On Dec 24, 4:28*pm, "
wrote:
I am considering installing two 12kw air source heat pumps to replace
my old oil fired boiler.

Is one for backup, or both simultaneously?

Both ASHP simultaneously...
- 48kW heat output @ CoP of 2.0
- 60kW heat output @ CoP of 2.5
- 72kW heat output @ CoP of 3.0

If you want 48-60kW to replicate a "massive kW combi"...
- Consider using a thermal store instead
- Then booster element if necessary for 45oC to 60oC

If you are using two ASHP serially to boost temps?
- Consider fitting underfloor heating
- Alternatively oversizing radiators to assist

I assume you do not have a pile of land you can stick GSHP pipes under
- there is no PP required for that as it comes under Permitted
Development whereas ASHP doesn't due to still undecided noise
standards at present.


The trianco activair units quote a start up current of 23amps
which then drops to 17.7amps for normal operation.

Invertor units probably, so gently on the startup - it may say it
needs Type-B circuit breaker rather than Type-C or even Type-D that
some HVAC systems require (which is fun to get EFLI low enough re
supply & final circuit, although RCDs are your friend here and 17th
likes them too).


Will a modern 100amp consumer unit be able to handle this?

Yes, indeed it is not an issue.
- If the units are invertor based their current draw is a) soft start
b) varies based on load. Invertor based units use DC compressor whose
speed varies rather than cycling on/off - better efficiency at a
higher price.
- Eventually CO2 air pumps will be cheaper which will boost your CoP
hugely even at very low temperatures (uses 2 stage DC compressor).

The problem might be your supply.
- If the units are invertor, the load is soft start & varies - so you
don't get "light dimming" which would be otherwise pretty severe.
- Your supply might be 60A by design, does not give you much headroom
re load.


How much current can you draw from a domestic supply cable? If one
consumer unit would be possibly overloaded could I simply have 2
100amp consumer units? One for the house and one for the air pumps.

They generally say 9kW electric boiler, or slightly more for a shower
(because a shower is not running continually). With appliances like a
cooker you have diversity, with a 9kW electric boiler there is no
diversity but the supply company assumes a certain diversity when
sizing supply cables, pole transformers and so on.

If the ASHP are invertor based their demand actually varies from
probably 4-12kW, rather than 12kW on/off.

So it depends on what your supply is.