What are the design principles informing the, ' which pump and UFH design' question

[Post and beam]

As per the title really guys and girls.

As a novice to the whole subject but trying to learn at a rapid rate, what matters when you need to make a purchase descision?

 

# Should you go for the smallest possible distance between centres of the UFH pipework.

# What bore size of pipe

# Pure water or glycol mix

# The smallest Heat pump that will do the job or the largest you can afford

# longest run of pipe before you hit issues

# is a particular manifold pair better than any other. If so, why?

 

That will do for todays homework

 

Thanks in advance

keith

 

[PeterW]

16 minutes ago, Post and beam said:

# Should you go for the smallest possible distance between centres of the UFH pipework.

No - it should be designed at optimal centres for the heat loss based on flow and heat requirements. 
 

17 minutes ago, Post and beam said:

# What bore size of pipe

16mm

 

17 minutes ago, Post and beam said:

# Pure water or glycol mix

Water & inhibited antifreeze 

 

18 minutes ago, Post and beam said:

# The smallest Heat pump that will do the job or the largest you can afford

125% of -5°C design load

 

18 minutes ago, Post and beam said:

# longest run of pipe before you hit issues

120m

 

19 minutes ago, Post and beam said:

# is a particular manifold pair better than any other. If so, why?

@Nickfromwales prefers Ivar, I prefer Wunda… (but he also likes some other very odd things…..)

[Post and beam]

Thats how this forum helps with rapid knowledge aquisition. Brilliant.

Supplemental question then....

With regard to the pipe centres question. In my ignorance i imagined using close pipe centres ( 150-200mm) such that the water temp could remain low and thus not stress the heat pump. Keeping my living room at a southern softie 22 degree C for example

[PhilT]

Commenting on item 4 it should be an objective decision falling somewhere between those two extremes, based both on accurate design calcs and real world experience. From my own, those of two friends with similar installations, and my son's new house, I would say it's better to go for around 30% over the theoretical size ASHP suggested by nominal target parameters eg heat loss quoted at -2 or -3degC, the reasons being:-

At some point in the future you may wish to extend your house. This was my main consideration in choosing an 11.2 instead of an 8.5.

Heat pump performance, like PVs and batteries, degrades over time even when regularly well maintained.

A larger unit working less hard should last longer and require less maintenance.

The recent extreme cold has highlighted a real double whammy. Not enough power for an extreme cold spell AND it can take up to 20 minutes for a heat pump to get back up to full power after a defrost cycle which can happen as much as twice an hour in humid (foggy) conditions.

It's been suggested that over-sized heat pumps work less efficiently in lower heat demand situations due to cycling. While that may be true in periods of relatively mild temperature (when power consumption is very low anyway), a heat pump sized accurately for nominal parameters will work MUCH less efficiently in extreme cold conditions.

There tends to be relatively small differences in price between two adjacent power rated units eg a 5 vs 7kWh or 8.5 vs 11.2kWh. IMO a few hundred pounds is a small price to pay given all of the above.

 

[ProDave]

A lot of it is led by the building.  e.g my maximum heat loss at +20 inside and -10 outside is a little over 2kW, so any heat pump you can buy will do it, and I bought a 5kW one about the smallest made.

 

200mm pipe centres for me which many consider to be wide spacing, it just so nicely worked with 400mm joist spacing (2 runs of pipe between each joist) why complicate it?  With the low heat loss any pipe spacing would work.

 

16mm pipe is pretty much the industry standard, we are unanimous on that.

 

I chose a cheap "no name" manifold and then changed the pump for a nice quiet Wilo pump and had no problems with them.  If mix and match buying individual parts, avoid the actuators where you screw a plastic ring on and the rest of the actuator clips to that.  they are utter rubbish.

[Post and beam]

Thanks Dave. I am still learning about calculating the heat loss. I expect the house to be well insulated, U=.15 walls and .12 slab.

Also found this in their spec' sheet ....Sloping ceiling (222mm rafter) 0.13 to 0.12 (dependent on rafter spacing) 2 layers 80mm Kingspan Kooltherm K107 and one layer 25mm Kingspan K107 Kooltherm.

 

Cold roof (cold space above e.g plane roofs and eaves) 0.11 2 layers 200mm loft roll 40

 

Putting the elements together for an accurate assessment is beyond me at the moment.

[JohnMo]

5 hours ago, PhilT said:

A larger unit working less hard should last longer and require less maintenance

that subject to not being so large it ends up cycling to much.

 

we are on 300mm centres and loop lengths matched to the heat loss of the room.  16mm tube max length used is 100m.  To date we have not had to flow any warmer than 34 degs even at -9 outside temp.  We have Ivar manifold and mixer.  Did have actuators, then Salus self balancing, now they have all be removed and run a open system on a single zone.

