Ducted A2A and floor level vents

[Crofter]

I'm leaning towards an underfloor ducted A2A system. The systems I'm looking at all seem to be designed for ceiling mount, which makes sense in air con mode. And it's dead easy to stick a vent in the ceiling.

But for heating, it makes sense to pump out heat as low as possible. Being in the north of Scotland I don't expect to need cooling often, if ever.

 

What's the standard practise for low level vents? I don't really want a grating on the floor itself. I'm hoping that it might be possible to fit something within the depth of the partition walls, but that's only 100mm and getting up through the wall plate will be a PITA.

 

The alternative would involve some sort of box-out affair, which doesn't seem very elegant.

 

I don't want to reinvent the wheel here...

[Beelbeebub]

Pump the air into the underfloor space and have gappy floorboards? 😁

 

 

[JamesPa]

2 hours ago, Beelbeebub said:

Pump the air into the underfloor space and have gappy floorboards? 😁

 

 

Isn't that what the Romans did?  Just saying!

[Crofter]

5 hours ago, Beelbeebub said:

Pump the air into the underfloor space and have gappy floorboards? 😁

 

 

The floorboards aren't anything like as gappy as the air bricks!!

Seriously, somebody else must have thought of how to do this.

Scratching my head a bit, I can hide box-outs for the bedrooms and bathroom, thanks the built in cupboards.

In the kitchen, vents could go in the plinth under the units.

The living room is trickier, no easy answer there...

 

What generally accepted as the best place to draw air in to the system?

[DanDee]

Your best bet is to look at the N. American systems and take ideas from there.

[Beelbeebub]

The big stumbling block is the low specific heat of air. 

 

If you assume an inlet temp of 40C (ie 20C above room) you need about 200m3/hr to provide 1kw. In context that's roughly the flow from a fairly powerful 125mm bathroom extractor. 

 

You could have hotter air (as I believe the Americans do as their air heating systems are gas powered) which drops the flow, or huge ducts.... 

 

[JohnMo]

13 hours ago, Crofter said:

Being in the north of Scotland I don't expect to need cooling often, if ever.

That's what I also thought, living between Aberdeen and Inverness, but had under floor cooling on most days when the sun came out, from May to September.

[SteamyTea]

Can you fit in something that exits via a shadow gap at floor level.

Maybe use a ceiling level shadow gap as a room extract to the MVHR.

 

200 (m³) x 1.25 (kg/m³) x 20 (K) x 1 (kJ/kg.K) = 5000 kJ (1.4 kWh)

 

I think.

 

Edited October 6, 2023 by SteamyTea

[Crofter]

8 minutes ago, JohnMo said:

That's what I also thought, living between Aberdeen and Inverness, but had under floor cooling on most days when the sun came out, from May to September.

Ah but that's the Costa del Moray, where they have this strage glowing orb in the sky.

[Crofter]

5 minutes ago, SteamyTea said:

Can you fit in something that exits via a shadow gap at floor level.

Maybe use a ceiling level shadow gap as a room extract to the MVHR.

 

200 (m³) x 1.25 (kg/m³) x 20 (K) x 1 (kJ/kg.K) = 5000 kJ (1.4 kWh)

 

I think.

 

No MVHR, this is for my 70s bungalow, not the cottage.

I'm pretty tempted to fit a conventional (non ducted) A2A in the cottage, but probably after I've fitted PV.

[Crofter]

24 minutes ago, Beelbeebub said:

The big stumbling block is the low specific heat of air. 

 

If you assume an inlet temp of 40C (ie 20C above room) you need about 200m3/hr to provide 1kw. In context that's roughly the flow from a fairly powerful 125mm bathroom extractor. 

 

You could have hotter air (as I believe the Americans do as their air heating systems are gas powered) which drops the flow, or huge ducts.... 

 

Standard appears to be 200mm diameter ducting. So not Bruce sized but not small either. Maybe this is why ceiling mount is much more common.

[JohnMo]

8 minutes ago, Crofter said:

Ah but that's the Costa del Moray, where they have this strange glowing orb in the sky.

Now I am on the computer I can see where you are.  Yes we have a strange orb in the sky

[Beelbeebub]

1 hour ago, Crofter said:

No MVHR, this is for my 70s bungalow, not the cottage.

I'm pretty tempted to fit a conventional (non ducted) A2A in the cottage, but probably after I've fitted PV.

