Posted by: @simonwhiteley

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@jamespa it is an 1850 solid wall 150sqm in London. I have heat loss calcs coming out of my ears, with a wide range of results- in summary every calc from physical fabric gives a bigger number than every calc from historical gas consumption. In HTC terms, I have answers ranging from 390W to 650W for the house as a whole.

I can reduce heat losses by up to 20% with some changes we are making. We are in a conservation area so a bit restricted.

The pump we are looking at is the 7kw Vaillant but whichever way it will be on the low side- at least in theory as here in inner London I am dubious about the bottom end temps, and we don’t keep the house at 20degC. Ventilation losses are also very guess-worky- even a pressure test would probably only show how much air we leak through the party wall!

i feel I do have a pretty good fix on the relative distribution of heat loss around the house. I have a room by room radiator upgrade plan which is OK but looks a bit more than absolutely necessary, which means £ and intrusion.

My simple reasoning is that radiator capacity at a given FT needs to exceed max heat pump output at that FT (which obvs needs to exceed max heat demand). But by how much? I trust my installer but the industry is coming from a background of cheap output and high FTs so some over-provision would be natural.

Bottom line is that the Vaillant is the largest HP we can fit within PD rights. We are installing a 6kw backup inline electric heater, which will be expensive to run but provides insurance if the higher heat loss estimates are nearer the mark. So my concern is to maximise the useful HP output at lower flow temps, and minimise backup use (where radiator size isn't a constraint as it just boosts FTs).

Ah, that makes sense and the 7kW Vaillant plus backup is a strategy a neighbour of mine has, so far successfully, adopted.  Note that at -2 the 7kW Vaillant actually claims 8.5kW output.   

Im not sure what you mean by 'In HTC terms, I have answers ranging from 390W to 650W for the house as a whole' though, do you mean 3900-6900W? and what does HTC stand for (I assume its basically house loss).

Posted by: @simonwhiteley

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I have heat loss calcs coming out of my ears, with a wide range of results- in summary every calc from physical fabric gives a bigger number than every calc from historical gas consumption. In HTC terms, I have answers ranging from 390W to 650W for the house as a whole.

Same here.  I had 2 full 3hr surveys and also did my own calculation, as well as any number of salesman estimates.   The problems with the 'professional' surveys were: - invisible fabric upgrades were ignored by surveyors, even though I made a point of telling them, surveyors double counted about 1kW, because they counted room to room losses but not room to room gains, surveyors assumed ACH=2.5 whereas my measured gas consumption suggests 0.5-0.8. Judging by my ASHP (7kW Vaillant) performance so far, the determination based on measured gas consumption was spot-on and the 16kW quoted by some of the professionals a load of baloney.

Assuming you are happy with the system sizing (what does you gas consumption suggest?) then you are right that rad total (at design FT) must exceed the house heat loss.  In principle it needs only just to exceed it and the rad in each room should be matched to the loss of that room.  If it turns out the rads are overall too big you just turn down the FT until they aren't and vice versa.  The most important thing is to get the relative sizing roughly right and the absolute sizing at least big enough for the FT you want to run at.

re rads:

I have one fan radiator (Mitsubishi Climaventa not Jaga, but a close choice) in a location where otherwise the rad just wouldn't fit.  Its our main kitchen/living room.  You can hear it when its at full chat, but you can also hear the (louder) fridge.  Im pleased with it however if you are planning on running at low FT (<30 at highest OAT) then you will get a problem that the Mitsubishi certainly, and possibly the Jaga, turn off their fans when FT,30, on the assumption that this means that no heating is taking place.  I may end up hacking the controller!

I also have one type 33 and the wife hates it.  They are definitely MUCH more intrusive out of the box but as others have said can be disguised particularly if the location 'fits'.  

Feel free to discuss/share your design ideas as they evolve

@downfield No I wasn’t. Thank you. Might be the answer for some places.

My quick scan of the Jaga product sheet left me confused (I get the concept). How is the fan controlled? It obviously needs a power connection but does it need a control connection to the HP controller? And why does the Watts output vary with “control voltage”? Sorry to ask!

Posted by: @sunandair

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Posted by: @cathoderay

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@jamespa - I did say there is more than one method! Not everyone is familiar exponents and reciprocals! The Stelrad (there are others) conversion factor table is a more accessible/understandable/intuitive way of getting to the results.

just to throw the proverbial spanner in…. So this is only dealing with 40c flow temps during higher demand days. Whereas this doesn’t the solve the more frequent days of 10c and above ambients when the flow temps might be 30c 32c or 35c. This is the biggest need to overcome cycling caused by emitters not being specified to cope with heat transfer at lower flow temperatures. 
I once did work out a correction factor down to 30c because the Stelrad charts didn’t go down that low.

