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 tvar had the compressor inside and all that was outside was the hext 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 diverted valve?

Edited Monday at 19:17 by Beelbeebub

[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 15 hours ago 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.