ASHP: monobloc, split or combi?

[AndySat]

After a long road through the planning process, I'm finally reaching the point where I need to make some heating system design decisions as input into the Building Warrant on our planned 130m2 timber-frame house.
Two local contractors have estimated the heating requirements at 3kW, based on a simple 24W/m2 calculation and are proposing monobloc ASHP solutions of 5kW.
I've used Jeremy Harris' spreadsheet to get a total heat loss of just over 2kW. LoopCAD, interestingly, estimates the total UFH load at 3.9kW but that may be down to user error on my part!  Either way, I'm assuming an ASHP around 5-6kW will more than meet the demands of UFH and hot water.

So far, so good, but I'm wondering if a split ASHP solution is better, something like the Hitachi Yutaki, especially the combi version. The price of the Hitachi combi is a lot lower than the Mitsubishi monobloc + DHW cylinder that the contractors are suggesting.
As I see it, the advantage of a split unit is that the piping between the units is smaller and easier to route (and less heat loss?), also the outside unit is smaller. The downside is that the system needs to be commissioned by a refrigeration engineer, but this shouldn't be a big problem.
Does it make any difference if the ASHP is monobloc, split or combi?

[JohnMo]

2 minutes ago, AndySat said:

split ASHP solution is better

Keep all the noise outside and just install a monobloc. Then a cylinder with a 3m²+ coil.

 

Then all you need in house is cylinder, diverter valve and simple UFH manifold. Plus the heat pump controller. Depending on screed thickness just simple batch charging against a thermostat works fine. Or weather compensation. But batch charging via the thermostat gets away from cycling and the floor acts as a huge buffer. Keep it simple.

[S2D2]

+1 to internal noise being an issue with split systems.

[Beelbeebub]

Where does the internal noise come from in split systems? 

 

I know there were some systems that had the compressor inside and all that was outside was the heat exchanger and fan. 

 

But do modern ones do that? I thought they kept all the gubbins outside and all that was inside was a plate heat exchanger, a pump, maybe an expansion vessel and a diverter valve?

[AndySat]

Thanks, I hadn't considered the noise aspect...

21 hours ago, JohnMo said:

Keep it simple

Agreed. I'm all in favour of keeping it simple and easier to maintain. I've only asked one contractor for a UFH quote so far and their design has multiple zones (one per room) which I understand, given that e.g. a bathroom needs a higher floor temp than a bedroom, but they also have one thermostat per zone, which seems excessive. I'm hoping that the flow in each zone can be set to achieve the required temp and then the whole house can be controlled by a single thermostat, as @JohnMo says.

 

18 hours ago, Beelbeebub said:

all that was inside was a plate heat exchanger, a pump, maybe an expansion vessel and a diverted valve

Yes, I think that is the case, at least in the Hitachi split that I was considering.

 

 

[Beelbeebub]

1 hour ago, AndySat said:

Thanks, I hadn't considered the noise aspect...

Agreed. I'm all in favour of keeping it simple and easier to maintain. I've only asked one contractor for a UFH quote so far and their design has multiple zones (one per room) which I understand, given that e.g. a bathroom needs a higher floor temp than a bedroom, but they also have one thermostat per zone, which seems excessive. I'm hoping that the flow in each zone can be set to achieve the required temp and then the whole house can be controlled by a single thermostat, as @JohnMo says.

 

Yes, I think that is the case, at least in the Hitachi split that I was considering.

 

 

It is fairly standard to design ufh as zoned per room.. It sort of comes for free with the physical components - all the pipes come back to a manifold. 

 

The good news is that it's trivial to "dezone" the system. You just remove the zone actuators (or never install them). You can then adjust the relative flows through each loop to account for the different demands per room. 

 

My old ufh was originally zoned per room but I ditched that abiut 5 years ago for the entire house being one zone controlled by a single thermostat. It was actually better and more stable than the individual zones. 

[JohnMo]

57 minutes ago, AndySat said:

only asked one contractor for a UFH quote

With an ASHP with no radiators you can, if you wish drive the UFH from the circulation pump within the heat pump. So no mixer or pump needed on the manifold. I would state to the UFH contractor you are going to run weather compensation.

 

I would install an electric towel rail in bathrooms and use that to dry towels when heating is off and drive the room temp up if needed. If you want the bathrooms warmer than the rest of the house on the same flow temp and on a single zone, just put more pipe (closer pipe centres) in the bathrooms.

 

Given a choice of heat pumps, I would just go Panasonic.