If it's a bungow, presumably with a loft, running the ducting in the loft is the obvious solution - far easier to install and maintain. 

 

TBH, why not just a mutki split with the pipework running through the loft and then to a standard head unit? Super neat and pretty standard. 

[SteamyTea]

13 minutes ago, Beelbeebub said:

Super neat

I think the wall units looks dreadful.

Reminds me of cheap motels.

[DanDee]

12 minutes ago, SteamyTea said:

I think the wall units looks dreadful.

Reminds me of cheap motels.

there are no limits to what a brain can think of, let it loose

 

Edited October 6, 2023 by DanDee

[Crofter]

33 minutes ago, SteamyTea said:

I think the wall units looks dreadful.

Reminds me of cheap motels.

Exactly.

I don't want to force air down from up near the ceiling, I want it to emerge at floor level where it's needed.

I expect a ducted system will be quieter than a mini split.

I'd also like to be able to duct warm air to every space, which gets expensive with splits. But ducting is cheap.

 

I don't really mind working under the floor. My loft is so full of junk that it's probably easier to work down there. Condensation drain is easier. Connection to outside unit is easier. Everything seems to point to a floor level installation, when the primary aim is heating.

[Beelbeebub]

11 hours ago, Crofter said:

Exactly.

I don't want to force air down from up near the ceiling, I want it to emerge at floor level where it's needed.

I expect a ducted system will be quieter than a mini split.

I'd also like to be able to duct warm air to every space, which gets expensive with splits. But ducting is cheap.

 

I don't really mind working under the floor. My loft is so full of junk that it's probably easier to work down there. Condensation drain is easier. Connection to outside unit is easier. Everything seems to point to a floor level installation, when the primary aim is heating.

If you have easy access to underfloor and it's clear just use the same insulated ducting and terminals for loft routing but under the floor. 

 

You'll probably need to put some boxing in underneath to support the ducts. 

 

If you have a timber suspended floor with a good clearance then lift some boards and create a plywood box between and under the joists. Then lay the duct into that. 

 

Then use standard duct terminations in corners and the like. 

[Crofter]

3 hours ago, Beelbeebub said:

If you have easy access to underfloor and it's clear just use the same insulated ducting and terminals for loft routing but under the floor. 

 

You'll probably need to put some boxing in underneath to support the ducts. 

 

If you have a timber suspended floor with a good clearance then lift some boards and create a plywood box between and under the joists. Then lay the duct into that. 

 

Then use standard duct terminations in corners and the like. 

Lifting part of the floor isn't a great option, it's continuous pine T&G boards with the internal partitions built over the top. I think it would be pretty destructive and the floor would never be the same again.I've also just laid laminate through about half the house and don't want to lift that again!

I really don't mind working underneath- there's about 2-3ft clearance everywhere. I replumbed the entire house recently and shifted everything from the loft to underfloor.

 

The ducting in looking at is the same idea as I used for MVHR in my other house- flexible, expandable, lightweight, so dead easy to work with. Will need support but nothing complicated.

 

I see in the US people use floor gratings. It must work, otherwise they wouldn't be doing it. But it feels a bit wrong to me, for some reason. Can imagine it working in rooms with hard flooring, I suppose, but not so much with carpet.

 

I think I may have to resign myself to building a box-out and just have it positioned where it's partially hidden, e.g. behind a shelving unit.

[Beelbeebub]

1 hour ago, Crofter said:

Lifting part of the floor isn't a great option, it's continuous pine T&G boards with the internal partitions built over the top. I think it would be pretty destructive and the floor would never be the same again.I've also just laid laminate through about half the house and don't want to lift that again!

I really don't mind working underneath- there's about 2-3ft clearance everywhere. I replumbed the entire house recently and shifted everything from the loft to underfloor.

 

The ducting in looking at is the same idea as I used for MVHR in my other house- flexible, expandable, lightweight, so dead easy to work with. Will need support but nothing complicated.

 

I see in the US people use floor gratings. It must work, otherwise they wouldn't be doing it. But it feels a bit wrong to me, for some reason. Can imagine it working in rooms with hard flooring, I suppose, but not so much with carpet.

 

I think I may have to resign myself to building a box-out and just have it positioned where it's partially hidden, e.g. behind a shelving unit.

If you are happy to work under the floor then go for it! Too claustrophobic for me (plus I'm over 6ft and tend towards solid than skinny). 

 

The Americans do use the dates with carpet. Usually several around the perimeter, especially under windows. 

