ᐅ Basic Information on Radiator Replacement in Older Buildings

Hello everyone,

We want to replace the radiators in the children’s rooms or rather install new ones. What exactly do I need for that?

I need some basic information.
The house was built in 1981, gas heating (condensing boiler), underfloor heating on the ground floor, radiators upstairs.
Niches were bricked up and the heating pipes were left about 50cm (20 inches) apart.
Should I buy radiators that are suitable for low-temperature heating systems?
Are DIY store radiators much worse than, for example, a Kermi x2 (is the higher price worth it)?

Thanks a lot!!

Finally, a simple and nice question again ;-)

Is there a heat load calculation or do you know what type of radiator was installed? Then you have two options.

1. Direct replacement (1:1 exchange)
2. Replacement with a heat pump-compatible radiator

Heat pump-compatible radiators that work well even with lower energy efficiency are typically type 33 radiators. It doesn’t really matter if they are off-the-shelf or branded products. Branded ones might have slightly better specifications. Recently, I simply ordered radiators online (for a whole apartment). Buderus (which is Bosch) radiators, which you can also often find in hardware stores.

You can also decide based on the appearance. Smooth or ribbed. Depending on the connection situation (where the pipes are located), there are different models. With suitable adapters or S-connecting fittings, you can connect pipes that are “at the wrong height,” for example. Mounting hardware is usually included.

If you plan to switch to a heat pump at some point or want to lower the supply temperature throughout the house, choose type 33 radiators right away. You can also check out or compare options at online retailers specialized in heating components.

You will never be able to keep your apartment reasonably warm in winter with an affordable electricity consumption and a heat pump, no matter what type of radiator you use.

It’s difficult to bypass the laws of physics. The heating surface of underfloor heating is as large as the floor area of your house including proper insulation. You simply can’t achieve this with a radiator.

And with a supply temperature of 30°C (86°F), it just won’t get warm enough. You would need to operate at 45 to 50°C (113 to 122°F) supply temperature to make it somewhat effective. But then the electricity meter will run at record speed.

It’s somewhat like with storage heaters, which were even banned in 2009. Now, they want you to heat directly with electricity again.
Nonsense in old buildings.

Buchsbaum schrieb:

You will never be able to heat your apartment reasonably in winter with an affordable electricity consumption using a heat pump. It doesn’t matter what type of radiator you use.

You can hardly overcome the laws of physics. The heating surface of underfloor heating is as large as the floor area or living space of your house, including appropriate insulation. You can’t achieve this area with radiators.

Do you really believe the nonsense you are saying?

1. Personal experience
Mid-1960s old building with new windows and modern insulation of the top floor ceiling. Heat pump flow temperature in winter up to about 35/36°C (95/97°F) at most.

2. The heating surface of underfloor heating alone means nothing. What matters is the heat output. This depends on pipe diameter, installation spacing, flow rates, and flow temperatures. First, a room-by-room heating load calculation is done to determine what kind of underfloor heating needs to be installed and how the system should be set. And if a room requires 1,500 watts, it doesn’t matter at all to the room whether that heat is supplied through underfloor heating or radiators.

The heat output of a radiator is specified by the manufacturer. For example, if you use an 11-section 60x80cm (24x31 inches) standard 08/15 radiator:
- at 75/55°C (167/131°F) it delivers about 650 watts
- at 55/45°C (131/113°F) it delivers about 420 watts
- at 40/35°C (104/95°F) it delivers about 190 watts

If I switch to a 33-section radiator, the values are:
- at 75/55°C (167/131°F) about 1,700 watts
- at 55/45°C (131/113°F) about 1,000 watts
- at 40/35°C (104/95°F) about 600 watts

Simply by changing the radiator, I can reduce the flow temperature from 75 to 55°C (167 to 131°F) and still have a reserve.

If I have some extra space in my radiator niche, I replace the 60x80cm (24x31 inches) type 11 radiator with a 60x90cm (24x35 inches) type 33 radiator with 750 watts heat output. Then I can lower the flow temperature from 75/55°C (167/131°F) down to 40/35°C (104/95°F) with reserves. Temperatures can be adjusted further if needed.