[JohnMo]

10 minutes ago, Beelbeebub said:

sort of comes for free with the physical components - all the pipes come back to a manifold

The trouble is if you hardwire, you pay the electrician to install wires, you end up paying £50 per thermostat (could be way more or a little less), then you get talked into auto balancing actuators at another £25+ each. So an 8 room house you end up spending several hundred on need less stuff, plus labour to install it... Leave the contractors they will also insist on mixers and pump. I have so much needless stuff I have removed it's unreal.

[joth]

19 minutes ago, Beelbeebub said:

It is fairly standard to design ufh as zoned per room.. It sort of comes for free with the physical components - all the pipes come back to a manifold. 

 

The good news is that it's trivial to "dezone" the system. You just remove the zone actuators (or never install them). You can then adjust the relative flows through each loop to account for the different demands per room. 

 

My old ufh was originally zoned per room but I ditched that abiut 5 years ago for the entire house being one zone controlled by a single thermostat. It was actually better and more stable than the individual zones. 

Yes to this, obviously you're incurring unnecessary cost of room stats, actuators and installation if you go this route. Also in a lower (BR) spec property you'd risk one outlier cold zone calling for heat too much and driving everywhere, but the calculated heat loss suggest this is a better than BR property so most likely work just fine without zoning. (Aside - do you have MVHR and planning for good airtightness? This is likely the difference between the heat calculation methods)

 

In some cases I find zoning useful to avoid bedroom overshoot - but the easiest and cheapest solution to that is simply not put UFH into bedrooms at all if they're upstairs. 

 

If you're sufficiently savvy then installing without zoning but with a plan for zoning should it become needed is the alternative tactic.

 

Edited Tuesday at 15:56 by joth

[IGP]

Don’t zone if at all possible. Adjust the spacing of the UFH loops to match the intended design temperature for particular rooms while running a single low flow temperature. 

[Beelbeebub]

19 hours ago, JohnMo said:

The trouble is if you hardwire, you pay the electrician to install wires, you end up paying £50 per thermostat (could be way more or a little less), then you get talked into auto balancing actuators at another £25+ each. So an 8 room house you end up spending several hundred on need less stuff, plus labour to install it... Leave the contractors they will also insist on mixers and pump. I have so much needless stuff I have removed it's unreal.

Yes, 100%. Though it does make sense to zone for the older non-WC gas boiler systems. 

 

But now, with WC and especially with HPs, don't zone - or onky have the absolute minimum.

 

The physical hardware (manifold etc) is identical just with fewer components. 

[Beelbeebub]

Also should add that if you are getting a particular room too hot using the single zone UFH, you can just turn the flow down to that room to reduce the output. Effectively your flow temp is driven by the worst loop in your house. 

 

 

[AndySat]

Thanks all, this is helping me get to a clear set of requirements that I can discuss with the contractors.

 

I did try to reduce the number of zones in LoopCAD by linking the cold end of the bathroom loop to the hot end of the adjacent bedroom loop and making the spacing tighter in the bathroom, but although it looked ok on paper, I think it's simpler to keep the loops separate and adjust the flows to get the right amount of heat into each loop. This assumes that the worst loop will put enough heat into the room with 35C flow, as @Beelbeebub says.

On 21/01/2025 at 15:45, JohnMo said:

I would install an electric towel rail in bathrooms

Good idea, especially since the floor area is quite small and it's a challenge to get enough UFH pipework down!

 

On 21/01/2025 at 15:54, joth said:

do you have MVHR and planning for good airtightness

Yes, MVHR is planned - that's another topic that I'll be raising soon! The house is designed to be quite airtight (around 0.5 ach/hr) but not to Passivhaus spec. Whether we get it as good as that in practice remains to be seen...

 

I'm still to decide on a preferred make/model of ASHP. But I think I'll separate that from the UFH design i.e. I'll get the UFH designed as far as the manifold and then do the ASHP/DHW cylinder as a separate exercise. I'm not eligible for any grants, so I don't need a full system certification, just a design that will be acceptable for a Building Warrant here in Scotland.

[JohnMo]

16 minutes ago, AndySat said:

I'm not eligible for any grants

Not that I bothered with grants, but why aren't you eligible? Any new house should be eligible and you get more money than England.

[Gary68]

In terms of internal noise and monoblocs I noticed you mentioned Mitsubishi, they don't have external pumps so the main pump and the noisiest one will be in your house, so I would avoid, I speak from personnel experience.

[AndySat]

22 hours ago, JohnMo said:

but why aren't you eligible? Any new house should be eligible

I might be missing something, but my reading of the Home Energy Scotland Grant and Loan terms is that the grants are for improvements to existing installations and don't apply to new builds.

