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Oxyvent Tank Preliminary Tests
Fluids and Heat Transfer Laboratory,
Trinity College Dublin

Objective
The primary objective of the current study is to provide experimental measurements that
confirm the claims that the a hot water heating system with an Oxyvent tank system installed will provide as good if not better space/comfort heating than conventional wet radiator systems. A conventional radiator system operates with a pump that circulates water to several radiators linked in a row with the radiator inlets attached to a common feed pipe and the radiator exits plumbed to a common return line (see Figure 1). With the boiler thermostat adjusted to a temperature which is nominally between 72 ºC to 85ºC, the system is ‘balanced’ by adjusting a valve located at the radiator exit to a nearly closed position, which virtually stagnates the flow of water within the radiator, until the temperature drop across the radiator is nominally 11ºC. The claim here is that the Oxyvent tank can provide like space/comfort heating with a lower boiler set point (~63 ºC) (without having to balance the system i.e. with the outlet valves on each radiator all fully open). For the former, the obvious benefit is less fuel usage. For the later point, the benefits are envisaged to be (i) improved long term reliability and maintenance of the system since anything that compromises the balancing of the system will negatively influence the heating characteristics and require re-balancing, (ii) since the volumetric flow of water through each radiator is larger one would expect better heat spreading, (iii) since the water is no longer nearly stagnant within the radiator it is less likely for air to become trapped requiring bleeding and (iv) each radiator along the line will have an inlet temperature that is nearly identical so that the heat throughput is ‘selfbalancing’.

Figure 1: Conventional hydronic space heating system.

Experimental Facility
An experimental facility was commissioned in order to perform baseline tests in a typical
balanced system as well as a system operating with the Oxyvent tank in place. The facility, shown in Figures 2 and 3 consists of a conventional boiler with diesel as the combustion fuel, an accumulator tank and 6 Quinn wet radiators. The inlet and outlet of each radiator was fitted with thermocouples to monitor the temperature. A turbine flow meter was situated at the exit of the last radiator so that the net power dissipater by radiator #6 (Rad #6) could be monitored. All of the measurements were acquired by a computer fitted with a data acquisition system. Measurements were taken every second for several hours. Further to this, a thermal imaging camera was utilized to provide thermal maps of the radiators to gauge the heat spreading.

Figure 2: Experimental facility

Figure 3: Oxyvent tank

Results and Discussion
Figure 4 shows the thermal images of radiator #4 and radiator #6 for a balanced system (no
Oxyvent tank: top images) and ‘self-balancing’ system (with Oxyvent tank: bottom images).
From the images it is immediately apparent that the temperature distribution of Rad #4 is
much more uniform for the case with the Oxyvent tank in place. This is clear when one
considers that the difference between the Tmax and Tmin is 16.6ºC for the balanced system with an inlet to outlet temperature difference of 11.7 ºC and an average temperature of 63.6ºC. For the scenario with the Oxyvent tank installed and the outlet valve of the radiator fully open the difference between the Tmax and Tmin for Rad #4 is only 8.8ºC with an inlet to outlet temperature difference of only 4 ºC and an average temperature of 60.1ºC. Since the average temperatures are very close and buoyant natural convection is the primary restriction to heat flow, it is likely that total power dissipated by Rad #4 will be comparable for both situations. The same general trend is observed for Rad #6, the last radiator in the system. However, one notices that compared with the balanced system in which the water inlet temperature (Tmax) has dropped by 3 ºC from Rad #4 to Rad #6, the inlet temperature of Rad #6 is only 0.8 ºC less than Rad #4 which illustrates the self balancing nature of the system with the Oxyvent tank installed.

Figure 4: Thermal images of radiator #4 and radiator #6 for a balanced system (no Oxyvent tank) and ‘self-balancing’ system (with Oxyvent tank)

This self-balancing is illustrated more clearly in Figure 5 where the inlet temperature to the
very first radiator in the system (Rad #1) and the inlet to the very last radiator of the system
(Rad #6) are plotted with time for an interval of approximately 16 minutes. What is
immediately apparent is that for the balanced system, without the Oxyvent tank, there is a
substantial difference in TR1 and TR6, in particular during the heating phase (following the
boiler on/off due to the thermostat). However, the Oxyvent inlet temperatures for the
Oxyvent system are nearly identical and within the experimental uncertainty of the
thermocouples. Secondly, it is clearly evident that there are massive temperature swings for the balanced system with the temperatures within the system following the ramp-up and falloff caused by the boiler turning on and off. However, the Oxyvent tank appears to
significantly dampen-out these large fluctuations, presumably by increasing the thermal
inertia of the system prior to Rad #1. These fluctuations are dampened even though the
frequency of switching of the boiler is not significantly different between the two cases as
indicated by the exhaust gas temperature readings illustrated in Figure 6.

Figure 5: Inlet temperatures to Rad #1 and Rad #6

Figure 6: Exhaust gas temperature in the flue

The total area traced under the respective curves in Figure 6 is qualitatively indicative of the amount of energy being consumed in the boiler by burning of the fuel, which is less for the Oxyvent system.

Figure 7 shows the power output of Rad #6 with time. Once again, the balanced system is
very responsive with very large amplitude fluctuations in the power output due to the rapidly
changing temperatures since the volumetric flow rate remains constant (~1.4 LPM). With the Oxyvent tank in place the amplitude of the fluctuations are reduced considerable due to the increased thermal inertia induced by the system. What is most important here is that, over time, the average power output, that being the thermal energy dissipated by Rad #6 into the room, is about the same with a small 8.8 % measured drop. However, this 8.8% is within the experimental uncertainty of the measurement system and is likely not significant. In reality, the tighter power control of the Oxyvent system would provide exceptionally improved comfort since it would maintain the temperature of a room at a much more steady temperature compared with the balanced system.

Figure 7: Net power output of Rad #6

Conclusions
The preliminary tests which have been carried out tend to indicate that the claims of the
Oxyvent system are apparently true with a lower operating temperature required, less fuel
consumed, improved temperature uniformity of the radiators, a system that is ‘self-balancing’ and improved comfort heating.

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