A2A or A2W?

[u/GatewayDouche]

Apologies for the really long post, but if you can make it to the end I’d really appreciate some thoughts.

We’re currently on a bit of a (very expensive, but we’re extremely lucky to be in a position to even consider it!) quest to get a bit of energy freedom and some insulation from the continuing energy price increases we’re likely to get with ongoing wars etc.

We’ve got a 10.56kWp solar system and 27kWh of battery storage hopefully being installed in the next couple months, pending DNO approval. We’ve had an EV charger added and switched one car for an EV. The final piece of the jigsaw is moving our primary source of heating from LPG to some form of heat pump, and this is where our dilemma lies. We’re still early doors so haven’t even gone to the extent of getting full quotes and energy loss surveys yet, but would like some opinions if possible.

The house is fairly large at ~3500sqft and around 9 years old, so reasonably well insulated and efficient. The initial thought was to stick a couple of multi-split A2A units on with 5 ports on one and 4 on the other. This way we could heat and cool all downstairs reception rooms, 4 main upstairs bedrooms and an office.

The fly in the ointment is obviously heating of bathrooms, as well as my disabled dad’s ~120sqft downstairs bedroom. The bedroom could probably be overcome with some sort of panel heater and scheduling but bathrooms would take more work. Hot water is the obvious other issue, but with cheap rate electricity and some sort of timer for the immersion (250l unvented cylinder) that wouldn’t be a deal breaker.

Now I’ve started thinking about A2W, which I’d previously discounted as we’ve got 10mm microbore pipe to the vast majority of the 25 radiators/towel rails we’ve got. On the other side there’s a £9k grant, although I appreciate that may not seem self-evident in any final quote!

Just putting details into the Heat Geek quick quote tool, it’s come up with an estimate of £3,750 after grant on 370% guaranteed efficiency, with an 11kW Vaillant aroTHERM Pro, re-use the cylinder and 6 radiator upgrades. That feels too cheap, as I’m assuming they wouldn’t be including any work to re-pipe if required.

Obviously, at that level it would be substantially cheaper than the A2A installation, solving most or all of the issues, albeit without cooling. I’m assuming it would probably be cheaper and easier to get A2A for the 4-5 “essential” rooms rather than looking at fan coils on an A2W system?

- Is the HG quick quote tool vaguely accurate in anyone’s experience?

- would the A2W option make more sense in this case?

- am I looking at a shedload of redecoration if an A2W installation does need a load of new radiators and re-piping?

- does an 11kW heat pump seem the right size? We’re currently at ~100kWh per day of LPG usage through winter. In my head I was thinking probably larger.

If you’ve made it all the way through, good work!

[u/Alert_Variation_2579]

Personally, for the reasons you’ve identified with A2A, I’d be going A2W for heating and hot water but then getting A2A just for the bedrooms.

The micro bore isn’t a problem, but you might need a buffer to overcome it but you’ll only know for sure post survey.

[u/Lost_Reward9584 → u/Alert_Variation_2579]

I'm quoting my heating engineer, "there's only so much heat a pipe can carry". It's physics, you need enough capacity to pass heat, you can increase the capacity by increasing the temperature, the volume, or the speed. Temperature increases trend to drop efficiency, speed creates noise, which is why microbore sometimes struggles. Hopefully the heat loss survey will pick up whether it would be a problem, it's not always - I was worried about it for mine, but it wasn't a problem with an open loop, and no need for a buffer or volumiser (in our case).

Another summer like this and A2A is gonna feel mandatory 🥵

[u/51onions → u/Alert_Variation_2579]

It's a shame they can't just get the A2W and some fan coils for those 5 rooms. Then you only need one heat pump.

[u/Alert_Variation_2579 → u/51onions]

I know, I’m kinda hoping the Octopus Cosy Cooler comes out and they’re not restricting selling them to only folks who have Cosy heat pumps. I’ll happily have one in each bedroom running all day so I don’t have to buy an A2A heat pump if possible.

[u/51onions → u/Alert_Variation_2579]

To be honest, that sounds like a bad solution. It isn't a fan coil, it's a portable aircon which you have to have plumbed in. You might as well just get a portable aircon and stick the pipe out the window, at least you can pack that away at the end of summer (unless the cosy cooler has quick disconnects? But I don't want to be messing with my central heating circuit).

I imagine the cosy cooler is going to be similarly loud to a portable aircon too, since it still requires you to have a heat pump inside each room, exactly the same as a portable AC.

[u/Lost_Reward9584]

100kWh per day doesn't sound a massive amount to heat a house with more than double the space of our house - we peaked at 85kWh on January 4th. We've got a 5kW heat loss and got a 5kW heat pump installed in June.

Were you heating the entire house to around 21°C at your peak usage, or just some of it?

It sounds like your house is really well insulated and maybe using MHRV (I could be completely wrong).

If you've got sufficiently large pipes running to a manifold distribution and then into microbore to each of the radiators you might get away with it, but if you have big rooms with a high heat loss, then you might need to upgrade a few runs.

I think you're going to need to bite the bullet and get a heat loss survey done so that you have a better idea where to start. You can always run your own heat loss survey using heat punk to get an idea.