 

Soon to install a 6kW pump, heating house and summer house.

 

The thing with UFH is your house doesn't need to be hot to feel comfortable.  Our is rarely above 19 deg unless the wood stove is on or we get solar gain.

 

Glycol would only be needed for a heat pump, or anti freeze valves in leu of glycol

[Adsibob]

59 minutes ago, Post and beam said:

Putting the elements together for an accurate assessment is beyond me at the moment.

Use @Jeremy Harris’s spreadsheet. It looks complicated, but isn’t.

[Nickfromwales]

4 hours ago, JohnMo said:

that subject to not being so large it ends up cycling to much

or unless it's mitigated against with a suitably sized buffer.

[JohnMo]

7 hours ago, Nickfromwales said:

unless it's mitigated against with a suitably sized buffer

But then your CoP takes a hit

[Nickfromwales]

52 minutes ago, JohnMo said:

But then your CoP takes a hit

Again, this can be managed, but you'll need a buffer if the areas are zoned. I also am considering much bigger buffers for longer single 'burns' and storing heat created extremely cheaply from ToU. Doubt the CoP losses would be that significant in real life, given what I've witnessed to date on live installs.

[JohnMo]

12 minutes ago, Nickfromwales said:

Again, this can be managed, but you'll need a buffer if the areas are zoned

Or a volumiser, in the flow or return piping, then there is no mixing of the flow and return water.  Or if installing a buffer install as a 2 port buffer between the flow and return, so its only engaged as zones close off.  Or stop zoning everything in to small areas/water volumes.

 

A recent test (simulation) was showing a reduction of 1 on CoP, with buffer compared to without buffer.  Both running WC, but with buffer HP required increased HP flow temps to overcome the mixing within the buffer.

 

If your are getting a couple of batches of cheap electric, a large buffer could be beneficial even with the hit in CoP. But you would need a low energy consumption house to make a real benefit or a very large buffer.

[ReedRichards]

2 hours ago, JohnMo said:

But then your CoP takes a hit

Does it?  I wonder if @JamesPacould model this as part of his weather compensation spreadsheet?

[Nickfromwales]

44 minutes ago, JohnMo said:

Or a volumiser, in the flow or return piping, then there is no mixing of the flow and return water.  Or if installing a buffer install as a 2 port buffer between the flow and return, so its only engaged as zones close off.  Or stop zoning everything in to small areas/water volumes.

 

A recent test (simulation) was showing a reduction of 1 on CoP, with buffer compared to without buffer.  Both running WC, but with buffer HP required increased HP flow temps to overcome the mixing within the buffer.

 

If your are getting a couple of batches of cheap electric, a large buffer could be beneficial even with the hit in CoP. But you would need a low energy consumption house to make a real benefit or a very large buffer.

All of my clientele are building to passive or just below, some above ( NZEB / ZEB etc ), but you’re right to point this out for the masses reading info here.

 

The bigger the buffer ( or buffers ) the closer to input temp you can store at. 
As I posted elsewhere, if there’s going to be a buffer plumbed in anyways, why not just make it a monster and load shift. 
Just needs to get you to midday for a cheeky bit of top-up off whatever solar is going begging.

[Nickfromwales]

16 minutes ago, ReedRichards said:

Does it?  I wonder if @JamesPacould model this as part of his weather compensation spreadsheet?

I think he should.
May mean his plans for the weekend go out the window, but he needs to consider our needs and really buckle down. 

[JamesPa]

2 hours ago, ReedRichards said:

Does it?  I wonder if @JamesPacould model this as part of his weather compensation spreadsheet?

Sorry, I wouldn't know where to start on modelling buffers and I have not seen much data on how much mixing occurs, which would be a basic necessity.

 

I guess worst case is that flow and return mix completely, so they are effectively at the same temp at the buffer op.  I think this might mean that the flow temp from the HP will need to increase by about half of the delta T across the emitters.  That would be a big hit (which my spreadsheet could easily model once the increase in required flow temp is known).

[JamesPa]

7 hours ago, ReedRichards said:

Does it?  I wonder if @JamesPacould model this as part of his weather compensation spreadsheet?

Thinking about this a bit more, to first order it can be modelled by just knowing the temperature drop on the flow across the buffer tank, which can easily be measured if you have one (a buffer tank that is).  This can then be used to offset the WC curve, and give an estimate of the effect.

 

If someone can measure that in a scenario or two, an estimate of the 'cost' of the buffer tank can quickly be generated.

 

Need some data!