For the conversion, there are a) professionals, b) manufacturer tables, and c) even online tools. This is just an example. You can also calculate for 35/30°C (95/86°F) or whatever your heating system requires.

You can’t bypass the laws of physics. But physics is quite straightforward and everything can be easily calculated. And my explanation here is just basic physics behind heating systems ;-)

Therefore, many claims like “You can never properly or economically heat old buildings with a heat pump” are, unfortunately, just baseless opinions repeated around the pub table.

With a 35-degree supply temperature, the pipe losses in the older building, and probably less-than-ideal insulation, the apartment will get cold in winter.

And please don’t use the last three winters as a benchmark. Oh, right, it doesn’t really get that cold anymore because of climate change and so on. You can believe that or not. Maybe that works in the Rhine Valley, the Palatinate, and other parts of Germany that aren’t particularly prone to cold.

In 1996, much of Saxony-Anhalt experienced continuous frost from mid-October to the end of March, sometimes down to minus 30 degrees Celsius (minus 22 degrees Fahrenheit). The ground was frozen to a depth of 1.50 meters (5 feet). There were hardly any days with temperatures above 0 degrees Celsius (32 degrees Fahrenheit).

Even Mannheim had 26 ice days. So, in such weather conditions, I wouldn’t want to live in an apartment heated by a heat pump with a supply temperature of 30 degrees Celsius (86 degrees Fahrenheit). Not to mention the electricity consumption.

Surely, the reports online about cold apartments and extreme electricity usage from heat pumps are just imagination.

For now, I’ll keep heating with oil; it’s currently very affordable, and my supply temperature remains at 70 degrees Celsius (158 degrees Fahrenheit).

Buchsbaum schrieb:

With a 35-degree (Celsius) supply temperature, the heat loss in older buildings, and definitely not optimal insulation, it will get cold inside during winter.

If about 23°C (73°F) indoor temperature at -10°C (14°F) outside feels cold, then I agree with you. Those were our winter values over the last two years (the heat pump hasn’t been running longer than that). And yes, the insulation is not optimal – actually, there is basically none on the exterior walls. Also, it was not 35°C (95°F) supply temperature but 36°C (97°F).

Buchsbaum schrieb:

And please don’t take the last three winters as a benchmark. Oh right, it doesn’t really get that cold anymore because of climate change and all that.
You may believe that, but you don’t have to. Maybe that’s true in the Rhine Valley, the Palatinate, and other parts of Germany that don’t experience very cold weather.

In 1996, large parts of Saxony-Anhalt had continuous frost from mid-October to the end of March, sometimes down to minus 30°C (-22°F). The ground was frozen down to a depth of 1.50 meters (5 feet). There were hardly any days with temperatures above freezing.

1. Heating systems are designed according to standards (heat demand according to location) and based on extreme winters from about 30 years ago.
2. I don’t want to imagine what costs come out of the chimney at current oil prices. It doesn’t matter whether you have high electricity, gas, or oil consumption.
3. ANY heat pump can operate at -30°C (-22°F) as an extreme temperature, of course using an electric heating element for those times. Normally, operation down to around -20°C (-4°F) is possible without electric backup if sized correctly. My heat pump produces only 55°C (131°F) “cold” water at -20°C (-4°F) without a backup heater.

Buchsbaum schrieb:

I will continue heating with oil for now; it’s currently super cheap, and my supply temperature stays at 70°C (158°F).

Go ahead. However, my tenant is very happy with her heat pump and the heating bills. After removing the oil heating, costs dropped by about two-thirds — and I’m talking about oil prices between 40 and 60 euros. That was the leftover oil we burned before conversion.

Oh, and do you also believe that the currently modest CO2 tax will remain like that in the coming years? The industry expects a factor increase of 7 to 10 starting in 2026 compared to today. Then, even heating at -30°C (-22°F) will really hit the wallet.