Octopus Heat Pumps

[rogerzilla]

10mm microbore is not the Work of Stan.  It is merely the product of one of his minor demons.

8mm microbore is the work of the Hornèd One himself.

[Pickled Onion]

Is there an issue with reduced water velocity (= heat transfer) when using larger diameter pipework? Velocity is inversely proportional with cross-sectional area and all that.

IANAPlumber, but AIUI friction has a great part to play in reducing velocity, especially on bends and especially as the bore gets smaller.

[fd3]

Mutter something about Bernoulli’s equation mutter

[drgannet]

Interesting that you didn't need any 28mm runs.  Given the size of our house and the additional insulation I've been busy installing over the last couple of years I suspect that 22mm would suffice for us too.  However, I figure that if I fit 28mm rather than 22mm in a couple of years time when we move to a ASHP we'll avoid have to rip up the floors that I'm about to lay if the installer specifies 28mil.

I'm considering replacing our current tank with a Mixergy tank. When we eventually make the switch to a ASHP we should be able to avoid fitting a buffer tank as the Mixergy uses plate exchanger to interface with the heat pump.

The buffer tank is a 50 l tank at the start of the CH circuit, to provide a buffer for when the ASHP goes through defrost cycle.

The pipework is 35mm between ASHP and interface unit, 28mm from the interface unit to hot water tank and also to plate heat exchanger on the CH circuit. This is effectively all one glycol circuit. 28mm from plate heat exchanger to buffer tank, and then 22mm to the CH circuit.

[Wowbagger]

Comments from son re ASHP experience:

It was very good, and very economical. It was only affecting one open square room though so it isn't fully representative.

For the house in Siuntio we are looking at two to heat the whole place, backed up by the existing radiators plus the hearth.

[Wowbagger]

More on ASHPs: my choir director has most of his heating from a fairly old ASHP. He doesn't think much of it. (I went round there once so that we could play Beethoven's 1st concerto on 2 pianos.)

From the limited amount of reading I've done in the matter, Mitsubishi ASHPs are reckoned to be the best around, but I would imagine that it's likely to change rapidly as different companies enter what is probably a pretty volatile market.

[rogerzilla]

What is the scope for improvements in CoP/SPF over time?  If it's just tweaking motor and compressor efficiency, probably not much?  I was always rubbish at thermodynamics at university, and refrigeration/HVAC wasn't covered anyway.

There is a Carnot cycle formula for maximum efficiency, which is indoor temp/(temp difference) but that's about as useful as an ideal Otto cycle, giving CoP of over 11 for 21^oC indoors and -5^oC outdoors.  So where are the biggest losses?  Is it in the phase changes?  Do different refrigerants make much difference?

It's a bit naughty to quote heat pumps as 300% efficient if CoP is 3; they're really under 30% efficient compared to the ideal cycle.

[graculus]

  I was always rubbish at thermodynamics at university

Ditto. I think you did Chem.Eng., same as me? Thermodynamics was one of 'my eyes glaze over' subjects' - half a dozen quantities all with the units kJ/kg.

[spesh]

Three Laws of Thermodynamics (paraphrased):
First Law:  You can't get anything without working for it.
Second Law: The most you can accomplish by work is to break even.
Third Law:  You can't break even.

[rogerzilla]

Fourth Law: your alloy mini-pump will be too hot to hold before your tyre is hard enough

[Joe.B]

Is there an issue with reduced water velocity (= heat transfer) when using larger diameter pipework? Velocity is inversely proportional with cross-sectional area and all that.

In so far as I understand it the increased pipe diameter demanded by ASHP's is all about the differential temperature between the supply and return.

A system boiler will happily run with a DT of 20C, a ASHP on the other hand requires a DT of around 6C.  In order to achieve the flow rates necessary for such a low DT larger diameter pipes are nearly always required.
 

[spesh]

Fourth Law: your alloy mini-pump will be too hot to hold before your tyre is hard enough

That was me on Wednesday.

[Kim]

Is there an issue with reduced water velocity (= heat transfer) when using larger diameter pipework? Velocity is inversely proportional with cross-sectional area and all that.

In so far as I understand it the increased pipe diameter demanded by ASHP's is all about the differential temperature between the supply and return.

A system boiler will happily run with a DT of 20C, a ASHP on the other hand requires a DT of around 6C.  In order to achieve the flow rates necessary for such a low DT larger diameter pipes are nearly always required.

Why is a low differential temperature desirable?  Shirley the heat pump would be more efficient if it were heating cooler water?

Oh, except the flow temperature would then be too low for the radiators.  Gotcha.

Seems to me that the whole radiators thing is a bad idea, but that way lies expesive to retrofit un-BRITISH technologies.

[rogerzilla]

Tbh, rads are a nuisance in tiny modern homes.  Nowhere left to put the sofa.  Supersize rads for a lower flow temp, even worse

[Mrs Pingu]

What's the alternative, forced air? No thanks. All that dust and shit (cat hair) being blown about, and I hate having air blown at my head and the feeling of my mucus membranes dessicating.
Let's face it, radiators are not the problem, shit housebuilders are.

[Joe.B]

Is there an issue with reduced water velocity (= heat transfer) when using larger diameter pipework? Velocity is inversely proportional with cross-sectional area and all that.

In so far as I understand it the increased pipe diameter demanded by ASHP's is all about the differential temperature between the supply and return.

