Next steps on my ASHP journey: Radiators and Pipework

I’m taking a slow and cautious step-by-step journey along the path towards getting an ASHP. Below are my thoughts on replacement radiators, whether my current pipework is sufficient, and whether my hot water tank should be replaced. I’m a research scientist by profession, so like to understand and work things out for myself. However, I am also new to this area, so would much appreciate feedback and advice, including pointing out any faulty assumptions or missteps on my part.

Insulation

Back in 2023, we added as much retrofit insulation as was practical to our 1970’s 4-bed detached (146 sqm). That included toping up the loft insulation, adding underfloor insulation including above the integral garage, switching to high-performance double-glazed units, adding PIR insulation to the walls of the garage, and insulating the timber walls of the dormer. The house is now as well insulated as it reasonably can be.

Heat Loss

I have recently done heat loss calculations, both a theoretical room-by-room approach and a practical approach based on data from the heat energy going into the house and the temperature difference that created. See:

https://renewableheatinghub.co.uk/forums/renewable-heating-air-source-heap-pumps-ashps/adventures-in-heat-loss-calculations/#post-66313

The heat loss worked out at about 6200W with at least 500W provided by other sources (electricity use, people, solar gain in the coldest months), so about 5700W needed from a heating source running for 24 hours per day.

Assuming hot water heating for two hours per day at 6000W, which is plenty for us, leaves 22 hours for heating, which comes out at about 6200W.

A 7kW heat pump would therefore appear to be sufficient. For example, a Vaillant Aerotherm 7kW, which provides its specified output or more at -5C or higher outdoor temperature and flow temperatures in the range 35-55C. It can also provide a flow temperature of 75C for heating hot water.

Radiators

The radiators are next on my to do list. The existing ones are type 10 and type 20 and have a combined output of about 12500W at DT50, at least they might have done when new. Most of them are the same age as the house, i.e. 55, and ready for retirement. Some are likely silted up, since a thermal camera shows the lower middle part of the radiator remains cold for a while on warm up. They often need bleeding and can be noisy – less so now that the gas boiler operates using weather compensation and flow temperatures are no more than 65C.

I have looked at the Stelrad radiator specs and chosen larger K2 radiators to match or slightly exceed the room-by-room heat loss at a flow temperature of 40C, i.e. a radiator temperature of 37.5C, assuming a 5C drop from flow to return. I calculated the radiator performance based on the published DT50 values, scaled according to (X/50)^1.3, where X is the difference between the radiator temperature and the room temperature. This gives a scaling factor of 0.29 for DT20 and 0.24 for DT17.5.

The new radiator for the hallway is under-powered by 15% as there isn’t space for a larger size, and fitting a K3 would be problematic due to the extra width. Taking a pragmatic view, if the hallway only reaches 17C on the coldest days of the year then that’s fine. In practice, I expect it will take some heat from the adjacent rooms, which have overcapacity. Aside from that I was able to match very well the room-by-room heat loss.

In total, the new radiators have an output of 26300W at DT50 and 6300W at DT17.5. By comparison, the lowest temperature that the existing radiators could possibly support is 50C, i.e. a flow temperature of 52.5C. Since, they are old and likely have quite a lot less output than new, that figure is no doubt optimistic.

For the new radiators, the total water content is 117 litres, with an estimated further 18 litres in the relevant copper pipes. Total water volume 135 litres, which works out at about 21.7 litres per kW. According to the info on the Heat Pump Water Volume calculator this is in the good range (>20).

https://renewableheatinghub.co.uk/does-your-heat-pump-have-enough-water-volume-to-run-efficiently/

At a design flow temperature of 40C a Vaillant Aerotherm 7kW is expected to give a SCOP of 4.13, which would be nice.

Pipes

Copper pipe is used throughout. This is 22mm for 10m on the flow and return from the gas boiler, splitting into two 15mm pipes (upstairs and downstairs) that are each about 40m long. The upstairs and downstairs 15mm pipes are not equal in terms of the heat energy they need to transport. The downstairs one needs to carry about 3700W and the upstairs one about 2500W.

The Vaillant Aerotherm 7kW operates at a maximum flow rate of 0.333 litres per second, with the difference between flow and return temperatures varying with output. At a 5C temperature difference, this corresponds to 7kW output.

