Heat Pump Radiators UK: The Complete T30 Sizing Guide (2026)
Your Heat Pump's Working Fine — So Why Are Your Radiators Stone Cold?
Picture this. You've just spent thousands on a shiny new heat pump. The installer left, the app says everything's running, but two hours later your living room still feels like a fridge. The radiator's lukewarm at best. You crank the thermostat. Nothing changes.
Here's what's actually happening — and nobody tells you this upfront. Your old radiators were sized for a gas boiler pumping out water at 70°C. Your heat pump runs at 40-45°C. That's not a fault. That's physics. And unless your radiators are built for those lower temperatures, you're going to be cold.
We've tested hundreds of radiators in our Coventry warehouse — shipping them across the UK since 2012 — and we've seen this exact problem land in our inbox dozens of times. This guide will sort you out. No jargon, no sales pitch, just the numbers you need and the radiators that actually work.
What ΔT Actually Means (And Why It's Everything)
ΔT — or "Delta T" — is just the difference between your radiator's average surface temperature and the room temperature. That's it. Nothing scary.
When you see a radiator rated at "2000 BTU" or "586 Watts," that number assumes a specific ΔT — and for almost every radiator sold in the UK, that's ΔT50. Here's what that means:
ΔT50 assumes your boiler sends water out at 75°C, it comes back at 65°C, and your room sits at 20°C. The average radiator temperature is 70°C, and 70 minus 20 gives you your ΔT50.
Now here's the problem. A heat pump typically runs at a flow temperature of 45°C, returning at 35°C, in a room at 20°C. That gives you an average radiator temperature of 40°C — and a ΔT of just 20°C. That's ΔT30 territory.
Your radiator hasn't changed. The water's just cooler. And because radiator output doesn't drop in a straight line, the difference is brutal.
The Output Drop — Real Numbers
Radiator heat output follows a power curve, not a straight line. The correction formula is:
Correction Factor = (ΔTtarget ÷ ΔT50)1.3
Here's what that looks like in practice:
| ΔT Rating | Correction Factor | Example: 2000 BTU → | Real-World System |
|---|---|---|---|
| ΔT50 | 1.00 | 2000 BTU | Standard gas boiler (75/65/20) |
| ΔT45 | 0.87 | 1740 BTU | High-temp heat pump or condensing boiler on low |
| ΔT40 | 0.75 | 1500 BTU | Mid-range heat pump setup |
| ΔT35 | 0.63 | 1260 BTU | Efficiency-optimised heat pump |
| ΔT30 | 0.51 | 1020 BTU | Typical heat pump (45/35/20) |
| ΔT25 | 0.41 | 820 BTU | Low-temp optimised system |
| ΔT20 | 0.30 | 600 BTU | Ultra-low-temperature design |
Let that sink in. Your 2000 BTU radiator — the one that kept your bedroom toasty with a gas boiler — delivers barely half the heat when paired with a standard heat pump. You haven't lost efficiency. You've just asked a radiator to do its job with water that's 30°C cooler.
That's why people rip out perfectly good heat pumps thinking they don't work. The pump's fine. The radiators are just too small.
Which Radiators Actually Work With Heat Pumps
Not all radiators handle low-temperature water the same way. Some types naturally perform better at ΔT30 — and some barely work at all.
Aluminium Radiators — The Heat Pump Champion
Aluminium heats up faster than steel. Much faster. Where a steel panel radiator might take 20 minutes to feel warm at the top, an aluminium radiator starts emitting usable heat within minutes — even with 40°C water flowing through it. The material's thermal conductivity is roughly four times that of steel, which means it transfers lower-grade heat far more effectively.
Aluminium radiators also hold less water than steel equivalents. Less water volume means the heat pump doesn't have to work as long to bring the system up to temperature, which improves the pump's COP (Coefficient of Performance) — basically, you get more heat per kilowatt-hour of electricity.
We stock a wide range of aluminium radiators in horizontal and vertical orientations, in white and anthracite finishes. Browse our aluminium radiators →
Column Radiators — Surface Area Wins
Column radiators have a secret weapon: surface area. The more surface area a radiator has, the more heat it can throw into a room — and at lower water temperatures, surface area matters more than anything else.
A four-column radiator might have two or three times the surface area of a single panel radiator of the same dimensions. That means it can deliver useful heat output even at ΔT30. Traditional cast-iron-style column radiators also retain heat longer, smoothing out the temperature dips between heat pump cycles.
