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We're looking into replacing our boiler. The current one is tankless and only provides hot water for heating on a closed loop. There is an entirely separate electric HW tank & plumbing for 'domestic' hot water (showers etc.)

We don't necessarily want to alter the DHW system at the moment, it works fine, and leaving it alone would be cheaper / simpler. But it might be nice to have the option of switching it to the boiler in the future.

Would it be possible to install a tankless combi boiler but leave the DHW side of it disconnected (capped off) and use it just for heating duty? I don't know if having "dry pipes" on that side might cause some problem? Or some other concern.

If there are specific types of boilers that would / would not allow this, that would be very useful information as well.

I can see that often a boiler will have two variants of the same model - one with DHW and one without. That makes me think this is likely to be possible. But I have looked in a few manuals of boilers but have not seen this addressed. Web searches haven't turned up anything either. Of course I may simply not have seen this information, but thought it was worth asking for advice here.

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  • Think DHW water is kept separate from boiler water. DHW is the same source as drinking water and some boilers use chemicals to prevent scaling and/or rusting.
    – crip659
    Commented Mar 12, 2022 at 15:20
  • I know this is old, but frankly, this sounds like something that should be addressed to the manufacturers. They're the ones who have the final say on whether your use would be covered by a warranty if there were a failure, so they're the ones who know if this is likely to cause a premature failure.
    – FreeMan
    Commented Sep 12, 2022 at 14:58

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Would it be possible to install a tankless combi boiler but leave the DHW side of it disconnected (capped off) and use it just for heating duty?

Yes, that is possible. The typical way that a combi boiler works is that it detects a flow on the DHW pipe and uses that as a trigger to start heating the DHW. If there is never a flow to detect, DHW mode will never be activated.

I don't know if having "dry pipes" on that side might cause some problem? Or some other concern.

I am not sure what the pressure difference on the plate heat exchanger will do with regards to lifetime. To overcome that you could (ask your installer to) create a small loop between the hot and cold water sides (with fill and drain valves, so that you can completely fill it with water).

If there are specific types of boilers that would / would not allow this, that would be very useful information as well.

To be able to keep the DHW disconnected, you would need to have a model that only heats the DHW on the basis of flow-through. Your boiler must not have any kind of hot water storage/buffer, not even a single liter.

As soon as there is any kind of buffer, the boiler will have logic to keep that heated, even if you don't actually use it. Sometimes such a buffer is advertised as providing hot water faster.

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Potterton Gold Combi boiler only has 2 modes: hot water only, or CH+hot water. There's no CH only mode, but I don't think that water will flow through to heat the plate heat exchanger if the hot water is not running, so it should be alright – no efficiency lost.

In hot water only mode, the burner only turns on when the hot tap is turned on, and the diverter valve is likely always in the hot water position so 100% of the water flows from the primary+secondary exchanger loop into the plate heat exchanger. In CH+hot water mode, the diverter valve lets 100% of the water from the loop into the plate heat exchanger when the hot tap is turned on (with 0% on CH; it's a diverter valve), and 0% when it is off (with 100% on CH; 100% of the flow is in the primary+secondary loop which now includes the entire CH system because the diverter valve is diverting to CH flow instead of the plate heat exchanger). The heat of the burner can be changed to change the temperature.

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