[Adsibob]

A lot of people on this forum are keen on buffers for UFH.

Just to say that there is an alternative which might be simpler: we have a low loss header and a boiler which can modulate to 1/17th of its maximum (Viessmann). We don’t have a buffer tank, and don’t have a problem with short cycling. Even though our house is nowhere near passive, I’ve been impressed by how little it costs to heat, so big vote of confidence for the low loss header.

[IanR]

23 hours ago, JohnMo said:

A recent test (simulation) was showing a reduction of 1 on CoP, with buffer compared to without buffer. 

 

Have you got a link to that study, or has it been discussed here previously? I remember reading an old study that found no effects on CoP of a buffer. iirc. there was a small negative effect from a 4 pipe buffer if the buffer didn't allow stratification due to size/shape. It would be good to catch up, if there's a new study that contradicts it.

[JohnMo]

 

6 hours ago, Adsibob said:

lot of people on this forum are keen on buffers for UFH.

Just to say that there is an alternative which might be simpler: we have a low loss heade

Basically a buffer and LLH are the same thing, a buffer just has more capacity, both give hydraulic sepereration between the the heat source(s) and heating system.  Both add inefficiency to a system; if added to a system that can be run without them being added.

 

Managing zone size, can in most case delete any requirements, in a domestic situation, for a buffer or LLH. Saving capital costs and adding simplicity and efficiency. No LLH or buffer, less thermostats and zone valves, easier system balancing

 

[JohnMo]

6 minutes ago, IanR said:

link to that study

https://renewableheatinghub.co.uk/how-to-correctly-install-heat-pumps-so-that-they-work-properly-and-efficiently

[IanR]

1 hour ago, JohnMo said:

https://renewableheatinghub.co.uk/how-to-correctly-install-heat-pumps-so-that-they-work-properly-and-efficiently

 

Thanks for that, I've not seen that one previously. Just to offer a different study that brings a different conclusion, I've added the link below.

 

I'd need to spend a bit more time looking the respective Test Methodology, but it seems the "Brendon Uys" study has the Buffer in a space that is at 7°C. If I've understood that correctly, then the message would be to install the buffer within the Thermal Envelope.

 

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/198850/hot_water_cylinders_buffer_tanks_heat_pumps.pdf

 

Buffer Tank testing is from Section 5 (page 44)

[JamesPa]

39 minutes ago, IanR said:

I'd need to spend a bit more time looking the respective Test Methodology, but it seems the "Brendon Uys" study has the Buffer in a space that is at 7°C. If I've understood that correctly, then the message would be to install the buffer within the Thermal Envelope.

 

I know of two reasons why a buffer tank may reduce system efficiency if installed in a system which can be run without one.  The first is heat loss from the tank, which can be mitigated by installing the tank within the thermal envelope.  The second is mixing, which increases the required flow temperature.  This cannot, to the best of my knowledge, be mitigated but may be more or less significant depending on the design and plumbing of the tank.  

 

If someone (or, better still, a few) with a buffer tank can measure the flow temperature either side (ie temperature drop across the buffer), it would be a simple matter to model the latter effect and get a first-cut feel for how significant it is.

[JohnMo]

3 hours ago, IanR said:

different conclusion

Not sure it does say a different conclusion, but that report is now very old and only really applied to fix speed heat pumps.

 

Report are written early 2000s when most heat pumps didn't modulate.

 

Recommendations states - for Installers Buffer tanks are unlikely to be required when the heat pump can modulate (i.e. if the heat pump is not fixed speed).

 

3 configurations of buffer, one a 4 port, the other 2 are actually not buffers but used as a volumiser in either the flow or return piping, so no mixing occured between flow and return water.

 

Fig 41 shows a lower return temp for all test when no buffer was installed.

 

Fig 42 shows a long on time time for all buffers, especially so for those in volumiser format. This really is showing the volume of the water system to be inadequate for the kW input. But a volumiser being better than a buffer to combat limited water.

 

Fig 43 clearly shows if you split your system in small zones you do need an extra volume of water. But why not save money by not splitting in small parts?

 

Fig 45, 46 47, when hysterisis was at 10, the highest CoP was in a no buffer configuration.

 

Fig 48 shows by just running one rad with no buffer, you are running low on system water capacity 

 

But generally the report shows how much heat pumps have moved on in the last 20 years, they were cops of less than 3.

[PhilT]

Interesting piece of research by Brendon Uys. Just to clarify does the third option suggest a monobloc ASHP can pump the heating flow by itself? Mine (Ecodan R32) has a pump just inside the house right next to it - is that in addition to it's own pump? There is also another pump near the LLH dedicated to the radiator circuit. Is that a sub optimal config?