IMO these rad schemes really should take some account of low output needs and not just design temps at -3c

 

Not a real problem.  

If the rad output is correct at the design FT and design OAT, then it will also be (almost) correct at the lower FT determined by the WC curve when OATs are higher (I say 'almost' because the relationship isn't quite linear and most WC curves are - but the max deviation is only about a degree (less if the heat pump dosnt bother to modulate the flow rate) so not really an issue)

As regards cycling, the heat loss from the house at higher OATs is what it is, and upsizing the radiators doesn't change that. If the heat pump cant modulate down to the house heat loss then it cant modulate down and will cycle.  Thats why matching the heat pump to the house heat loss (and not oversizing it too much) matters.

Posted by: @simonwhiteley

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@downfield No I wasn’t. Thank you. Might be the answer for some places.

My quick scan of the Jaga product sheet left me confused (I get the concept). How is the fan controlled? It obviously needs a power connection but does it need a control connection to the HP controller? And why does the Watts output vary with “control voltage”? Sorry to ask!

Each Jaga rad has its own temp sensor and fan controller.  There is no central connection to the heat pump.  Same with Mitsi and just about every other fan radiator out there.  It simply turns the fan up and down until it gets the room temperature right (well, thats the theory).

Posted by: @jamespa

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Im not sure what you mean by 'In HTC terms, I have answers ranging from 390W to 650W for the house as a whole' though, do you mean 3900-6900W? and what does HTC stand for (I assume its basically house loss).

HTC stands for Heat Transfer Coefficient - heat loss per C degree difference between IAT and OAT. It’s advantage is that it is independent of design temperature and desired room temperature, which vary depending on location and personal choice. 

@jamespa thanks again. HTC saves getting into debates about assumed indoor and outdoor temps- it is just the heat loss for the house at 1degC temp diff. It allowed me to corral all the various estimates done with different inside and outside temps. I was doing my head in before I came across it as a concept.

Thanks for the feedback on heat loss estimates generally. I am pretty sure the fabric estimates have too high U values for our (well made) solid walls, and the ventilation losses are wild.

My concern with consumption-based estimates are: what was my average (ie after setbacks) daily house temp? I have assumed 1-2 deg below thermostat but hard to guess. And what was the solar gain benefit? The other uncertainties eg cooking, hot water, what HDDs to use, I persuaded myself were marginal to the calculation.

I have plumped for somewhere in the middle between fabric and consumption estimates, and my installer is broadly on side- he is just constrained by the MCS procedure from being explicit.

The places I have in mind for a fan rad the noise wouldn't be a big issue, and if the FT is down to 30deg it wouldn't matter if it didn't kick in- other rads would do the work well enough. Being a tall thin house heat flows quite nicely around anyway.

Posted by: @jamespa

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Posted by: @cathoderay

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The sums for correcting radiator outputs for lower flow temps are straightforward

Many manufacturers give an equation in the following form:

output is proportional to (deltaT)^n

where

deltaT is the mean difference between the rad temp and room temperature 

^ means raised to the power of and

n (the exponent) is invariably 1.3 or very close

 

So if they quote outputs at DeltaT=50, then at deltaT=17.5 the output will be (17.5/50)^1.3 =0.255 times as much.

The reciprocal of this figure (1/.255=3.9) is sometimes called the 'oversize factor'.  This is the number by which you have to multiply the calculated load to get the DT50 output required.  So if the calculated load is 500W and the oversize factor 3.9, you need a radiator rated for 1950W at DT50.  Calculating the oversize factor makes looking through the catalogues easier!

thanks both. I think that calculation gives a lower heat output by about 20% than the HG rule of thumb table I linked above. As he is the one guaranteeing the system performance, I will ask him how he sized the radiators! 

All noted, I hadn't encountered HTC before, but now you have explained it it makes perfect sense. 

Your concerns about consumption based estimates are fair and of course there is also the question of the efficiency of the boiler.  I put everything I had (including 2 years of smart meter readings) into a big 'melting pot' to determine my choice and then, like you, found an installer (two actually) that would also take other data into account whilst of course conforming fully to the MCS rules.  Im happy with the outcome so far, Ive only had the heat pump 6 weeks but we have had a good range of outdoor temps.  The onset of cycling is ~ 11.5C and it copes at +2 (cold and wet) and -2 (colder but dry).  We haven't had -2 for several days in a row yet (last time that happened was two winters ago) but Im not particularly worried, once it reaches temp it coasts at ~40-50%% compressor even at lowest temps.

