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above best practice radiant heating system

wastl | Posted in Energy Efficiency and Durability on

Hello there!
We have here in Germany rather high energy prices and the standard heating system is starting to become a heatpump “x to water” driven radiant heating.
Now, builder are not always on track and “meant well” does not always mean “done well”..

So to give an owner some easy to check design rules we have a set of rules called “flow30” to know that the system is efficient and prolongs the life of the heatpump. These rules are “above best practise” in some details to make understanding the design simple again with a owner/customer in mind.
Yes, efficient designs do not need these design rules, but IF you meet these here then you know the system will be “ok” and as future-proof as can be – and – they are rather easy to verify.
It helps that our climate is more like CZ4-5-ish.

Here it goes…:

(disclaimer: no toasty feet and you need near-PGH – levels of insulation, also modulating heat pumps preferred…)

– 30C (86°F) water temperature at design load
– 30 mbar (0.45 psi) pressure loss in the worst loop
– 30% max length difference from shortest to longest loop
– max. 30 % of all loops have a dedicated temperature controller. (More below)
– 30 mm (1 1/4″) min. diameter of the main supply line to the loop manifold

Room temperature target is 68°F with 75°F in the bathrooms.
Outdoor reset is used to control the internal temperature in the “normal” rooms and only special rooms receive a dedicated controller. Due to the low design temperature extra energy (sun through the window) cannot let the radiant floor do the overheating anymore – if the room temperature climbs by 3-5°F then it will have radiant floor temperature and that floor will not supply heat anymore (At least in 95% of the year – maybe not at design load.)
Bathroom temperature (75°F) normally can only be achieved if not only a floor but at least a wall or the ceiling are also radiant – depending on layout, insulation etc.. If someone does not agree to that extra, an electric heater (radiant panel) to boost the heating system for a few minutes would also work (dam the COP..)

– Best COP by lowest possible system temperatures
– simplify the control system by using outdoor reset and a proper fixed regulation of the flow valves of the loops to maintain room to room temperatures. Having all loop of a similar length helps in this regard because hydraulically the loops are already similar. By having only some loops with controlled valves, the WP can continue supply water as per outdoor reset and is not starved/shut off by closed valves –> that helps to prevent short cycling.
– reduce circulator consumption by having low pressure losses.

For the US and the energy prices involved that is today maybe over the top, but with 33 ct/kwh here its not so crazy.. Also, radiant floor as a standard for the trades makes it more affordable than in the US.


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