 

 

They also (sometimes) have them in walls low down, you could come up from below inbetween some studs and then through the wall with a decorative grate... 

 

 

Though, if you were a fan of the X files you might not be too keen! 

 

Edited October 7, 2023 by Beelbeebub

[Crofter]

1 hour ago, Beelbeebub said:

If you are happy to work under the floor then go for it! Too claustrophobic for me (plus I'm over 6ft and tend towards solid than skinny). 

 

The Americans do use the dates with carpet. Usually several around the perimeter, especially under windows. 

 

 

They also (sometimes) have them in walls low down, you could come up from below inbetween some studs and then through the wall with a decorative grate... 

 

 

Though, if you were a fan of the X files you might not be too keen! 

 

That low level wall outlet is basically what I'm hoping to do. But fitting a 200mm duct in to a 100mm stud wall will be interesting. I guess I can split it in to four 100mm ducts...

[Beelbeebub]

3 hours ago, Crofter said:

That low level wall outlet is basically what I'm hoping to do. But fitting a 200mm duct in to a 100mm stud wall will be interesting. I guess I can split it in to four 100mm ducts...

How about this type of kit? 

 

https://www.epicair.co.uk/collections/airflow-airflex-semi-rigid-ducting

 

There are 75mm round and 115x51 oval ducts. They have ceiling and floor plenum outlets. 

 

We have the older version with metal  plenums, you could wall mount them between studs and run 2x 75mm round (or use oval) to each one in a radial scheme. 

 

Or you could mount them through the floor (I think they have specific ones for that) 

 

It was a pretty easy system to use, just drag the pipe off the coil, cut to length and stick in socket. 

 

The radial scheme has more pipes, but they are smaller and you don't have to worry about sizing down as the run goes on or about inter room sound as much. 

[Crofter]

So 

This should all be a solvable problem. But I'm realising it's fairly involved.

 

I've not heard great things about the system design for A2A by the 'experts' and I'd like to get my head around the principles.

 

So I guess I need to do a rough heat loss analysis first, to size the unit. For example, using Heatgeek's rules of thumb I'm getting maybe 60w/M2, which for my 93m2 property is about 5.5kw. That's about half what I was expecting. What happens if I oversize a good bit, up to say 10kw? Will the system cycle too much and lose efficiency?

 

Next, pick a suitable ASHP unit and start playing around with flow rates and temperatures. That should let me start getting a handle on duct and vent sizes, and layouts.

 

I'm finding very little info on all of this online. I suppose some of the principles will be the same as MVHR, but I guess it's a much larger volume of air being moved, and I'm going to want different rooms to be at different temperatures.

[sharpener]

3 hours ago, Crofter said:

I suppose some of the principles will be the same as MVHR

 

Yes, I looked into adding heat from a HP to the existing MVHR system using basic "specific heat of air" calcs. But the air flow required was from memory about 5x what I have at present so clearly not a runner. (The electric inline heater in my Villavent central unit is only 2kW, but it is disabled anyway.)

 

WRT your original query I have been thinking that the 110 x 50 or 204 x 60 rectangular PVC ducting you can get for tumble driers might have a role to play, though obvs you would need several outlets in a typical living room.

[JohnMo]

A pretty good rule of thumb. Passivhaus heating specs are designed around around heating via the MVHR system.

 

The 10W per m2 value, specified for the max heating requirement, is the maximum value of heat transfer you can provide to the house when the the MVHR is providing 0.3 ACH. Any more than that the air smells burnt.

 

So if you want to provide 60W per m2, you will need a minimum air flow of 6 x 0.3 or 1.8 ACH, based on your house volume. So 93m2 X 2.4 X 1.8, so 400m3/h.

 

If you want to provide 10kW you are looking at the best part of 800m3/h or around 3.6 ACH. 

 

That's a lot of air blowing around.

 

 

[SteamyTea]

2 hours ago, JohnMo said:

 

So if you want to provide 60W per m2, you will need a minimum air flow of 6 x 0.3 or 1.8 ACH, based on your house volume. So 93m2 X 2.4 X 1.8, so 400m3/h.

 

If you want to provide 10kW you are looking at the best part of 800m3/h or around 3.6 ACH. 

 

That's a lot of air blowing around.

Could you put in a secondary system that recirculates the air, but also heats it. More vents and pipes I know, but cheap to do.

 

Double the pipe diameter and you quadruple the mass flow rate for any given flow rate.