Like you 7kW was the biggest Vaillant model that would fit, I could have gone Mitsubishi 11.2kW or Samsung 12kW, but the quoted noise levels are 5dB higher, the Samsung controls are rather basic, both are bigger and the Mitsubishi in particular is taller so would rise above the window that I really want it sitting below!  Because I was pretty confident my loss is less than the 8.5kW the Vaillant claims at -2C/45C (being my design flow temp) I didn't bother with the backup heater and reasoned that, if there was a problem, it can be retrofitted.  But thats my case of course.

It sounds to me like you have a pretty sound plan and have done your research, as I say feel free to post any design choices for comments/ideas.

@jamespa also: I assume the fan radiator has a lower water volume? We are looking at a simple one zone, no TRVs, no buffer/volumiser system, so need enough radiator volume to do defrosts. We do have 15mm/22mm pipes and a c12 radiator system with one or two rooms on UFH- too many fan radiators might complicate that judgement? Probably only one or two fan rads would be needed.

Posted by: @simonwhiteley

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How is the fan controlled? It obviously needs a power connection but does it need a control connection to the HP controller?

Yes each Jaga needs a power supply but there is room inside the casing for the small transformer - the controller and fans use 24V DC.  I installed a 240V spur for each rad.  see pics attached

There are temp sensors that switch the fans on when warm water is detected - you can adjust the trigger temp level.  So no connection to the heat pump - each unit operates independently.  There is a small control pad that fits into the grille on top:  this allows you to set the fan speed to slow medium or fast.  This is manually operated and does not self-adjust, but varying the fan speed changes the heat output.  In practise we have found that the lowest setting is adequate, even in sub-zero temps, but of course that depends on a multitude of other factors.

We have TRVs on the rads to control the room temps as normal - I know this is a no-no but seems the only way to keep the bedrooms cooler.  We leave them in a mid-position.

All the TRV's downstairs in living areas are fully open or not fitted.

We also have a Jaga mini canal trench rad fitted in front of a large 3.6m square window (see picture).  This has a series of longitudinal fans and also works well.

All the units are almost silent when the lowest setting.  Above that you can hear them but they quickly blend in to the background.

There are two other settings:  breeze function and cooling function.

Breeze:  this just circulates air irrespective of the water temps.

Cooling:  this turns on the fan when the flow temp falls below a trigger level, which you can adjust.

We live in a barn and cooling was an important reason for getting an ASHP.  Note that Jaga describes this as "light cooling" because you need to keep the temp of the chilled water above the dew point to avoid condensation.  We normally use 16C or 17C.  So this isn't "air conditioning" which uses much cooler flow temps.  On hot days we leave the cooling on in the bedrooms and it reduces the temps there by 6 - 8C typically.

You can fit the Strada rads without the fan units- they have emitters in the bottom third of the rad only (like old finned car radiators) and most of the casing acts as a "chimney" to draw the air through.  But the heat outputs are obviously lower without fans and cooling doesn't work at all.

Posted by: @simonwhiteley

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I assume the fan radiator has a lower water volume?

Yes much lower volume in the rads.  So the system heats up quickly.  We have a 100l buffer but I think that was in the default design anyway.  You can mix and match regular rads with the Jaga units.  We started off with 5 (3 upstairs and 2 down) but changed the other 4 over as soon as we were happy that they worked as specified.

Even with just the buffer and low pipework/rads volume (not sure of the total) our 14kW Ecodan doesn't cycle much.  Overall heat loss is about 10kW at -3C.

Posted by: @simonwhiteley

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@jamespa also: I assume the fan radiator has a lower water volume? We are looking at a simple one zone, no TRVs, no buffer/volumiser system, so need enough radiator volume to do defrosts. We do have 15mm/22mm pipes and a c12 radiator system with one or two rooms on UFH- too many fan radiators might complicate that judgement? Probably only one or two fan rads would be needed.

as @downfield says fan radiators (with the exception, I think, of the Myson Ulow-2 which is literally a more or less standard radiator plus fans) have much lower volume.  A 50/100l buffer volumiser in flow or return (arguments both ways) is a possible work around if system volume turns out to be a problem.