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.
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.
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:
Item | Approx 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.
Table of SCOPs for 5kW and 7kW versions, from MCS:
Flow temperature | SCOP 5kW | SCOP 7kW |
---|---|---|
35°C | 4.48 | 4.36 |
36°C | 4.41 | 4.32 |
37°C | 4.34 | 4.27 |
38°C | 4.27 | 4.23 |
39°C | 4.2 | 4.18 |
40°C | 4.13 | 4.13 |
41°C | 4.05 | 4.09 |
42°C | 3.98 | 4.04 |
43°C | 3.91 | 4.00 |
44°C | 3.84 | 3.95 |
45°C | 3.77 | 3.91 |
46°C | 3.7 | 3.85 |
47°C | 3.63 | 3.80 |
48°C | 3.56 | 3.75 |
49°C | 3.48 | 3.70 |
50°C | 3.41 | 3.65 |
51°C | 3.34 | 3.60 |
52°C | 3.27 | 3.54 |
53°C | 3.2 | 3.49 |
54°C | 3.13 | 3.44 |
55°C | 3.06 | 3.39 |
60°C | 2.71 | 3.10 |
65°C | 2.37 | 2.80 |
70°C | 2.04 | 2.48 |
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.
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.
Vaillant provide data for the following temperatures (first 2 columns are from Vaillant, the other columns are my calculations):
Flow temperature °C | Return temperature °C | Radiator temperature°C | Flow - radiator temperature °C |
---|---|---|---|
35 | 30 | 32.5 | 2.5 |
45 | 40 | 42.5 | 2.5 |
55 | 47 | 51 | 4 |
65 | 57 | 61 | 4 |
COP figures obtained by interpolating Vaillant tables for the powers we need at various outside temperatures:
Radiator temperature °C | COP @ -2°© 51.kW | COP @ 7°C 3.3kW | COP @ 10°C 2.7kW |
---|---|---|---|
38 | 4.2 | ||
39 | 4.0 | ||
40 | 3.9 | ||
41 | 3.8 | ||
42 | 3.0 | 3.7 | 3.8 |
43 | 3.0 | 3.6 | 3.6 |
44 | 2.9 | 3.5 | 3.5 |
45 | 2.8 | 3.4 | 3.4 |
46 | 2.8 | 3.3 | 3.3 |
47 | 2.7 | 3.2 | 3.2 |
48 | 2.6 | 3.1 | 3.1 |
49 | 2.5 | 3.0 | 2.9 |
50 | 2.5 | 2.9 | 2.8 |
51 | 2.4 | 2.8 | 2.7 |
Radiator temperature °C | COP @ -2°C 5.1kW | COP @ 0°C 4.7kW | COP @ 2°C 4.3kW |
---|---|---|---|
51 | 2.4 | 2.5 | 2.6 |
52 | 2.4 | 2.5 | 2.6 |
53 | 2.3 | 2.4 | 2.5 |
54 | 2.3 | 2.4 | 2.5 |
55 | 2.2 | 2.3 | 2.4 |
56 | 2.2 | 2.3 | 2.4 |
57 | 2.1 | 2.3 | 2.4 |
58 | 2.0 | 2.2 | 2.3 |
59 | 2.0 | 2.2 | 2.3 |
60 | 2.0 | 2.1 | 2.2 |
61 | 1.9 | 2.1 | 2.2 |
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.
Plan | Lounge extra | Dining room | Back room extra | Hall | Study | Bathroom | Front bedroom | Back bedroom | Radiator temperature @ -2°C | Flow temperature @ -2°C | Radiator temperature @ 7°C | COP @ 7°C | SCOP | Radiator cost |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
C | 59 | 63 | 50 | 2.9 | 2.92 | - | ||||||||
J | K2 700x800 £129 | K2 600x700 £89 | K2 600x400 installed | K2 600x700 £89 | K2 450x900 £87 | K2 450x700 £68 | 54 | 58 | 45 | 3.4 | 3.22 | £462 | ||
L | K2 700x800 £129 | K2 600x800 £94 | K2 600x400 installed | K2 600x800 £94 | K2 450x1000 £100 | K2 450x800 £87 | 51 | 55 | 45 | 3.4 | 3.39 | £504 | ||
I | T21 600x600 reused | K2 700x800 £129 | T21 600x600 reused | K2 600x800 £94 | K2 600x400 installed | K2 600x800 £94 | K2 450x1000 £100 | K2 450x800 £87 | 47 | 51 | 40 | 3.6 | 3.60 | £504 |
H | K2 600x700 £89 | K2 700x800 £129 | T21 600x600 reused | K2 600x700 £89 | K2 600x400 installed | K2 600x700 £89 | K2 450x900 £87 | K2 450x700 £68 | 46 | 49 | 41 | 3.7 | 3.70 | £551 |
K_si | K2 600x1000 £109 | K2 700x900 £140 | T21 600x600 reused | K2 600x1000 £109 | K2 700x400 £87 | K2 600x800 £94 | K2 450x1000 £100 | K2 450x800 £87 | 43 | 46 | 38 | 4.2 | 3.85 | £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.