It's not permitted to install oil or gas boilers in new builds, so perhaps there's no need for an incentive to install heat pumps?

 

36 minutes ago, Gary68 said:

they don't have external pumps so the main pump and the noisiest one will be in your house

I have to admit that the question of noise never occurred to me when I started researching ASHP. I'm aware that some older external units can be a nuisance to the neighbours (not in our case though as the location is on the opposite side of the house) but I assumed that internal noise wasn't a problem. One of the ASHP contractors offered to show me around one of their recent installs, so that might give me an idea of how noisy it is in practice.

[joth]

20 hours ago, AndySat said:

the grants are for improvements to existing installations and don't apply to new builds.

It's not permitted to install oil or gas boilers in new builds, so perhaps there's no need for an incentive to install heat pumps?

That...... actually makes sense 

[Nickfromwales]

21 hours ago, AndySat said:

I might be missing something, but my reading of the Home Energy Scotland Grant and Loan terms is that the grants are for improvements to existing installations and don't apply to new builds.

It's not permitted to install oil or gas boilers in new builds, so perhaps there's no need for an incentive to install heat pumps?

 

I have to admit that the question of noise never occurred to me when I started researching ASHP. I'm aware that some older external units can be a nuisance to the neighbours (not in our case though as the location is on the opposite side of the house) but I assumed that internal noise wasn't a problem. One of the ASHP contractors offered to show me around one of their recent installs, so that might give me an idea of how noisy it is in practice.

One I visited was just beyond what I’d accept in my house, but was in their basement. 
When firing up from cold it was ‘not quiet’, a bit like a washing machine shivering into life then starting a spin cycle is the best way I can describe it. 
Lots of buzzing, whirring, and vibrations etc, a Daikin iirc. 

[PhilT]

22 hours ago, AndySat said:

I might be missing something, but my reading of the Home Energy Scotland Grant and Loan terms is that the grants are for improvements to existing installations and don't apply to new builds

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

[Beelbeebub]

5 hours ago, Nickfromwales said:

One I visited was just beyond what I’d accept in my house, but was in their basement. 
When firing up from cold it was ‘not quiet’, a bit like a washing machine shivering into life then starting a spin cycle is the best way I can describe it. 
Lots of buzzing, whirring, and vibrations etc, a Daikin iirc

Was it a new system? I know some of the older designs had the compressor inside. Which would be noisier, but I was under the impression that all the newer split systems had the compressor outside. 

[Nickfromwales]

2021 iirc. I'm not sure what they do with these things nowadays, but I really can't see the benefit of a split unless you need very hot water eg to run existing rads etc.

[JohnMo]

2 minutes ago, Beelbeebub said:

Was it a new system? I know some of the older designs had the compressor inside. Which would be noisier, but I was under the impression that all the newer split systems had the compressor outside. 

You just have to be careful the terminology is correct. A lot of systems can be hydro split (water split), while others and more appropriate to the term of split, are refrigerant split. Both are available.

 

The Viessmann reference above is a hydro split.

[Beelbeebub]

11 hours ago, JohnMo said:

You just have to be careful the terminology is correct. A lot of systems can be hydro split (water split), while others and more appropriate to the term of split, are refrigerant split. Both are available.

 

The Viessmann reference above is a hydro split.

But in both cases the compressor is often outside. 

 

The internal bits of both refrigerant and hydro split only contain standard central heating circulators, diverter valves etc which should be fairly quiet. 

[Beelbeebub]

11 hours ago, Nickfromwales said:

2021 iirc. I'm not sure what they do with these things nowadays, but I really can't see the benefit of a split unless you need very hot water eg to run existing rads etc.

I think the benefit is from the manufacturer pov. The outdoor unit is a standard aircon unit (manufactured in vast numbers) . It doesn't care if it's connected to a common indoor fan unit or a hydro box that dumps the heat to water. 

 

There are some secondary benefits about not freezing outside and typically smaller and easier runs to the outside unit. 

 

The downside is the need for refridgerant handling during install. 

[Nickfromwales]

8 hours ago, Beelbeebub said:

I think the benefit is from the manufacturer pov. The outdoor unit is a standard aircon unit (manufactured in vast numbers) . It doesn't care if it's connected to a common indoor fan unit or a hydro box that dumps the heat to water. 

 

There are some secondary benefits about not freezing outside and typically smaller and easier runs to the outside unit. 

 

The downside is the need for refridgerant handling during install. 

Yup, I think mostly aimed at / suitable for more adverse installs, particularly where freezing is going to be a known issue.