How do you assess a product like Oxyvent?

Here's an interesting press release that just landed on my desk. It's for a new plumbing product called Oxyvent. It's a box that you add onto your radiator or underfloor heating system that helps it run better, and it promises huge savings in fuel burned.

According to the PR lady, Paul Worswick, director of Oxyvent, is "very keen that Oxyvent isn't seen as some magic box of tricks as the physics behind the product is straight forward when explained." There's lots of information about on the website and a YouTube video sequence showing Paul himself with a Pimlico Plumber who is busy installing Oxyvent in someone's house. There's an FAQ and there is a brief summary of some tests carried out by Dr Tony Robinson, a lecturer at Trinity College, Dublin (the kit hails from Ireland). And there are some ringing endorsements from satisfied customers.

But what there isn't is any simple to understand explanation of what makes Oxyvent so special. It seems to turbo charge the flow rates of water through the radiators, which enables them to run cooler, but to my mind that doesn't equate to making them more energy efficient. Some where in this system there must be a trade off - an extra pump or two, another heat exchanger, something unexplained.

I'm quite prepared to accept the claims at face value, but only if there is a coherent explanation of how it all works. As it stands, the publicity poses more questions than it answers. Installing one of these is going to set you back the best part of a grand: that's an awful lot of money for something being sold on trust.

So I looked up Tony Robinson, the Dublin academic who has been testing Oxyvent. He was easy to track down so I emailed him. He got back to me in a couple of hours and what he wrote was very illuminating.

The simple answer is that under the conditions that we tested in my lab the Oxyvent system did something useful; and there is no questioning this because our experiments were well thought out, they were accurate and we are experts in this field.

We observed two things: (i) when the radiators were balanced (to give approximately 11°C temperature drop at around 75°C boiler setting) the radiators we tested showed major non-uniform distribution in temperature, i.e. very large cold regions, due to the low water flow rates, and (ii) there were large fluctuations in the radiator temperatures and power outputs due to the boiler switching on and off which caused the inlet water temperature to cycle hot and cold.

He went on. For this scenario the Oxyvent system made a difference by smoothing out the fluctuations in the main inlet water temperature and thus the power output of the radiators. It basically added thermal inertia to the system so that the radiators did not react to the switching of the boiler. Now, the water flow rate can be increased i.e. unbalancing the radiators, without large excursions in the power output, so that for a given water set point temperature the radiator power output is nearly constant with time. The knock-on effect is that with the higher flow rate the temperature distribution of the radiator is much more uniform (we used thermal imaging to show this) so that, for a given inlet temperature, the unbalanced radiator would output more power since it would be, on average, hotter. Thus, the even temperature over the radiator provides more heat (for a given water temperature) and the Oxyvent tank ensuring that there are no severe cycling of this heat combine nicely, in the sense that the boiler water temperature can be reduced, which reduces fuel consumption, whilst still outputting adequate heat that is not pulsing over time.

One might ask then why use the Oxyvent tank for this; why not just reduce the water temperature (thus saving on fuel) and turn up the flow rate (thus improving the heat spreading on the radiators and thus the power output)? The answer is that for this case, the water inlet temperature is still cycling due to the on-off nature of the boiler so that the radiators may well reach the same peak power output but will also drop to a very low one, so that on average over time the power output is much lower than the case with the Oxyvent tank which provides a much more constant inlet water temperature to the radiators, even though the boiler is cycling.

Are you any the wiser? I'm not sure I am. It makes it look like it does something, but what exactly is still hard to tell. Other experts I know had reservations, but perhaps the best comment I got was from Michael Holmes of Homebuilding & Renovating magazine. He wrote I suspect that this product is a large heat exchanger/store, so the boiler flow and return go direct from this box, which acts as a thermal flywheel. Any benefit in terms of energy saving is likely to come from increasing the amount of time the boiler is in condensing mode, and by setting the boiler to a lower output temperature so it gradually heats up this thermal store. It is not clear whether it offers direct DHW too.

I suspect there are ways to achieve the same using controls. A boiler with a modulating burner that has a second low temp output for UFH etc. might achieve the same results without the expense. A thermal store cylinder can also work as a thermal flywheel, and provide DHW on demand.

So there you have it. A product that does something but we are not really sure what. It doesn't come with any 3rd party accreditation, like a BBA certificate, so we are left with lots of customer feedback and the observations of a Dublin academic. My hunch is that there are some installations where Oxyvent may make a huge difference, but others where it may do very little. And I realise that's not very helpful either.