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Heating and cooling: Warm air central heating - Options for a low carbon house

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    • CommentAuthorPaulEdwin
    • CommentTimeSep 1st 2023
     
    When we bought our house 12 years ago, I was impressed by the absence of radiators that were a feature of the 1980s warm air heating system. Rising energy costs were to reveal the disadvantages of the system: reliance on gas fuel and the inability to regulate different areas of the house (eg bedrooms). Johnson & Starley are UK manufacturers of warm-air heaters, and they claim that their products will be able to use a future gas mix including 20% hydrogen. This is not an efficient use of energy even if the national gas specification is changed in this way.

    I will leave the refurbishment of our heating system for a future owner but wanted to start a discussion to explore how this heating product is best dealt with for a low carbon house.

    There are three options that I have thought of, starting with the first – rip out the ducting and replace the gas heater with an air source heat pump and new radiators (and all the new pipework!). This would be a disruptive building operation, not only due to the routing of new pipework, but also the removal of the vertical ducting between ground and first floor would then require substantial redecoration.

    Could a heat exchanger replace the gas heater? This would need new heat extraction ducts installed in the loft and ground floor ceiling spaces. The permeability of the building envelope may have to be checked and remedied if leaking. There is also a question of whether the existing supply ducts are correctly sized.

    For my third option I wondered if an air-to-air heat pump could replace the gas heater and supply warm or cool air through existing ducts? This would avoid the need for an extraction system as above.

    I forgot to mention that we have 8kW capacity solar panels installed on a SSW facing roof.
    •  
      CommentAuthordjh
    • CommentTimeSep 1st 2023
     
    Hi Paul and welcome.

    My first house was new and had gas-fired warm air heating, which suited me fine at the time. The heater was in a column with the stairs wrapped around it. I don't know how the ducts ran to other rooms, presumably through the floors. If I was replacing it, I'd look to take out the heater and gubbins in the stairs column but probably leave the ducting in place and perhaps reuse it. I've no idea what has happened to that particular house in reality.

    The main design problem with warm air systems is that there is a limit to how hot you can heat air before you get a burning smell. This is generally accepted to be about 50°C. The passivhaus heating limit is set so that it can be achieved by heating air to this temperature and circulating volumes of air appropriate to a ventilation system. (I live in a PH heated in this way now)

    For any house that doesn't come up to the passivhaus standard, you're still limited to the maximum temperature so you have to increase the flow volume to be able to deliver enough heating power. Hence the much greater airflow rates used in warm air systems.

    Air-to-air indoor units are designed to produce warm air at lower temperatures, so use even larger flow rates proportionally. They are generally designed to be much lower power than a warm air unit though and are intended to heat a single room.

    I'm not sure what sort of heat exchanger you are thinking of?

    If I had the problem, I would first start by thinking how much insulation I could add to the house to reduce the heating power required. It might be that a suitably sized heat pump could use a fan coil unit to replace the gas warm air heater. But some people don't want the noise of a warm air system, so might prefer to install a wet system with radiators or underfloor heating. It depends how major a refit they were undertaking at the same time.
    • CommentAuthorMike1
    • CommentTimeSep 1st 2023 edited
     
    Posted By: PaulEdwinCould a heat exchanger replace the gas heater?

    Yes. I've actually been in contact with Johnson & Starley who tell me that they are developing new versions of their Aquair heat interface unit (http://johnsonandstarley.co.uk/aquair-heat-interface) with larger heat exchangers, suitable for interfacing with ASHPs. So if you can hold on until next year, there may be an easy solution using the existing ducts.

    Alternatively there are a few in the USA who have built their own heat exchange units, if you fancy some DIY. For example https://www.youtube.com/watch?v=zEax_mR1Ogo

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