Vaillant aroTHERM plus heat pump

Updated Oct 2023.


This is a monobloc design, but it does need some indoor parts. There are 3 possibilities for the indoor parts:

I initially thought that the best solution would be the hydraulic station, together with our existing HWC, although we wouldn't particularly want the backup heater. It costs an extra £974, which seems far more than the individual bits would cost.

However, I have since discovered that the Vaillant range of uniStor hot water cylinders includes a  slimline version that would fit where the gas boiler currently is. It's pre-plumbed, and comes with the 3-way diverter valve, pressure vessel and immersion heater. It's 435mm diameter and 1642mm height. With all the pre-plumbed bits it should fit in a width of 550mm. This could be used with the Control interface module, which could be mounted on the wall to the left of the kitchen outside door. This solution would have the advantage of not needing our water pump to get high pressure water.

Heat exchanger

Vaillant have an optional heat exchanger with circulating pump, which allows the heat pump to be used like a water-split unit. This limits the use of glycol antifreeze to the plumbing between the heat pump and the exchanger. No antifreeze is then needed in the radiator/DHW plumbing. Dimensions are 500H x 250D x 360W mm, and price is £690 exc VAT. I don't believe glycol is expensive enough to warrant using this, but there may be other reasons to use it.

In particular, our radiator circuit is vented, i.e. there is a feed & expansion tank in the loft. It's likely that the heat pump is designed to be used with a sealed circuit. I suppose that it could be converted to a sealed system by simply removing the F&E tank from the system, but there's a risk that the increased pressure could prduce leaks.

Water volume in heating circuit

A minimum water volume in the heating circuit of 55l is needed for de-icing cycles, and a buffer vessel (price £560) is needed if the radiators do not provide this volume. Only radiators without TRVs can be counted for this purpose (if a TRV is closed, the water in the radiator is inaccessible), so this means just the lounge and dining area radiators. According to my rough calculation, these would only provide about 9l of water. If we removed (or permanently opened) the TRVs on all the ground floor radiators, we would increase the water volume to about 31l.

The Vaillant "buffer vessel" seems to be what is also known as a "low loss header". Bombinho is very critical of these ("one of the worst things ever invented for hydraulic connection"), so we probably ought to avoid using it.

The minimum water volume of 55l can apparently be reduced to 20l if there is a backup resistive heater in the circuit. The Vaillant hydraulic station contains a backup heater, so if we had one we probably wouldn't need a buffer vessel. The hydraulic static is 974-560 = £414 more expensive than the buffer vessel, so its cost might be justified if the other bits in the hydraulic station would cost around £400 if bought separately. These bits are:

ItemApprox price
3-way diverter valve£100
10 litre expansion vessel£70
Expansion relief valve£20
Total£190

So the hydraulic station would add about £200 to the component cost. But it should reduce installation cost.

SCOP

Table of SCOPs for 5kW and 7kW versions, from MCS:

Flow temperatureSCOP 5kWSCOP 7kW
35°C4.484.36
36°C4.414.32
37°C4.344.27
38°C4.274.23
39°C4.24.18
40°C4.134.13
41°C4.054.09
42°C3.984.04
43°C3.914.00
44°C3.843.95
45°C3.773.91
46°C3.73.85
47°C3.633.80
48°C3.563.75
49°C3.483.70
50°C3.413.65
51°C3.343.60
52°C3.273.54
53°C3.23.49
54°C3.133.44
55°C3.063.39
60°C2.713.10
65°C2.372.80
70°C2.042.48

The last 3 rows of the table are from my extrapolations.

The SCOP for 5kW falls to 2.80 at a flow temperature of 58.7°C. The SCOP for 7kW falls to 2.80 at a flow temperature of 65.0°C. So these are the highest flow temperatures that could be used to be eligible for the BUS grant.

In order to minimize radiator replacements I want to use a flow temperature of 65°C (in cold weather only). This would require using the 7kW heat pump.

Performance data

The following tables use performance data from the Vaillant Product Information, section 9.7.2 starting on page 359 for the 5kW heat pump, and section 9.7.4 starting on page 363 for the 7kW heat pump.

The heat loss and required radiator temperature figures are from my heat model - see table in my Radiators document.