TODO: Compressor speeds and COPs are approximate since I haven't accurately interpolated the Vaillant tables.
Outside temperature °C | Heat loss kW | Radiator temperature °C | Flow temperature °C | Compressor speed rps | COP |
---|---|---|---|---|---|
-7 | 6299 | 58 | 62 | - | - |
-2 | 5249 | 51 | 55 | 80 | 2.4 |
0 | 4857 | 50 | 54 | 70 | 2.5 |
2 | 4450 | 48 | 51 | 60 | 2.6 |
7 | 3454 | 45 | 48 | 40 | 3.5 |
10 | 2825 | 41 | 44 | 30 | 3.7 |
15 | 1792 | 36 | 38 | cycles | 5.8 |
TODO: Compressor speeds and COPs are approximate since I haven't accurately interpolated the Vaillant tables.
Outside temperature °C | Heat loss kW | Radiator temperature °C | Flow temperature °C | Compressor speed rps | COP |
---|---|---|---|---|---|
-7 | 6231 | 50 | 54 | 97 | 2.0 |
-2 | 5235 | 47 | 51 | 80 | 2.4 |
0 | 4822 | 45 | 49 | 60 | 2.5 |
2 | 4428 | 44 | 47 | 60 | 3.3 |
7 | 3419 | 40 | 43 | 40 | 3.5 |
10 | 2788 | 37 | 40 | 30 | 4.6 |
15 | 1780 | 34 | 37 | cycles | 5.8 |
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.
Room | Room temp °C | MCS heat loss W | My heat loss W | Radiator power for MCS heat loss W | Possible radiator |
---|---|---|---|---|---|
Lounge | 21.0 | 1007 | 1417 | 1963 | K2 vertical 1800x600 2376W |
Dining room | 21.0 | 790 | 607 | 1360 | K2 700x800 1568W |
Back room | 21.0 | 1137 | 812 | 2216 | +++ K2 600x900 1558W + T21 600x600 833W (old lounge) |
Hall | 18.0 | 385 | 533 | 663 | --- K2 600x700 1212W installed would do |
Study | 21.0 | 389 | 554 | 758 | +++ K2 600x400 693 |
Front bedroom | 18.0 | 648 | 500 | 1115 | K2 450x900 1234W |
Back bedroom | 18.0 | 586 | 367 | 1009 | K2 450x800 1096W |
Ensuite | 22.0 | 393 | 189 | 800 | |
Bathroom | 22.0 | 379 | 403 | 771 | --- K2 600x800 1385W |
Total | 5876 | 5383 |
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.
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.
Room | Room temp °C | Octopus heat loss W | My heat loss W | Radiator power for Octopus heat loss W | Possible radiator |
---|---|---|---|---|---|
Lounge | 21.0 | 1993 | 1589 | 4059 = 2376+1683 = 2376 + 1385 + 298 | K2 vertical 1800x600 2376W + whatever |
Dining room | 21.0 | - | 616 | K2 600x800 1385W | |
Back room | 21.0 | 1134 | 884 | 2310 = 1558+752 | K2 600x900 1558W + T21 600x600 833W (old lounge) |
Hall | 18.0 | 352 | 585 | 631 | T21 500x700 972 |
Study | 18.0 | 270 | 471 | 484 | K2 700x400 784W |
Front bedroom | 18.0 | 687 | 283 | 1231 | K2 450x900 1234W |
Back bedroom | 18.0 | 493 | 133 | 884 | K2 450x700 959W |
Ensuite | 21.0 | 531 | 231 | ||
Bathroom | 22.0 | 396 | 309 | 1132 | K3 600x400 956W |
Total | 5856 | 5101 |
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:
Model | Dimensions W x H mm | 1444 - 0.85 * H - W | Price £ exc VAT |
---|---|---|---|
Telford Tempest 150l | 510 x 1060 | 33 | 461 |
Vaillant uniSTOR 150l | 585 x 1000 | 0 | 730 |
RM Cylinders 150l | 545 x 1102 | -38 | 588 |
Joule Cyclone 150l | 540 x 1190 | -65 | 488 |
Main 150l | 550 x 1114 | -53 | 484 |
Joule Invacyl 150l | 545 x 1190 | -36 | 505 |
Gledhill StainlessLite Plus 150l | 550 x 1118 | -56 | 550 |
So the only cylinders I've found that would fit are the Telford Tempest and the Vaillant uniSTOR.
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.