We’re going to have to address this very issue with our system. Meeting Ken Bone this week was incredibly enlightening. One key takeaway from our chat was the importance of getting kW down the pipe, which boils down to velocity. Our system also uses microbore, but that's far from a deal breaker. I'll keep everyone updated on our progress, because our system—whether as a whole or in parts—is like many other retrofit systems across the UK, which means there's a lot to share and learn.

There are workarounds to use microbore with LT heat pumps. Where are you based @graeme?

As for HT units, they are significantly more expensive to run than LT units. I’ll post some comparison data tomorrow.

Posted by: @editor

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As for HT units, they are significantly more expensive to run than LT units. I’ll post some comparison data tomorrow.

I'm not sure that's strictly true.  What is true is that its more expensive to run a heat pump at a higher flow temp than at a lower flow temperature, but a HT pump and an LT pump run at the same temperature will be roughly the same cost (actually the HT one might be a bit cheaper because they tend to be more recent designs).

Also switching to an HT pump won't overcome microbore pipe problems unless you increase the delta T across the emitters (which may or may not be possible). But if you design for the normal 5C DeltaT the kW capacity of the pipe is the same whether you are running at 35C or 65C.  Increasing flow temp increases the emitter output (so will reduce the number of radiator upgrades) but not the pipe capacity.

Heat geek has a 'cheat sheet' for pipe capacity Vs diameter here https://www.heatgeek.com/does-my-pipework-need-upgrading-for-a-heat-pump-with-cheat-sheet/

If you the microbore is all plumbed into a manifold so each rad has its own run you are in with a chance, it all depends on the the architecture and losses.

It's wrong to assume pipe upgrades are necessary, you need to do the calculations.  Do you have a heat loss survey and/or know the system layout?

Posted by: @graeme

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Last winter we could generally get away with having only two or three radiators on at any one time (

This is an interesting statement.  It suggests your current total emitter capacity is well in excess of demand.  So by switching on more emitters you might well get away with no or a modest upgrade.  It really does need the calcs. 

Posted by: @editor

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Posted by: @editor

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As for HT units, they are significantly more expensive to run than LT units. I’ll post some comparison data tomorrow.

 

I'm not sure that's strictly true.  What is true is that its more expensive to run a heat pump at a higher flow temp than at a lower flow temperature, but a HT pump and an LT pump run at the same temperature will be roughly the same cost (actually the HT one might be a bit cheaper because they tend to be more recent designs).

If @heacol is around, he's the best person to answer this because he has conducted a lot of tests and has data to that effect.

As an example I checked the specs for the 6kW R32 (medium temp) and R290 (HT) Ecodans.  They have the same claimed scop at 55C.  

Obviously there will be detailed variations between manufacturers.

Posted by: @editor

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As for HT units, they are significantly more expensive to run than LT units. I’ll post some comparison data tomorrow.

Hi Mars,

I am running a Samsung 12Kw High Temperature Quiet heat pump in a 5 bedroom house which has a heat loss calculation of 10Kw at ambient -2C.

It just so happens I am tweaking the settings as I write this but I am in the position to give you some example running figures.

The heat curve (which seems to be the sweet spot for my property) is 35C flow temp at ambient +10C and +45C flow temp at ambient -2C. As I write this it is +7C outside and the pump in a steady state is pulling 1.4Kw (read from the electricity meter) to maintain a room temp of +21C. I am currently getting a daily COP of 4.3 based on the pumps own controller figures. (Glynn Hudson's analysis shows this maybe 30% lower than reality due to the sensor positions etc) Sadly I do not have heat meter equipment installed which I wish I had the forethought to do last year.

The vast majority of the time the pump works in the low temperature range however the advantage I think is a) it is capable of heating DHW to +55C autonomously, b) if the weather gets really bad it has the horsepower available (all be it more expensive) in reserve and c) vast majority of the time the pump is not working too hard so relatively quiet.