A system boiler will happily run with a DT of 20C, a ASHP on the other hand requires a DT of around 6C.  In order to achieve the flow rates necessary for such a low DT larger diameter pipes are nearly always required.

Why is a low differential temperature desirable?  Shirley the heat pump would be more efficient if it were heating cooler water?

Oh, except the flow temperature would then be too low for the radiators.  Gotcha.

Seems to me that the whole radiators thing is a bad idea, but that way lies expesive to retrofit un-BRITISH technologies.

Can’t really disagree with any of that, if the future of (wet) heating is going to be low temp we’d all be much better off with under-floor heating. Radiators are what most of us are stuck with though.

Going off on a radiator related tangent; I’m about to hit buy on Dryton-Wiser smart controls.  Cost is around £500 but it’s going to pay for itself fairly quickly given where we are with the cost of gas

[Mrs Pingu]

I have a Wiser but only the base system, do not yet have the smart TRV's but am thinking about it, especially if I can fit them myself.

[CarlF]

If you already have TRVs, replacing the heads with the Wiser ones is really straightforward.
Unless the existing TRVs aren't a type that fits one of the two different adapters that come in the box- in which case you need to work out which adapter you need and find somewhere that can actually sell it to you. (Probably Amazon, but you need to go through that thing where the search gives you dozens of vaguely described adapters for different systems *that are not the one you clearly specified* and you have to sift through the listings then cross your fingers that you ordered the right thing...). But once you do have the right adapter, it's dead easy.

If your radiators don't have TRVs already then in principle it would be a pretty straightforward plumbing job to add them, but if the system is old enough not to have TRVs then a) it's the sort of job where something seized or corroded could ruin your whole day and your whole carpet and b) you should consider swapping the radiators for new ones- they're much more efficient these days.

[rogerzilla]

I paid BG £500 to supply and fit 6 TRVs.  I'm sure it can be done much cheaper.  Not sure why I bothered, because I then discovered a 5kW wood burner happily does the whole house, and I only use the heating for a couple of hours a week.

[Cudzoziemiec]

Is there an issue with reduced water velocity (= heat transfer) when using larger diameter pipework? Velocity is inversely proportional with cross-sectional area and all that.

In so far as I understand it the increased pipe diameter demanded by ASHP's is all about the differential temperature between the supply and return.

A system boiler will happily run with a DT of 20C, a ASHP on the other hand requires a DT of around 6C.  In order to achieve the flow rates necessary for such a low DT larger diameter pipes are nearly always required.

Why is a low differential temperature desirable?  Shirley the heat pump would be more efficient if it were heating cooler water?

Oh, except the flow temperature would then be too low for the radiators.  Gotcha.

Seems to me that the whole radiators thing is a bad idea, but that way lies expesive to retrofit un-BRITISH technologies.

Can’t really disagree with any of that, if the future of (wet) heating is going to be low temp we’d all be much better off with under-floor heating. Radiators are what most of us are stuck with though.

Going off on a radiator related tangent; I’m about to hit buy on Dryton-Wiser smart controls.  Cost is around £500 but it’s going to pay for itself fairly quickly given where we are with the cost of gas

Someone who was looking at it for a sort of shop-sized building told me that underfloor heating is basically the only way to make heat pumps worthwhile.

[rogerzilla]

It's a case of utilising low-grade heat.  Quite the opposite to a 2ft x 2ft wood stove at 200 deg C.

[Pickled Onion]

Someone who was looking at it for a sort of shop-sized building told me that underfloor heating is basically the only way to make heat pumps worthwhile.

Many years ago, we installed an ASHP system in a set of commercial/shop units in a converted chapel. There were limited options for insulation, so it was very thermally sup-optimal. Heat/cooling distribution was microbore pipes (I don't know what liquid was in them) to either standard aircon fan units or floor-standing units in traditional radiator style enclosures.

I suspect it was over-specced, but it was incredibly good at heating and cooling - it could bring the whole building up to 20 degrees on a winter Monday morning in about 10-15 minutes. The current talk about low-grade heating and not working without massive insulation are probably down to a combination of retro-fitting to old systems and keeping the price and size down. The whole system came in at about 20 grand, and the running costs were up to a grand a month plus a grand a year to service, covering 5 units (at a guess a space equivalent to 2.5 houses). I shudder to think what the current occupants will be paying later this year, but they're still open.

[rogerzilla]

I think it's down to efficiency.  You can make an adequately-sized heat pump raise temperature quickly, but the CoP drops towards 1.  Ok, worst-case scenario is that it's the same as a fan heater, but not many people could afford to heat their house with a fan heater.

[chrisbainbridge]

Stupid question. What is CoP? Have I missed it being defined upthread?

[rogerzilla]

Coefficient of Performance, how many watts of heat you get for each watt of electricity.

SPF (Seasonal Performance Factor) is basically CoP averaged over a year, and is what to use when calculating the economics. 

SPF needs to be > (electricity ppkWh/gas ppkWH) to make replacement sensible, but in practice you need more electricity for water heating (anti-legionella) and potential top-ups when the heat pump struggles.

Average real world UK SPF is 2.45 according to https://greenbusinesswatch.co.uk/do-air-source-heat-pumps-work-in-the-uk, and that's for a good, well-designed installation.

By all means get an ASHP to save the planet, but only if you can afford 40-50% more on the heating bill.