Flow temperature

Vaillant provide data for the following temperatures (first 2 columns are from Vaillant, the other columns are my calculations):

Flow temperature °CReturn temperature °CRadiator temperature°CFlow - radiator temperature °C
353032.52.5
454042.52.5
5547514
6557614
COP data

COP figures obtained by interpolating Vaillant tables for the powers we need at various outside temperatures:

Radiator temperature °CCOP @ -2°© 51.kWCOP @ 7°C 3.3kWCOP @ 10°C 2.7kW
38
4.2
39
4.0
40
3.9
41
3.8
423.03.73.8
433.03.63.6
442.93.53.5
452.83.43.4
462.83.33.3
472.73.23.2
482.63.13.1
492.53.02.9
502.52.92.8
512.42.82.7


Radiator temperature °CCOP @ -2°C 5.1kWCOP @ 0°C 4.7kWCOP @ 2°C 4.3kW
512.42.52.6
522.42.52.6
532.32.42.5
542.32.42.5
552.22.32.4
562.22.32.4
572.12.32.4
582.02.22.3
592.02.22.3
602.02.12.2
611.92.12.2

Possible radiator selections

SCOP figures are extrapolated/interpolated from MCS data, and assume that the heat pump flow temperature is the radiator temperature at -2°C outdoors plus 4°C.

PlanLounge extraDining roomBack room extraHallStudyBathroomFront bedroomBack bedroomRadiator temperature @ -2°CFlow temperature @ -2°CRadiator temperature @ 7°CCOP @ 7°CSCOPRadiator cost
C







5963502.92.92-
J
K2 700x800
£129

K2 600x700
£89
K2 600x400
installed
K2 600x700
£89
K2 450x900
£87
K2 450x700
£68
5458453.43.22£462
L
K2 700x800
£129

K2 600x800
£94
K2 600x400
installed
K2 600x800
£94
K2 450x1000
£100
K2 450x800
£87
5155453.43.39£504
IT21 600x600 reusedK2 700x800
£129
T21 600x600 reusedK2 600x800
£94
K2 600x400
installed
K2 600x800
£94
K2 450x1000
£100
K2 450x800
£87
4751403.63.60£504
HK2 600x700
£89
K2 700x800
£129
T21 600x600 reusedK2 600x700
£89
K2 600x400
installed
K2 600x700
£89
K2 450x900
£87
K2 450x700
£68
4649413.73.70£551
K_siK2 600x1000
£109
K2 700x900
£140
T21 600x600
reused
K2 600x1000
£109
K2 700x400
£87
K2 600x800
£94
K2 450x1000
£100
K2 450x800
£87
4346384.23.85£726

Plan: implement plan L initially, with option to upgrade to plan I later.

The above assumed a ventilation rate of 30l/s.
With a ventilation rate of 60l/s at an outside temperature of -2°C, planL would need a radiator temperature of 57°C and a flow temperature of 61°C, resulting in a SCOP of 3.04. At 7°C outside, the radiators would need to be at 47°C, resulting in COP of 3.2.
With a ventilation rate of 60l/s at an outside temperature of -2°C, planI would need a radiator temperature of 51°C and a flow temperature of 55°C, resulting in a SCOP of 3.39. At 7°C outside, the radiators would need to be at 43°C, resulting in COP of 3.5.

Plan L performance

TODO: Compressor speeds and COPs are approximate since I haven't accurately interpolated the Vaillant tables.


Outside temperature °CHeat loss kWRadiator temperature °CFlow temperature °CCompressor speed rpsCOP
-762995862--
-252495155802.4
048575054702.5
244504851602.6
734544548403.5
1028254144303.7
1517923638cycles5.8
Plan I performance

TODO: Compressor speeds and COPs are approximate since I haven't accurately interpolated the Vaillant tables.


Outside temperature °CHeat loss kWRadiator temperature °CFlow temperature °CCompressor speed rpsCOP
-762315054972.0
-252354751802.4
048224549602.5
244284447603.3
734194043403.5
1027883740304.6
1517803437cycles5.8

Radiator sizes using MCS heat loss figures

For a radiator temperature of 51°C, the correction factor is 0.513 for a room temperature of 21°C, 0.581 for 18°C, and 0.491 for 22°C.

