heating to 48C once a day at 1pm and once a week to 69C for a legionnaire cycle which then stays above the 43C limit set, despite use the following day. Your exact COP will depend a lot on detail settings which depends on use required and we all differ. We have a 200L tank and there’s two of us who don’t shower for ages.

The potential suppliers might be considering worst case of a larger tank heated hotter to last longer (or more times per day) or your household details. 1.75 is low and 3.x credible.

@judith thanks! I think Eon basically haven't bothered to work out then DHW COP for any pump or design presented, which is slightly concerning... However, they've had sensible conversations with me about the design and the two year monitoring period seems reasonable or ensure it's set up properly. 

I know actual COP would vary, I just assumed I'd be able either calculate the estimate myself or find something from the manufacturer online like the published SCOP data for heating 

Posted by: @lakey

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I know actual COP would vary, I just assumed I'd be able either calculate the estimate myself or find something from the manufacturer online like the published SCOP data for heating 

Estimating COP for DHW is complicated by the fact that the flow temperate is not constant, but rises steadily over the heating cycle.

Manufacturers will publish data that allows you to calculate COP at a given outside ambient temperature for a given flow temp. This, for example, makes it easy to look up what the COP should be when it's 7C outside and you have a flow temp of 35C.

But when heating DHW to 55C, the flow temp may start at 20C and finish at 65C, and the COP will vary (decrease) as the flow temperature increases to allow more heat to be transferred to the DHW tank. This makes it far harder to estimate the overall COP for heating the DHW tank, which probably accounts for the wildly differing values you have been quoted. Then we will have the seasonal variation due to changing outside ambient temperatures throughout the year.

I don't normally measure COP individually for DHW alone, but I'm guessing somewhere around 3 or just below is not unreasonable, but will depend on other factors such as how hot you heat the hot water, and the starting temp it has cooled to each day. I'd be surprised if it's as low as 1.75, unless you are heating it at 4am each day when it's 0C outside, and you reheat your DHW to 60C. Heating it at 2-3pm when it's warmest would be a lot more efficient.

@old_scientist Thank you! Right, so I shouldn't consider the manufacturers stated or estimated values when comparing my quotes, they're basically all much of a muchness... I was hoping they'd differ based on size of heating coils in the cylinder or something tangible.

Like I say, I don't think Eon has calculated it, if you download the MCS spreadsheet it's the default value. I'm just surprised they put it on the quote when they haven't thought about it... Possibly signs to steer clear of them as installers. 

Posted by: @lakey

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Eon are proposing either a Vaillant Aurotherm plus 7kW on brackets (not a fan)

It would be a lot quieter without a fan  😉 

Two other points:

1: A DHW temp of 55°C is on the high side.

By way of comparison, it I were to buy a Temperature Regulating valve to protect against scalding,
the default temperature setting would most likely be 42°C.

OK - so I've moved mine a bit higher.
But it doesn't even go as high as 55°C because that's very close to the point where the human body reflex causes you to withdraw!

2: Yes the surface area of the heat-exchange coil in a DHW tank does matter more when it's being supplied from a lower-temperature source.

Greater surface area is usually offered by tanks which have external Plate Heat Exchangers.

An installer who is aware of this is likely to be more skilled/experienced.

@transparent thank you! 

I was planning to turn it down from 55 myself and monitor the daily changeover so I could work out how best to minimise excess legionella treatment runs.

All four quotes I've received have had DHW of 55 (local heat geek, local Vaillant supplier, Eon and then another local installer heat geek accredited but I haven't gone through heat geek). That said, the first HG had my flow at 55 for heating (single room heatloss and radiator issues I made another post about). 

If the efficiency of an external plate heat exchanger is greater than an internal coil, then why are the majority of installs done with manufacturers own cylinders that have a coil with ~3 - 4 m2 surface area? Are external heat exchangers significantly more expensive?

Posted by: @lakey

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If the efficiency of an external plate heat exchanger is greater than an internal coil, then why are the majority of installs done with manufacturers own cylinders that have a coil with ~3 - 4 m2 surface area? Are external heat exchangers significantly more expensive?

 

PHE plus UVC direct cylinder (no coil) plus pump is cheaper than most heat pump cylinders.  However its 'different' and people dont like 'different'.  Plus its an extra active component.  

I can't really comment on pricing @lakey

I'm a self-builder rather than a heating installer.
My intention is merely to introduce you to the alternative approaches whilst you're still exploring what you want.

Mixergy are a well-known brand who offer DHW tanks with Plate Heat Exchangers.
They have a page of technical explanations which is worthwhile reading.

As for why installers take particular courses of action...
... who knows?!

If they all implemented best practice, then this Forum wouldn't need to exist 🙂 

@jamespa @transparent, Thank you both! 

What do you mean by an extra active component? Surely it's the same, just the location of the heat exchanger is different? Just to clarify, that's a Plate Heat Exchanger with an Unvented Cylinder with no coil is cheaper than one with a coil typically and made to go with the heat pump? What's the direct/indirect section regarding? 

Mixergy page is very useful, thank you. I'll try probing each of the installers a bit further to help in understanding the DHW side too which seems to be mainly omitted/a by-product of the rest of the installation/design. 

That Mixergy internal HP & cylinder certainly looks interesting, shame it's unpriced/out of stock! 

Posted by: @lakey

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What do you mean by an extra active component? Surely it's the same, just the location of the heat exchanger is different? Just to clarify, that's a Plate Heat Exchanger with an Unvented Cylinder with no coil is cheaper than one with a coil typically and made to go with the heat pump? What's the direct/indirect section regarding? 

You need an extra pump that pumps the water from the DHW tank through the heat exchanger which is external to the DHW tank.  The other side of the PGE has water from the heating circuit pumped through (by the ASHP pump) it as normal.

A direct cylinder has only an immersion,  An indirect cylinder has immersion plus coil.  You can buy a direct unvented cylinder for about £400 and a PHW plus pump for about £400.  You will struggle to get an indirect cylinder with a heat pump coil for that.  

The vast majority of installers will fit a UVC with a heat pump coil.  Often they will fit the manufacturers 'pre plumbed' UVC which has the diverter valve and a few other components included; this may or may not be a good fit to your circumstances and if it isnt they will have to do unnecessary plumbing to get it to work.  In particular those installers that employ rookie plumbers to do the actual work will do this, because it makes the plumbing job nearly foolproof.  The same installers will often fit LLHs/Buffers which you almost certainly don't want, so beware.