Our column radiators come in traditional and modern styles, from classic 2-column to deep 4-column configurations. Browse our column radiators →
Type 21 and Type 22 Panel Radiators — Convectors Help
A Type 11 (single panel, single convector) radiator at ΔT30 is a sad sight. The single panel just doesn't have the surface area, and a lone set of fins can't compensate. But step up to a Type 21 (double panel, single convector) or Type 22 (double panel, double convector), and things change.
The second panel doubles your radiating surface, and the convection fins pull cooler air up through the radiator, warming it as it passes. At low water temperatures, this convective boost matters — it's the difference between a radiator that barely warms the wall behind it and one that actually heats the room.
If you're keeping existing radiators on a heat pump retrofit, single panel Type 11s are usually the first to go. They're the weakest link. Browse our panel radiators →
What to Avoid
Traditional single panel radiators without convectors (Type 10/11). Designer flat-panel radiators with minimal surface area that rely entirely on high water temperatures. Anything undersized for the room it's in — and "undersized" at ΔT30 means about twice the BTU rating you'd need at ΔT50.
How to Size Radiators for a Heat Pump — A Practical Method
You don't need to be a heating engineer to get this right. Here's the straightforward method we use with customers every day:
Step 1: Calculate your room's heat requirement the normal way — use our BTU calculator, or get a figure from your heat pump installer's room-by-room heat loss calculation. This gives you the actual watts or BTUs your room needs.
Step 2: Take that number and divide it by the ΔT30 correction factor: 0.51. This tells you what the radiator's ΔT50 rating needs to be so that it delivers enough heat at ΔT30.
Radiator ΔT50 Rating Needed = Room Heat Requirement ÷ 0.51
Worked example — a typical UK living room:
Say your living room (4m × 5m with standard insulation) needs 5,500 BTU to stay warm. At ΔT50, you'd pick a radiator rated around 5,500-6,000 BTU and call it done. But at ΔT30:
5,500 BTU ÷ 0.51 = 10,785 BTU (ΔT50 rated)
You need a radiator with a ΔT50 rating of roughly 11,000 BTU — nearly double the gas boiler figure. That could mean a larger single radiator, or two smaller radiators splitting the load. Either approach works; what matters is that the total ΔT50 rating across all radiators in the room hits that figure.
The same logic applies in watts. A room needing 1,600W at ΔT30 needs radiators with a combined ΔT50 rating of around 3,140W.
Quick reference:
| Room Heat Needed (ΔT30) | Radiator ΔT50 Rating Required | Typical Radiator Solution |
|---|---|---|
| 1,000 BTU | 1,960 BTU | Small single aluminium or compact Type 21 |
| 3,000 BTU | 5,880 BTU | Medium column radiator or large Type 22 |
| 5,000 BTU | 9,800 BTU | Large 3-4 column radiator or two Type 22s |
| 8,000 BTU | 15,690 BTU | Multiple radiators or extra-deep column configuration |
Running Costs: Heat Pump Radiators vs Gas Boiler
Heat pumps win on efficiency (300-400% vs a boiler's 90-95%) but electricity costs more per unit than gas. Here's how it shakes out at current UK average prices:
| Metric | Gas Boiler | Heat Pump (SCOP 3.5) |
|---|---|---|
| Fuel cost per kWh | 6.24p (gas) | 24.67p (electricity) |
| System efficiency | ~92% | ~350% (SCOP 3.5) |
| Effective cost per kWh of heat | ~6.78p | ~7.05p |
| Annual heating (12,000 kWh heat demand) | ~£814 | ~£846 |
The running costs are close — within a few percent at current prices. The difference narrows further if your heat pump achieves a higher SCOP (Seasonal Coefficient of Performance), or if you're on a heat pump tariff like Octopus Cosy or OVO Heat Pump Plus. Those tariffs drop your electricity rate during off-peak hours, sometimes below 15p/kWh. At 15p, the heat pump costs around £514 per year — a clear winner.
The real savings come when you pair the heat pump with properly sized radiators. Undersized radiators force the heat pump to run at higher flow temperatures to compensate, which tanks the COP. A heat pump running at 55°C flow instead of 45°C might drop from a COP of 3.5 to 2.5 — adding hundreds of pounds to your annual bill. Get the radiators right and the economics work. Skimp on them and you're paying for it every month.
