UFH controls - do I need an interlock?

28 Oct 2008
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United Kingdom
I am planning a new house (for me) and I am trying to work out how my heating is going to work.

I intend to have wet underfloor heating (UFH) powered by an air source heat pump (ASHP). The ASHP will be connected to a buffer tank for the UFH to stop the heat pump cycling and will also be connected to the domestic hot water (DHW) tank.

My understanding is that the flow for the UFH will come from the buffer tank. Each manifold (there will be one on each floor) will have its own pump and control box. The room thermostats will be connected to the relevant UFH control box and there will be one overall time clock that will be connected to the UFH master control box.

The ASHP will have its own control box that will be connected to the buffer tank and the DHW tank. If the temperatures in either the buffer tank or the DHW tank drop below the set level then the ASHP control box will start the ASHP.

As far as I can see, the control units for the UFH zones don't need to have any connection to the ASHP control unit as they have completely independent functions. Am I right in thinking that Part L of the building regs says that they must be connected? I thought that this was supposed to stop boilers firing up when there is no demand from the heating system but I can't work out how this applies to a heat pump / buffer tank set-up.

Anyone got any ideas?

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The stats on the ufh and buffer vessel are the interlocks
Various Skools of Thought regarding heat pumps & buffer tanks. Many, myself included, would advacate not using a buffer tank, but just pump all the heat into the floor & use a compinsation system that monitors external temperatures & UFH return temperature. As a HP will cycle regardless of having a buffer tank or not!!
There is certainly lots of confusing advice around about buffer tanks. Some sites say don't do it because it reduces the overall efficiency (just make sure that there are enough open circuits) whereas some of the manufacturers say that you absolutely must have a buffer tank. The ability to have some rooms off and some on, regardless of the outside weather, will be critical. The house will be on a south facing hillside in Shetland. When the sun is out, the south facing rooms will heat up quickly (even on very cold days) whereas the rooms on the north will probably still need heating late into the spring.
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With a glazed south facing elevation, you'll need some floor sensors.
If it was me, I'd be building some tomb walls & covering the roof in solar panels. I'd also consider a heat recovery heat pump, the Nibe one is very good.

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