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Underfloor Heating Efficiency and Wire to Water Pump Efficiency

Wire to Water Efficiency Diagram

Underfloor Heating Efficiency and Wire to Water Pump Efficiency

When trying to calculate underfloor heating system efficiency, people usually look at just heating outputs and U-values of building materials. What about the wire to water pump efficiency? It’s important if your underfloor heating system requires installation of additional pumps or especially if we’re talking about an integrated system with a heat pump. The latter is gaining popularity in the recent years due to its environmental credentials.

In this article we’re only looking at wet underfloor heating systems (powered by heated water rather than electrical heating elements). To make it easier for consumers, most sales literature quote heating output in Watts per square metre.

Maximum Heating Output of Underfloor Heating

The maximum heating output you can get from a concreted underfloor system is 100W/sqm while wooden flooring can return 70W/sqm maximum. This is due to the U-value of building materials. The lower this value, the more power is converted to efficient heat.

This table shows U-values of the most popular building materials. For a full list of U-values, go here. From the scientific perspective, U-value measures the amount of energy lost through a square metre surface of that material for each Kalvin degree difference between the inside and outside. Lower U-values indicate lower heat conductivity.

Material U-value
Steel Sheet 1.20
8” Brick 0.41
16” Brick 0.25
2” Concrete 0.99
8” Concrete 0.67
1” Thick Wood 0.64

There are lots of different factors to consider when planning underfloor heating – square footage of the area, U-value of materials, length of piping, requirements for additional pumping and of course, what is going to power the system. As it’s impossible to “tweak” an underfloor heating system, customers need to think at least 5 years ahead. If they foresee installing a ground source heat pump in the future, the installer needs to know about it now.

When installing coils, the standard spacing between each coil is 200mm as it provides the optimum heating efficiency. However, if a heat pump is involved, the spacing will have to be reduced to 100mm otherwise it will negatively affect the whole system’s efficiency.

Wire to Water Efficiency of Underfloor Heating Pumps

But now let’s go back to the main topic of our article – wire to water efficiency of pumping systems for underfloor heating.

The method used for determining wire to water efficiency takes into consideration the efficiency of the pump and losses in the pipe fittings that connect the pump. The method is important as it allows the underfloor heating engineer to evaluate the efficiency of the whole system.

In scientific terms, wire to water efficiency is the energy that is passed on to the water used to heat the coils divided by the energy that is fed into the system.

For a single pump with constant speed, the formula to find wire-to-water efficiency would be:

WWE = Em x Ep x 100 (%)

where Em is motor efficiency and Ep is pump efficiency.

This WWE usually fluctuates between 30 and 90% depending on the size and technical abilities of the pump. Smaller pumps will usually have a lower WWE.

Now, to make things more complicated, we’re going to look at how water friction affects the system. That’s why major underfloor heating manufacturers spend so much time and effort perfecting their coils. Less fraction means better efficiency.

WWE = ((Hs – Hpf)Em x Ep)/Hs

where Hs I total friction loss of the water system, Hpf is friction loss of the pump and its fittings and valves.

Of course, the efficiency of a motor largely depends upon the load on a motor, which is calculated by the system head area.

To calculate system head, we use this formula:

Sh (ft) = Ch + Hf x (Q/Qd)n

Where Ch is constant water pressure at the end of an underfloor heating loop

Hf – variable friction of head for the system at its nominal load

Q – Flow in the water system, gpm

Qd – Nominal design flow in the water system, gpm

n – Friction coefficient, normally 2

Wire to Water Efficiency Diagram

The diagram shows a uniform multi-storey building to model a system head area calculation and a uniform head curve for the same building. For trial purposes we’re looking at a 10-storey building where each storey houses an underfloor heating unit and 160 gpm. Taking the unit, the valve and piping into consideration, each storey requires 25 of head that equals 105 Kpa. System head coefficient is 1.85.

If we switch of the top 5 floors and let the building’s underfloor heating system run at 50% load, the friction loss will be less than for the fully loaded system.

If, however, we switch off the bottom 5 floors and let the top 5 floors run as normal, the friction loss will be considerably greater than when the bottom floors were operational and top floors switched off.

Let’s take an example building that has three water pumps working at a 50% capacity each of 800 gpm (50 L/s). The pump head equals the building head of 60 feet (179 kPa) with an addition of the pumping system friction loss, Hpf, of 8 feet (24 kPa) for a total head of 68 feet (203 kPa). Wire-to-water efficiency calculations are done at system head curve coefficients of 0.37, 0.74, 1.11, 1.48, 1.85, and 2.59, and 3.70 using this formula:

WWE = ((Hs – Hpf)Em x Ep)/Hs

That gives quite a good coverage around the entire system head.

Why is this important? Well, for big buildings operating a complicated underfloor heating system incorporating multiple pumps, you may need to be able to develop a system that switches pumps on and off as and when required. This approach would lead to significant savings in operation cost, which is, in most cases, the main reason for installing underfloor heating in the first place.

This short calculation demonstrates that wire-to-water efficiency technology is useful through digital computer programming for small and medium pump systems. As more installers become proficient in computer software, it will become standard for any underfloor heating system with multiple loads and total pump horsepower in excess 50HP.

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