Old CH system; Combi plus external heat exchanger; any good?

Discussion in 'Plumbers' Talk' started by MWi, May 31, 2021.

  1. MWi

    MWi New Member

    Our gas boiler needs replacing. We have an old (one pipe) CH system with a separate hot water tank and old radiators. I understand that a Combi boiler will cause leaks from the radiators. One CH engineer suggests adding an external heat exchanger to a (Baxi) Combi boiler to bring down the pressure in the water circulating to the radiators. Has anyone experience with such a system please?

    (FWIW: 5 bedroom house, one bathroom, gas cooker).
  2. Mike83

    Mike83 Screwfix Select

    A combi won’t necessarily cause water leaks.
    The pipework should take the extra pressure.
    But the system won’t work that well on a one pipe system.
  3. techie

    techie Active Member

    Re pipe it, you know it makes sense.
  4. The Happy Builder

    The Happy Builder Screwfix Select

    Do you know what type pipe is it, copper or steel?
  5. MGW

    MGW Screwfix Select

    The one pipe system would work great with fan assisted radiators, but not with TRV heads, what has changed is the modern boiler monitors the return temperature of the water and adjusts output biased on that. The fan assisted radiator does not restrict the water flow, it varies output by varying fan speed, all the water goes through the radiator there is no TRV or lock shield valve only stop cocks so radiator can be removed for maintenance.

    But with the normal radiator you have a by-pass pipe, and the idea was that the whole pipe work got hot, and the radiator took what was required, the return water was not much cooler to the supply, and with a modern boiler this would cause the boiler to modulate (turn down) so the water temperature would be reduced, so it would not work.

    Theory you could have a by-pass valve at each radiator on the by-pass pipe, but in practice this would cause a really high pressure system so would not work.

    I can't see how a heat exchanger would help? Often called a calorifier their job is to transfer heat between potable and non potable water, potable means drinking water, often used in boats so the engine cooling water can heat the domestic drinking water. A combined heat store and heat exchanger may work, these are used in the main to combine the output of multi heat sources, so solar panels hot coil at bottom, and then solid fuel, then gas etc. But it means the very cleaver system inside the gas boiler in real terms no longer works, it is only really used when the gas boiler is just a back-up.

    There is also the issue that the lock shield valves for a non pressurised system tend to leak with a pressurised one, so all would need replacing.

    So only real way is convert to two pipe, yes the Myson would work, but to be frank think they are great in the living room, but would not want fans in bedrooms.
  6. Jimbo

    Jimbo Screwfix Select

    I have a heat pump system which has a heat exchanger to provide hydraulic separation. There is a separate pump in it so I think it would do the job. However, the system water will need to be clean as the plate heat exchanger will block up just as easily as the main heat exchanger in the combi boiler.
    MWi likes this.
  7. Jimbo

    Jimbo Screwfix Select

    I meant to say, the primary side is 2 bar and the radiator side is 0.7 bar in my case.

    Bear in mind that there will be a temperature drop across the heat exchanger, typically about 5K. This means that bigger radiators, or less heat loss through better insulation, will be needed to have the boiler operate in condensing mode all else being equal.

    A weather compensator will help by lowering the flow temperature most of the time.

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