A flow rate of 0.333l/s down a 22mm pipe corresponds to a flow speed of about 1.0m/s. A flow rate of 0.333l/s down two 15mm pipes in parallel, i.e. 0.167l/s each, corresponds to a flow speed of about 1.15m/s, which is in the standard range of 1.0 to 1.5m/s, and a little above the quiet range of 0.5 to 1.0m/s. The UK good practice limit is 1.2m/s for central heating circuits, so this would be met.

The pressure drop for 0.333l/s along one 10m section of 22mm pipe followed by two parallel 40m sections of 15mm pipe, then one 10m section of 22mm pipe is theoretically: 3.6 + 25.8 + 3.6 = 33 kPa = 0.33 bar, or 3.3m head. However, this value is for straight pipe, without junctions and corners that cause significant extra resistance to flow. Taking a pessimistic view that junctions and corners may add up to 50% gives a pressure drop of 0.5 bar or 5m head. This appears to be within the acceptable range for most central heating pumps, including my current one that delivers 6m head.

The 22mm hot water pipes separate off the main flow and return pipes after about 3.5m and run for a further 6.5m to the hot water tank. As we only ever do space heating or hot water heating, not both at the same time, this arrangement appears to be fine with flow rates and pressures (1.0 m/s and 0.07 bar or 0.7m head) for the hot water circuit below those for the heating circuit.

Hot water tank

Unlike the radiators, our hot water tank is not in need of replacement. Hence an economic argument would be needed to do so. Heating the hot water in the existing tank could be done using 75C flow from the heat pump with a COP of about 2.4. Whereas with a new heat pump specific tank, this could be done at lower temperatures, with a once-a-week anti-legionella cycle, with an average COP of about 3.2.

Our annual energy requirement for hot water is about 1600 kWh. The annual energy saving with a new hot water tank would therefore be about 167 kWh due to the increased COP. Assuming current cheap overnight electricity prices this amounts to a saving of about £15 per year. Assuming a Vaillant UniSTOR hot water tank and installation at £1500, which is perhaps optimistic, this equates to a payback period of 100 years, and so does not make economic sense.

Conclusions and questions

I aim to replace all the radiators next summer, but not the hot water tank, and then see how I get on trying out a 40C flow temperature over the following winter using my gas boiler as the heat source. It appears that my pipework can support a 7kW heat pump, but not a 10kW one, so I’ll need to make sure that any survey prior to fitting a heat pump concludes that a 7kW unit is the correct size.

Question 1: Rather than match the calculated heat loss for each room, should I up-size the new radiators further, e.g. 70cm high instead of 60cm in some rooms? The idea being that they could then be balanced down to the right output. The extra cost would be relatively low, extra water volume would be added to the system, and there would be more scope to turn them up if needed to get the correct overall balance. Or is this a waste of money?

Question 2: Feedback would be most welcome on pipework considerations. For those with lots of experience in this area, would the normal expectation be that my 22mm/15mm pipework is likely fine for use with a 7kW heat pump? And are the upstairs and downstairs 15mm pipes capable of carrying 2500W and 3700W respectively?

Question 3: As an alternative to what I propose doing, is there an argument for getting a heat pump first and then changing the radiators later? Are there any advantages in doing the radiator changes at the same time as a heat pump is installed (aside from draining and filling the system one less time)?

Why do insulation first?

If I had not upgraded the insulation first, then the heat loss would be about 9400W, 52% more than it currently is. Hence the need for a 10kW heat pump instead of 7kW, at an additional cost of around £1000 and possibly higher installations costs too. Instead of 40C flow temperature with the new radiators, it would be around 47C, reducing the SCOP. The maximum flow rate for 10kW is 0.574l/s. This flow rate down a pair of 15mm pipes equates to 1.97m/s, which is well over the practical limit of 1.5m/s, likely to cause excessive noise and erosion of the lining of the pipes. Hence, the main 15mm pipework both upstairs and downstairs would need replacing, at significant cost and disruption. The pressure drop for the existing pipework would be 9.9 + 70.7 + 9.9 = 90.5 kPa, 0.9bar, or 9m head. This is again an underestimate due to junctions and corners. Even an optimistic uplift of only 10% gives a pressure drop of over 1 bar or 10m head, which is too high.

As expected, there are strong arguments for a "fabric first" approach, insulating as much as possible first. This reduces the energy required for heating due to the lower heat loss, and more so with a heat pump by enabling lower flow temperatures and hence higher efficiency. Insulating first potentially avoids the need for a more costly higher output heat pump, that may require expensive and disruptive changes to pipework that would not otherwise be required.