RoomRoom temp °CMCS heat loss WMy heat loss WRadiator power for MCS heat loss WPossible radiator
Lounge21.0100714171963K2 vertical 1800x600 2376W
Dining room21.07906071360K2 700x800 1568W
Back room21.011378122216+++ K2 600x900 1558W + T21 600x600 833W (old lounge)
Hall18.0385533663--- K2 600x700 1212W
installed would do
Study21.0389554758 +++ K2 600x400 693
Front bedroom18.06485001115 K2 450x900 1234W
Back bedroom18.05863671009K2 450x800 1096W
Ensuite22.0393189800
Bathroom22.0379403771--- K2 600x800 1385W
Total
58765383

If the back bedroom temperature is raised to 21°C, the back room loss reduces slightly to 1068W.

If the air changes per hour in the back room is reduced from 1.5 to 0.5 the back room loss reduces to 822W, which require a radiator rated at 1602W. This is only 3% greater than the installed radiator. This change would also reduce the total heat loss to 5399W - close to my figure.

Radiator sizes using Octopus heat loss figures

For a radiator temperature of 51°C, the correction factor is 0.491 for a room temperature of 21°C, 0.558 for 18°C, and 0.469 for 22°C.

RoomRoom temp °COctopus heat loss WMy heat loss WRadiator power for Octopus heat loss WPossible radiator
Lounge21.0199315894059 = 2376+1683 = 2376 + 1385 + 298K2 vertical 1800x600 2376W
+ whatever
Dining room21.0-616
K2 600x800 1385W
Back room21.011348842310 = 1558+752K2 600x900 1558W + T21 600x600 833W (old lounge)
Hall18.0352585631T21 500x700 972
Study18.0270471484K2 700x400 784W
Front bedroom18.06872831231K2 450x900 1234W
Back bedroom18.0493133884K2 450x700 959W
Ensuite21.0531231

Bathroom22.03963091132K3 600x400 956W
Total
58565101

Hot water cylinder

Since the Vaillant heat pump can produce flow temperatures up to 75°C for DHW (up to 70°C when outside temperature is below zero), we don't need a special "heat pump" cylinder. However, the installer might not be happy using our current small vented cylinder, and an unvented cylinder would allow us to remove our "shower" pump. One reason that the installer might not like our cylinder is the lack of a temperature sensor (and no slot to take one), but I expect that an external sensor could be fitted.

We don't want the cylinder in the kitchen, so we need one that will fit in the airing cupboard. See my notes on how to tell if a cylinder will fit. Basically, 1444 - 0.85 * H - W should be greater than zero, where H and W are the height and width of the cylinder in mm.

Available cylinders:

ModelDimensions W x H mm1444 - 0.85 * H - WPrice £ exc VAT
Telford Tempest 150l510 x 106033461
Vaillant uniSTOR 150l585 x 10000730
RM Cylinders 150l545 x 1102-38588
Joule Cyclone 150l540 x 1190-65488
Main 150l550 x 1114-53484
Joule Invacyl 150l545 x 1190-36505
Gledhill StainlessLite Plus 150l550 x 1118-56550

So the only cylinders I've found that would fit are the Telford Tempest and the Vaillant uniSTOR.

Prices

Installation of heat pumps and their ancillary equipment (including radiators) is zero-rated for VAT from 1 April 2022 until 31 March 2027.

Prices below are from PlumbNation, The Heat Pump Warehouse, and Warmsafe Plumbing and Heating  - click on the price to see the relevant page on their websites. The prices exclude VAT.

5kW heat pump with control interface£2998
7kW heat pump with control interface£3368
5kW heat pump with hydraulic module£3971
7kW heat pump with hydraulic module£4342

Replacement radiators would add between £550 and £750 excluding VAT. If we bought and installed the radiators ourselves we would probably have to pay VAT, and the additional cost would then be £650 to £900 including VAT.

There would various other bits and pieces to add to this, e.g. cable, pipes, connectors, consumer unit. These could easily add up to another £500 or so.

So we are looking at a total cost for the components of about £5300 to £5800.

Octopus quoted £5300 for "Goods and equipment" - so similar to my estimate, but for an inferior system. They also quoted £5700 for installation! Hopefully other installers will charge less than that.