ASHP: monobloc, split or combi?

[AndySat]

On 25/01/2025 at 07:09, Beelbeebub said:

I think the benefit is from the manufacturer pov. The outdoor unit is a standard aircon unit

I agree. Having a standard unit is probably a good thing in terms of price and reliability, but my current thinking is to go with a monobloc - simpler and uses less space in the plant room. Plus it puts any potentially noisy bits outside.

The UFH will be a single zone and based around WC instead of thermostats. But I may hedge my bets and at least run the wiring for a thermostat in the bedrooms...

 

[JohnMo]

1 hour ago, AndySat said:

But I may hedge my bets and at least run the wiring for a thermostat in the bedrooms...

Just do wireless thermostat, easy, battery lasts a couple of years. You can also move it about.

 

We are running WC but use a thermostat to trigger a second operating point, basically WC plus 2 degrees. Use the thermostat to force the heat pump to run hotter during cheap rates period and also shelly relay across the terminals of the receiver to drive the heat pump hotter during excess solar production periods. Converted 3kWh of excess solar in to 11.5kWh of heat today for free.

[AartWessels]

On 24/01/2025 at 15:44, PhilT said:

Surely it's similar to the English scheme whereby you can apply for the 7500 grant for a self build if it meets certain criteria

No, I don't think so. As far as I am aware there are no grants available here for self build. 

[JohnMo]

28 minutes ago, AartWessels said:

No, I don't think so. As far as I am aware there are no grants available here for self build. 

I looked also, but from I could see, the £7500 is England and Wales. Grants for heat pumps is for people on benefits in Scotland. Nothing for new builds anyway.

 

I got my ASHP from eBay, there are plenty of bargains coming up. There were a couple of 6kW for £600, New unused.

[AartWessels]

1 hour ago, JohnMo said:

I looked also, but from I could see, the £7500 is England and Wales. Grants for heat pumps is for people on benefits in Scotland. Nothing for new builds anyway.

 

I got my ASHP from eBay, there are plenty of bargains coming up. There were a couple of 6kW for £600, New unused.

That's what I've been looking for as well. Not been lucky yet, but I guess I will be at some point. 

[AndySat]

Well, almost a year later and a lot has happened...

 

Build has started and the groundworks are more or less done. Timber frame kit is due to arrive in March. So I'm looking at the ASHP & UFH design again.

I'm a bit reluctant to delegate the design work to a specialist ASHP contractor as the two I've spoken to so far have suggested standard designs that either have multiple zones (one per room, complete with separate thermostats) or are massively over-specified for the house. Its only 140 m2, single level and very well insulated with MVHR, but one of the designs has a 8.5kW ASHP! Presumably these contractors would make a more accurate assessment once some money changes hands, but they don't fill me with confidence!

 

My own room-by-room calculations and Jeremy's spreadsheet suggest a total heat requirement of about 2.8kW worst case, dropping to about 1kW best case. A 4kW ASHP seems about right, with some margin at the top end (and for DHW), but how many will modulate down to 1kW? I've read every datasheet I can find, but very few manufacturers specify their minimum outputs. The 4kW Haier R290 that @JohnMo has just installed does indeed give this info: it will modulate down to 996W at +10C OAT and 35C flow, so this looks like a good fit for my case - it should be able to run without cycling for most of the time.

 

I'm interested to know how an ASHP actually modulates its output. Does it reduce the flow temp, while keeping the flow unchanged, or does it reduce the flow and keep the temp unchanged, or both?

 

[SimonD]

2 hours ago, AndySat said:

I'm interested to know how an ASHP actually modulates its output. Does it reduce the flow temp, while keeping the flow unchanged, or does it reduce the flow and keep the temp unchanged, or both?

 

Both. Most of the new ones have pwm pumps. It's also useful to confirm minimum flow temperatures as some are 25C some are 20C, but in some cases this really doesn't matter.

[AndySat]

13 hours ago, SimonD said:

Both. Most of the new ones have pwm pumps. It's also useful to confirm minimum flow temperatures as some are 25C some are 20C, but in some cases this really doesn't matter.

Thanks. That's what I was hoping (although it makes the UFH calculations more complex). The Haier ASHP will flow down to 20C, but this is below my 22C target temp of the bathrooms.

 

I think I've got enough data now to choose a suitable ASHP. Next step is the UFH design, but I'll take that into one of the existing threads to keep things simpler.

[JohnMo]

5 hours ago, AndySat said:

ASHP will flow down to 20C

Not sure how useful that is in the real world. I currently have my compressor restart hysterisis set at 4 degs, any lower than that you don't get a successful restart because floor slab has then equalised in temperature and the heat pump has no where to dump the heat. So at a flow temp of 20, in real world you need what ever you are heating to to drop temperature down to 16 - that just will not happen.

 

My WC curve calculation says I need to flow 22 degs at 10 OAT, but I think realistically I may need to set a higher temperature. It's programmed like that for now, if we get a mild day any time soon I can test it.

 

5 hours ago, AndySat said:

but this is below my 22C target temp of the bathrooms.

I always find ensuites difficult to heat. We have the bedrooms cool, so heat from the ensuite just bleeds onto the bedroom. But even at around 20 they are comfortable as floor is radiating heat at you anyway 

[saveasteading]

9 minutes ago, JohnMo said:

heat from the ensuite just bleeds onto the bedroom

Don't you just need to  close the door?

[JohnMo]

47 minutes ago, saveasteading said:

Don't you just need to  close the door?

That's easy - but when there are more than two in the house, almost never happens.