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ASHP - a few (probably dumb) questions about the lived experience

LickOfPaint · 2024-09-18T20:25:31Z

First time poster here looking for advice from people who have a heat pump as we are a trying to make the decision to take the plunge having got a quote from Octopus. I know heat pumps are most efficient when working at a low level to keep a house at a constant temp rather than the peaks and dips from a traditional boiler. But in reality we don't want our house heated at 21 degrees overnight or in the day when no one is home. Does anyone actually deviate from the constant temp for 24 hours and if so how does that work for them and the efficiency of their heat pump? If we wanted a cooler house overnight would this mess up the efficiency and cost us money? Also does anyone use a heat pump whilst ventilating their property - we crack windows open to avoid condensation and I worry this will mess up all the figures from the heat survey?

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Air Source Heat Pumps (ASHPs)

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(@transparent)

Illustrious Member Moderator

Joined: 5 years ago

Posts: 3463

21/09/2024 11:41 am

Posted by: @lickofpaint

↑

I just wanted to say a quick thank you for all the answers and discussion.

And thank-you for having the courage to post in the first place.

I hope you will do so again as you further consider a heat-pump installation.

I don't yet have a heat-pump...
... but would almost certainly self-install rather than use an MCS-accredited company.
That would mean foregoing the £7500 government grant, but I'm now confident that others here on the Forum could provide the knowledge required.

(I can buy an ASHP suitable for my 300sq.m house direct from a Chinese manufacturer for less than £3000 incl shipping.)

I already have a self-installed whole-house ventilation system with a heat exchanger in the loft (MVHR).

And I have underfloor heating... again DIY installed a room at a time.

Feel free to pick up whatever information you require here, no matter how 'dumb' you may think the question to be.
I can assure you we will provide a sensible and polite response!

Save energy... recycle electrons!

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Anonymous 5011

(@Anonymous 5011)

Joined: 4 years ago

Posts: 562

22/09/2024 9:11 am

Heat pump sizing does matter, but not sure it matters as much as made out, with modulating heat pumps. An example above where a 12kW unit turns down 3.5kW is a prime example, that's the same figure as my 6kW turns down to.

One issue with oversize is the flow rate still needs to be accommodated, a 12kW heat flows at twice the rate of a 6kW heat pump. So bigger pump, bigger pipes etc are needed with big over sizing.

But taking consideration of setting and operating the unit they can be run cost effectively and in some ways add flexibility. We have a house heat demand at -9 of just over 3kW. We have a 6kW heat pump. Last year we batch heated the floor mostly in 7 hrs of E7. The colder days this extended well in to the day to get the required kWh's into the floor. A bigger heat pump would have reduced the time spent in the expensive part of the day.

This year running pure WC, even on very low demand the 6kW works just fine, it runs for 20 to 40 mins and is off for an hour or many more. Just adds heat in as required. Coping really well even adding heat at an average of 0.1 to 0.4kWh over the past few weeks. When running we seem to get a CoP of between 5 and 6+. Has run down to 2.5 degs OAT so far.

Important bit is understanding exactly what short cycling is and when it's occurring and mitigation, this will vary from one house and heating system to the other. Plus most manufacturers recommend that a heat pump should not run routinely less than 10 mins as this results in lubrication issues.

👍

👌

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