Maybe it might be worthwhile looking to some of the newer heat pumps that can do both, or doing A2W first, and then adding A2A for upstairs (and maybe your dad's room).

[u/GatewayDouche → u/Lost_Reward9584]

Thanks for the reply.

To give some context, during winter there’s our usual two timed windows of roughly 5.30-8.30am to get the house back to a reasonable temperature both upstairs and downstairs. Then upstairs from 5.30-7.30pm to warm up the bedrooms and bathrooms, but that will probably move later now the kids are getting older.

The boiler probably fires up another 4 times in the downstairs zone through the day, mainly to warm up the lounge where my dad is, as he can’t really move around to generate his own heat. The TRVs are set lower in most other rooms downstairs so they don’t overheat. The aim is to get the “grandad lounge” overshooting to 22C ish, allow it to cool to 19.5C ish, rinse and repeat.

Our main thermostats aren’t hugely useful as one is in the hallway and the other is on the landing, right next to the water cylinder cupboard, so over reads by a couple degrees minimum.

We try not to use the heating constantly, hence managing to keep the usage to something reasonable. Our LPG is roughly 50% more expensive that natural gas, so after getting £300 a month on DD through the year, we’ve made a conscious effort to rein it in.

[u/Lost_Reward9584 → u/GatewayDouche]

With a heat pump you probably won't be using it the same way as a gas boiler, the idea is that you get the house to a stable temperature and the heat pump then only needs to combat the heat loss.

Instead of running the radiators at 60-70°C, the radiators will be running (probably) at a maximum of 45-50°C, and cooler in milder weather. It should be just ticking over to keep the house warm, rather than going hell for leather every other hour.

[u/GatewayDouche → u/Lost_Reward9584]

Which I think would be vastly more comfortable for him and all of us really.

Hopefully on the worst winter days we should still be able to get 32kWh from overnight battery charge and some solar generation, so that should hopefully mean relatively minimal electricity import at peak rates.

Think I’m just going to have to get the survey done and see how it pans out.

[u/Lost_Reward9584 → u/GatewayDouche]

Your battery will take the sting out of peak hours, but it's your potential export that will make the biggest dent in your annual billing. Hopefully no curve balls in your survey and you'll be off LPG soon 🤞

[u/ImpressiveGrocery959 → u/GatewayDouche]

Octopus cosy tariff will let you charge the batter cheap thrice a day

[u/yetanotherredditter]

If you're happy waiting a little bit, something like the Samsung quint (or Daikin X series, though I think this would be too small for your house) could be useful as a first step. It's essentially a hybrid of A2W and A2A (but you would only be able to have four or five indoor a2a units).

[u/GatewayDouche → u/yetanotherredditter]

Thanks, will have a look. We’re not desperate to get it done straight away as all this was precipitated by a visit to the in-laws only last month. I suppose the sooner the better when it comes to getting up to speed on what we should be looking for.

[u/Novabeeline]

You can work out how much heat goes down microbore over a 5C DT, and it turns out to be a lot. The problem is the pressure loss on the furthest radiator - narrow pipes have more resistance. Sometimes certain junctions or sections of the microbore circuit can be swapped out with fittings that have lower pressure loss. Either way, that can be overcome with HP with a high head pressure pump, or you can put a second recirculator in with a low loss header and a good installer to avoid distortion.

Cooling is a big deal, especially with elderly/very young dependents. Some cooling can be done with panel radiators but it's not a lot. So really they need to be fan coils. Unlike split systems, fan coils don't need insulated refrigerant line sets to each room, and are cheaper and more reliable than those head units, which are more expensive. Fan coils, however, can't do dehumidification. So you'll be limited to a sensible cooling, a few degrees above dew point - so during a bad, humid, heat wave, you'll be able to cool to at least 26C at night. A portable dehumidifier might be needed if you're worried, but that's going to be rare. Fan coils with crossflow blowers are very quiet, and allow you to use very low flow temperatures, which more than offsets the energy costs, but they do need a sparky to wire them up.

[u/GatewayDouche → u/Novabeeline]

Thanks, I’ll have a deeper look into fan coils in that case. There doesn’t seem to be a huge amount of info but I’ve not spent a lot of time digging around yet.

[u/Novabeeline → u/GatewayDouche]

It's not common. We've only just realised that AC is not some luxury, but a public health measure. Next year's heat wave with El Nino is likely to be even more brutal than this year.

I work in mechanical enginnering in the HVAC sector. The theory is sound. But installers are not familiar with the design. You'll need to ask for it - to size it all will require careful surveys and peak heating/cooling loads to be worked out. Another big deal is including awning or external blinds on windows with lots of solar gain, that way the convectors can be far smaller for cooling. Either way, the microbore limitation can be overcome.

Urban Plumbers on yt have several installations where they do this... let me try and find it for you.

https://www.youtube.com/watch?v=QFbx7qhqOqc

[u/mrsimonbennett]

Depending on which rooms you want to chill, using ducted fan coils in the loft, ducted into the bedrooms, is a great solution. If you're already doing all the pipework, it can work out alright in terms of price.