Our Heat-Pump-Ready Radiators
We've been shipping radiators from our Coventry warehouse (CV7 9NH) since 2012. Everything we stock is held right here in the UK — no dropshipping, no three-week lead times from a container somewhere. If you're local to the Midlands, we deliver same day or next day with our own fleet. Nationwide, it's free delivery on orders over £200 (and £9.99 under).
Here's what works best with heat pumps right now:
- Aluminium Radiators — Fast heat-up, excellent conductivity, low water content. The top performer at ΔT30. Available in horizontal and vertical layouts, white and anthracite.
- Column Radiators — Maximum surface area per wall footprint. Traditional 2, 3, and 4-column options that deliver real heat even with 40°C water.
- Panel Radiators — Type 21 & 22 — The practical upgrade. Double panels with convectors give you much better ΔT30 performance than single panels, and they fit the same pipe centres as your old Type 11s in most cases.
Not sure what you need? Give us a ring or drop us an email. We talk people through this every day, and we'd rather you get the right radiator first time than deal with returns.
Frequently Asked Questions
Do I need special radiators for a heat pump?
Not "special" as in a different technology — but you almost certainly need bigger radiators, or radiators made from materials that perform better at low temperatures (aluminium, multi-column steel). Your existing radiators were sized for ΔT50. At ΔT30 they output roughly half as much heat. If they were generously oversized to begin with, you might get away with it. For most UK homes, the answer is you'll need to replace at least some of them.
Can I use my existing radiators with a heat pump?
You can try — but test first. On a cold day, run your existing radiators at a 45°C flow temperature (most gas boilers let you dial down the flow temp). If your rooms reach a comfortable temperature within an hour or two, your radiators are sized well enough for a heat pump. If the rooms never quite get warm, or it takes four hours, you'll need larger radiators. This is the cheapest diagnostic you can do — costs nothing and gives you a real-world answer.
Are aluminium radiators better for heat pumps?
Yes. Aluminium's high thermal conductivity means it transfers heat from low-temperature water far more effectively than steel. An aluminium radiator running at ΔT30 will feel warmer faster and deliver more usable heat per square metre than an equivalent-sized steel panel. They also contain less water, so the heat pump reaches its target flow temperature quicker, improving overall system efficiency. We stock aluminium radiators across a wide range of sizes — see the full collection here.
What size radiator do I need for a heat pump?
Take your room's calculated heat requirement (use our BTU calculator or your installer's heat loss report) and divide by 0.51. The result is the ΔT50 BTU rating your radiator needs. For example, a bedroom needing 2,000 BTU of actual heat needs a radiator rated at roughly 3,920 BTU at ΔT50. If in doubt, go bigger — an oversized radiator on a heat pump is just a radiator that lets the pump run at an even lower, more efficient flow temperature. There's no downside.
Do heated towel rails work with heat pumps?
They can — but standard towel rails have very little surface area and rely heavily on high water temperatures. At ΔT30, a typical 600mm × 800mm towel rail might only emit 250-400 BTU, which is barely enough to take the chill off a small bathroom. If you want a towel rail on a heat pump system, go for the largest dimensions your wall can fit, or pair it with a secondary heat source (underfloor heating, a small aluminium radiator). Pipe centres on our towel rails follow the standard formula: rail width minus 50mm.
Will bigger radiators make my heat pump more efficient?
Yes — and this is the single most overlooked point in heat pump design. Larger radiators (or more of them) mean you can run your heat pump at a lower flow temperature and still hit your target room temperature. Every 5°C you drop the flow temperature improves the heat pump's COP by roughly 0.3-0.5. Over a 15-year lifespan, properly sized radiators can save you thousands in electricity — far more than they cost to buy.
Ready to Get the Right Radiators?
We've been helping UK homes stay warm since 2012. Our radiators are in stock at our Coventry warehouse, ready for same-day local delivery or next-day nationwide shipping. Every aluminium radiator comes with a 25-year warranty. Towel rails get 5 years, electric elements 1 year — and stainless steel radiators are covered for 25 years too.
- Browse Aluminium Radiators — The best heat pump performers we stock
- Browse Column Radiators — Maximum surface area, classic looks
- Browse Panel Radiators — Type 21 and 22 options for retrofit projects
- Use Our BTU Calculator — Work out your room's requirements
- Read More Guides — Radiator sizing, installation tips, and buying advice
Free UK delivery on orders over £200. Need advice? Get in touch — we answer real emails from real people, same day if we can.