Pumped air heat recovery systems for stoves

1 Improving the efficiency of stoves
2 Pumping air and recovering heat
3 Experiences from existing installations
4 Heated air volume and distribution
5 Distribution and condensation
6 Computational Fluid Dynamics
7 The practicality of traditional heating
8 Summary
9 Related articles on Designing Buildings
10 External Links

[edit] Improving the efficiency of stoves

Many older properties rely on a wood burner for their primary heat source, because the building materials and lack of insulation mean older homes are expensive to heat with oil or electricity. Solid fuel stoves are a great way to heat a room, and offer efficiency and comfort levels far beyond those offered by open fires.

For all their advantages though, stoves do still share some of the drawbacks of fires in that the heat they produce depends on air flows to move around a home. A fire induces air to flow from the cold air source offered by doors, windows and vents, into the heat source and then up the chimney. The result is the ‘hot face but cold back’ effect so familiar to people standing in front of their fire or stove.

Back boilers can mitigate this effect by circulating heat via radiators, and thus significantly increase the output and efficiency of the burning, but they are expensive and unsuitable for many homes simply because of all the plumbing required.

[edit] Pumping air and recovering heat

A novel solution to improve the efficiency of the common place stove is to instal a pumped-air heat recovery system. The system couples an air pump to a heat-recovery element set in the bottom of a stove flue, forcing a jet of heated air from the front of the flue just above the stove.

This then entrains and circulates the heat rising from the stove to enormously increase the volume and distribution of warmth around the room. The positive pressure this creates in the room can then be distributed out into the rest of the house to create convection flows of warm air that will circulate to a far greater extent than can ever be produced by the stove on its own.

[edit] Experiences from existing installations

Recoheat is an existing pumped-air heat recovery system produced in the UK by a company of the same name. Trust Pilot reviews give details of customers reporting that their existing stoves are now their sole form of heating, with some giving detailed data on house configurations and temperatures.

The product was launched commercially in November 2020, and having sold more than 400 units has built up a sufficient customer base to form a robust picture of performance in various contexts, from bungalows to barn conversions, yurts and mountain cabins. Sales to the US and Canada have also added temperature extremes to the case studies, while more temperate use in Ireland in the summer has brought its own challenges.

Anecdotally, the units are very effective and have particular characteristics that distinguish them from other forms of heating. Since 2022, the company has employed engineering consultants Cambridge Engineering Analysis & Design in order to understand the particular effects that have been reported.

[edit] Heated air volume and distribution

The first thing of the note is the volume of air the units heat, and how far it spreads. Many customers report heating their whole homes from a single stove with a Recoheat fitted, and while the initial year of sales saw customers with smaller two-bedroom cottages reporting this, the largest homes now reporting this are 4 bedroom properties with three storeys.

People in homes with very cold walls, including old stone houses, a couple of yurts, timber cabins and boats, report that the warm air flowing from the units form a pocket of air against the walls, effectively insulating them and allowing the rooms to be heated to the edges. Customers have reported changing the furnishings in their rooms because they are comfortable moving seating areas away from the stove.

[edit] Distribution and condensation

A secondary effect of more even heat distribution has been that condensation is reduced. This has been observed with an installation on a boat where previously condensation was a constant in cold weather, indication was that the Recoheat unit removed this with the evenly distributed flow of warm air it created within the internal space. The first customer to report this did so from their 70 foot by 14 foot wide beam canal boat, so the area in question was large for a canal boat.

A particular and connected effect that is noticeable but hard to explain, is heat distribution to the floor. As heat rises, and cold air flows along the floor in order to feed the fire in the stove, this seems peculiar, but is attested by many customers in the UK and overseas. A customer in Nova Scotia even reported it from the uninsulated steel container he uses for the office of his timber business during a winter where the external temperature dropped to below -30C.

[edit] Computational Fluid Dynamics

To understand how the device works, CEAD built a Computational Fluid Dynamics (CFD) thermal model of the system within an enclosed room of 4m x 3m x 2.5m. The environment had 1.6 million temperature monitoring points in order to build a really detailed picture of the thermal changes.

