Is an oversized Vaillant a problem?

Hardware Heatpump
1

Following on from the Daikin thread (Is an oversized Daikin really a problem?) do we have any experience in the community suggesting that swapping out an “over-sizied” Vaillant for a unit that better matches actual heat loss provides a performance improvement?

@Zarch highlights in his blog post here Smaller Heat Pumps will never reach Top of the SCOPS | Energy Stats UK how the larger units can achieve higher performance than the smaller units at the same flow, return & outside temperature and the same RPS of the compressor:

image
image1707×1261 127 KB

The performance of the 7 kW arotherm improves at lower RPS as well, so if you need ~45RPS on the 5kW and only ~35RPS on the 7 kW to satisfy the same heat demand, the COP should be a little higher.

The performance impact of oversizing seems to depend on how well a particular make and model of heat pump deals with cycling or/and rapid changes in operating conditions, e.g the internal refrigeration cycle control algorithm and probably physical implementation as well. With the Daikin 9-16 kW range we’ve discussed how it seems to take quite a bit of time to settle into an efficient operating mode, but I dont think this is something we’ve noticed with the Vaillant units?

Can we see anything in the HeatpumpMonitor.org data for the Vaillants that are being monitored?

ID Location Rating COP Oversizing Factor Weighted average flow - outside temperature % Carnot
46 Mytchett, Surrey 7 4.63 1.5 22.9 50
53 Stroud 5 4.47 1.3 45
75 Beddgelert, Gwynedd 5 4.4 1.8 24.8 48
5 Camberley, Surrey 5 4.38 1.5 48
286 Leicester 5 4.29 1.9 50
124 North London 7 4.29 1.1 53
33 Telford 5 4.29 2.4 21.4 44
140 Buckinghamshire 10 4.27 1.1 28.5 51
51 Rushden, Northamptonshire 7 4.18 1.5 23.5 45
115 Hatfield, Hertfordshire 5 4.16 2 25.1 47
9 Sheffield 5 4.12 1.5 26.0 47
8 Fife, Scotland 7 4.1 1.7 27.7 50
34 Banbury 7 4.05 1.7 25.3 46
109 St Albans 5 3.99 1.3 25.3 47
26 Stratford-upon-Avon 7 3.95 1.1 28.6 49
64 Newport 7 3.83 2.3 29.3 48
59 Oxford 5 3.78 1.8 28.2 48
79 Reading 7 3.78 1.5 27.8 46
81 Haywards Heath 3.5 3.78 1.4 28.2 46
152 Gwent 7 3.76 44
77 Woodham, Surrey 5 3.74 1.2 30.8 48
41 St Albans 5 3.71 1.4 31.1 49
24 North Hertfordshire 5 3.68 1.8 28.2 44
90 Kidlington 5 3.66 1.7 28.3 45
272 Shetland 8 3.65 1.5 30.8 47
43 Edinburgh 7 3.41 2 32.0 45
125 Cuffley, Hertfordshire 5 3.4 1.1 40

Table link: https://heatpumpmonitor.org/?filter=VAILLANT&add=oversizing_factor,id,weighted_average_flow_minus_outside,running_prc_carnot&rm=installer_name,installer_logo,training,learnmore,hp_type,hp_model,mid_metering,data_flag,boundary

1 Like
2

A couple of interesting examples in that list:

  • There is no apparent drop in performance between System 109, St Albans 5kW, 1.3x over-sizing factor, 25.3C weighted system temperature, SPF: 3.99 and System 115 Hatfield, 5kW, 2.0x over-sizing, 25.1C weighted system temperature, SPF: 4.16.

  • There is very little difference (less than 0.1 COP) between system 26 (oversizing 1.1) and system 64 (oversizing 2.3) again at similar weighted system temperatures.

  • System 33 Telford should have a better performance than System 46 Mytchet based on system temperature but is 0.3 COP lower, this example may indicate oversizing penalty (2.4 vs 1.5) or perhaps its something else?

3

My Vaillant is not in your list (non-MID & less than 290 days) but I think my monitoring is solid and my observations valid. My Vaillant 10 kW unit in its current state cycles down to -10 °C :sweat_smile: but my conclusion is that oversizing is not a big problem on Vaillant because of their Energy Integral cycling approach. This makes the cycles really long and spaced out, efficiency during a cycle is really close to datasheet specs for steady state continuous operation. I’m getting maybe 1-2 cycles per hour. Depending on what your emitters are you might see room temperature fluctuation though. Reading the horror stories of other oversized heatpumps here makes me happy I have an oversized Vaillant.

3 Likes
4

Yes that’s my impression as well. Can you remind me which one your system is? I really should remember!

5

The Berlin 10 kW system.

1 Like
6

It’s also interesting that system 140 has higher performance than system 26, even though over-sizing factor and system temperature is the same, perhaps an example of the higher performance from the larger 10kW unit as @Zarch observes:

ID Location Rating COP Oversizing Factor Weighted average flow - outside temperature % Carnot
140 Buckinghamshire 10 4.27 1.1 28.5 51.4
26 Stratford-upon-Avon 7 3.95 1.1 28.6 49.4

There’s a few 5 kW models with very similar system temperatures 28.2C, yet perfformance is slightly worse, these do have higher oversizing factors, but my guess would be that the difference is more likely other factors such as @Zarch point or perhaps something else as well like primary pipework length…

ID Location Rating COP Oversizing Factor Weighted average flow - outside temperature % Carnot
90 Kidlington 5 3.66 1.7 28.3 44.56
59 Oxford 5 3.78 1.8 28.2 47.87
24 North Hertfordshire 5 3.68 1.8 28.2 44.26
81 Haywards Heath 3.5 3.78 1.4 28.2 45.58