Running with flow temperatures of +50C and above, certainly with the Samsung, you are looking at COP's of 3.4 and less (as the flow temp increases to +70C) for an ambient temp +7C.

Right now the ASHP is running 15% cheaper than my oil boiler

Interesting to see how these figures compare with other HT and LT pumps. I guess the point is if HT pumps are run with temps as low as possible then they are possibly no more expensive but have the extra horse power when needed.

Regards

@jamespa I have (some) heat loss stats, though I am not convinced. eg some of the sizes given for windows are greater than their actual size! My gut feeling though is that the house is well insulated and I have a concern about overspec'ing - but on the other hand I dont want a system that doesnt work. Also generally the house is bigger (4 bed - 3 public) for our usual needs - 2 people. So I am not keen in heating rooms which are not used. But we do use different parts of the house at different times...

@graeme I was watching the HeatGeeks YouTube video yesterday regarding heat pumps and microbore pipes for my Father-in-Law's potential project. Well worth a watch, bit techie but there are possible alternative options to replacing the piping it appears. One option that springs to mind is reduce your heat load with increased insulation which reduces the Kw required to be fed down the pipes etc. However as has been mentioned careful calculations and assessment maybe needed to decide on the best solution

Regards

Posted by: @graeme

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@jamespa I have (some) heat loss stats, though I am not convinced. eg some of the sizes given for windows are greater than their actual size! My gut feeling though is that the house is well insulated and I have a concern about overspec'ing - but on the other hand I dont want a system that doesnt work. Also generally the house is bigger (4 bed - 3 public) for our usual needs - 2 people. So I am not keen in heating rooms which are not used. But we do use different parts of the house at different times...

It's questionable whether zoning saves money with a heat pump.  The saving in lost energy is quite small, and because you need a higher ft as a result of less emitter area the reduced efficiency can outweigh any savings. Heat geek have a worked example to prove this.  If your house is broadly cubic zoning is unlikely to save money, if it's long and thin and you can heat only one end then it might.

You are right to be concerned about over speccing, it appears to be baked into the industry.  If you can determine accurately your whole house loss eg from existing consumption, you can sense check the room by room calculations.  This should allow you to check which upgrades are necessary and which aren't.  

At @jamespa, I understand your perspective on both High Temperature (HT) and Low Temperature (LT) heat pumps operating below 50°C potentially having comparable running costs. HT heat pumps are often installed as boiler replacements in systems designed to deliver a Delta T of about 20°C. So if you're paying to a HT unit, the expectation would be to run it hotter to deliver the heat that the property needs. Essentially, when operating a heat pump, we are moving heat energy to a higher temperature for practical use, which in turn demands more energy. The greater the temperature elevation, the higher the operational costs. To quantify, the number I've seen shared, for every 1°C rise in temperature from the heat source, we see an estimated increase of 2.5% in energy expenses.

Posted by: @editor

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HT heat pumps are often installed as boiler replacements in systems designed to deliver a Delta T of about 20°C. So if you're paying to a HT unit, the expectation would be to run it hotter 

I don't think that ot is necessarily the case that you buy a HT pump for it's HT capability or that the expectation is to run a HT pump at ht. 

Manufacturers are tending to move from R410a (lt) to R32 (mt) to R290 (HT) which reduces the gwp of the refrigerant.  As the make this transition they are making other changes which are also desirable such as improving appearance and noise.  Furthermore many are abandoning the older models.  So if you want one of the quieter, more recent and less ugly models with a refrigerant that has lower gwp, you get HT capability as a 'bonus'.  You may or may not use it. 

The statement 'HT pumps are more costly to run' helps some manufacturers to shift older, noiser, uglier and sometimes inferior stock, but is not actually true, albeit that it is true that pumps run at a higher temperature are more expensive to run.  It's a subtle but vital difference.  Similarly touting the HT capability helps manufacturers 'sell' their latest and more expensive product.  The reasons to buy may be very different though.