The animated simulations produced by the model are fascinating, and the contrast in behaviours with the stove heating without and with the Recoheat is highly impressive. A video with two simulations running alongside each other can be seen at https://www.youtube.com/shorts/I6TFB8cac04

The simulations confirmed that a Recoheat brings a room up to a temperature in 10 minutes that a stove alone produces in an hour. The average temperature in the model room after one hour is also 4 degrees warmer, but that figure doesn’t describe the change in comfort levels provided because of the way that average is achieved: the stove alone produces a very hot area around the stove and flue, with that heat circulating along the ceiling as time passes, but the majority of the room being significantly cooler. In contrast, the Recoheat creates a temperature that is even throughout the volume of the room, including floor to ceiling.

[edit] The practicality of traditional heating

Crucially, the model demonstrates how that’s achieved. It shows the element heating, expanding and accelerating the air pumped through the heat exchange so that a jet of hot air is propelled from the outlet. This air entrains the hot air rising from the stove and directs it into the room. It also entrains the cold air in front of the stove, and as this mixes with the jet, it destabilises it – so instead of rushing out in a steady flow, the jet starts to oscillate until it’s sweeping around in a rough circular motion in front of the stove. That motion stirs and mixes the air in the room floor to ceiling as observed like a giant fan blade composed of warm air.

Defining how well the device will succeed in circulating the heat around a property is much more difficult because of the number of variables at play. Essentially, the heated air in the primary room appears to work very well in warming and augmenting existing convection flows, but will not reverse an existing flow.

This means that a room with the stove that gives onto a hallway with staircase will succeed in heating right up the house. Bungalows, workshops and open-plan houses will also typically allow heat to be spread throughout. In the small number of reported cases (fewer than 1% so far, or 3 cases out of more than 400) where the units have failed to move the heat beyond the confines of the room, the layout of the house has usually (2 out of 3) clearly created a negative convection flow of cold air towards the heated room in sufficient volume that the additional heat could not overcome it. Only in one case have the homeowner and the company been unable to explain a failure without specialist engineering consultancy.

A key rider in this is that the stove itself needs to produce sufficient heat, and while 5kw has been sufficient for some homes, an 8kw or larger has been used to achieve total home heating in larger, colder houses. One of the homes that failed to heat was in fact working until the owners reduced the stove size from an inefficient 8kw (that not only produces more heat, but releases a higher proportion up the flue where it can be captured and circulated) to a new 5kw. Since the house was a Grade 2 listed cottage with single glazing and little insulation, this didn’t produce enough heat for the volume and heat egress.

Something else to note, though, is how efficiently the Recoheat device exploits the heat produced by the fuel combustion, so how much less fuel is required to produce how much more heat. Part of this is a result of the heat capture during the burn, but as much is because of the time period that the heat from combustion is circulated for.

[edit] Summary

Generally heat from combustion is only circulated once the stove body has itself been heated. Once the stove is hot, the radiant heat creates a convection flow that circulates the heat into the room. Whilst the stove is hot, this flow is maintained, but as soon as the surface temperature drops, the convection slows then stops fairly rapidly. At the same time though, there is still a great deal of heat in the stove that then passes up through the flue for many hours as the stove cools.

The Recoheat captures and circulates the combustion heat as soon as it starts to pass up the flue, so the heat is being exploited immediately. It then continues to circulate heat through the entire heating and cooling cycle, which is likely to be four to eight hours longer than the stove is being fuelled for: stove users will know that the embers in their burner will still be hot the morning after a fire. Even if this only succeeds in heating the air from the Recoheat to 30 degrees, that air is still circulating and can make an enormous difference to the temperature of the property. The reason for this is that heating a house back up from an unheated temperature requires far greater calorific investment the lower it’s required from. Nor is it a steady curve because brick has no insulating qualities once its temperature drops below 15 degrees, so if it can be maintained above that temperature, far more of the heat in the home can be retained.

As an inexpensive device that can be added to the majority of stove installations, the Recoheat therefore offers a scale of heat improvement that is unmatched by other far more complex technologies. It’s definitely something to consider if you’re looking to achieve a warmer home.