Ducted A2A and floor level vents

[Crofter]

1 hour ago, SteamyTea said:

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.

The system does recirculate. You need an inlet and outlet for each space. It doesn't draw in external air- it's a heating system, not ventilation.

[SteamyTea]

8 minutes ago, Crofter said:

The system does recirculate. You need an inlet and outlet for each space

Right.

Do you really need an in and out for each space. Surely it depends on what temperature you want in each space. You may find that bedrooms can have extracts only, and living areas inlets only.

 

I still think that using shadow gaps is the way forward, especially after looking at my dusty skirting boards earlier today.

[Crofter]

3 hours ago, JohnMo said:

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.

 

 

I did find an online calculator on engineering toolbox, and was playing around with that. Worst case scenario of 10kw heat loss, 21⁰ room temp, and 40⁰ supply temp, works out at 0.44m³/sec or a whopping 1584m³/hr. Yikes.

Best case with 5kw, 19⁰, and 55⁰ supply, drops that to 417m³/hr. Which is in line with your own figures.

I have yet to do a proper heat loss calculation, but a radiator sizing tool suggested about 7kw, making some hefty assumptions.

 

One unit I'm considering (a Mitsubishi 10kw) has a max flow rate of 2640m³/hr. It certainly is a lot of air! Maybe that's massively oversized after all.

 

Four 200mm ducts is about 0.4m². I should be able to work out the flow rate needed... after a cup of tea...

 

[Crofter]

1 minute ago, SteamyTea said:

Right.

Do you really need an in and out for each space. Surely it depends on what temperature you want in each space. You may find that bedrooms can have extracts only, and living areas inlets only.

 

I still think that using shadow gaps is the way forward, especially after looking at my dusty skirting boards earlier today.

It's not on the same scale as MVHR.

Then again, the more conventional A2A units without ducting seem to get on ok, with people using a single unit in the hallway to heat adjacent rooms.

 

Shadow gaps could be really elegant, but when you look at the total vent area required I think a couple of dedicated vents might make more sense.

[Crofter]

Ok, I've had my cup of tea now.

2640m³/hr is 0.733m³/s

With a total duct outlet area of 0.4m² I make that 1.8m/s or about 4mph.

That's the 10kw unit running full tilt, possibly double the actual heating requirement. Although there will also be losses with friction and bends.

 

That doesn't sound too scary. But it shows that you definitely need some big ducts and vents to make this work.

 

I'm making this up as I go along so more than happy to be corrected!

[Crofter]

An another thing...

 

It seems like a good idea to blow the warm air in at floor level.

But where's the best place to put the return vents?

I'm wondering if a ceiling vent, as far as possible from the inlet, is ideal. Or, given the air flow involved, would that turn the room in to a wind tunnel?

[SteamyTea]

20 hours ago, Crofter said:

Ok, I've had my cup of tea now.

2640m³/hr is 0.733m³/s

With a total duct outlet area of 0.4m² I make that 1.8m/s or about 4mph

How big is the volume it is entering, will there be furniture in the way?

Can you distribute the inlet into 2 or 3 in a line?

Or is the 733 lt/s (0.92 kg/s) the total airflow for the whole building?

The kinetic energy, is very small, so would not think it is noticeable.

[Crofter]

I've been crunching some numbers.

I don't think I need a 10kw unit.

Heatgeek's rules of thumb suggest around 5.5kw, and an onlinev radiator sizing tool returns a range of 4.5-6.5kw, depending on how exactly I answer it.

So I think a 7.1kw unit ought to be ample?

At full chat this is delivering 780m³/hr, or 216l/s. Which seems proportionally far less airflow than the bigger unit, and much easier to handle.

This flow rate tallies with the numbers from engineering toolbox.

Splitting it through four 200mm diameter ducts results in a max flow speed of 1.7m/s (3.8mph).

 

I think I messed up earlier when calculating cross sectional area of the ducting, so despite the much lower flow rate the 7.1kw still works out to have about the same airspeed.

 

[SteamyTea]

@Crofter

 

I am taking a keen interest in what you are doing.

For many years now I have thought that directly heating the air is the better way to heat a space, the downside has been that most of the systems available are US systems, so generally oversized.

There is an argument to be made that UFH is better, but there are downsides to it i.e. heat loss to ground, fixed position, repairs may be expensive.

There are problems with air handling, main ones I see are the size of ductwork, flowrates and limited temperature, and then the big one, sound transmission between spaces.

When I was a kid, our neighbours has a hot air system, and it was almost as good as an intercom system for attracting attention.  I can imagine a drunken conversation where one half hears everything from another room.  Would probably end badly.

[Crofter]

One of the big advantages I see with A2A is the faster response time.

For a house like ours, which is either used by a working family and empty through the day, or at other times used as a holiday let, the current storage heaters are about the worst possible type of heating it could have. I don't think UFH would be the best match either. Anyway, I've got a suspended timber floor and there are limits to the amount of work I'm willing to do.

[JamesPa]

2 hours ago, Crofter said:

One of the big advantages I see with A2A is the faster response time.

For a house like ours, which is either used by a working family and empty through the day, or at other times used as a holiday let, the current storage heaters are about the worst possible type of heating it could have. I don't think UFH would be the best match either. Anyway, I've got a suspended timber floor and there are limits to the amount of work I'm willing to do.

Slightly off-topic (but only slightly) I've been giving some thought to a related topic recently. 

 

Although A2A is not a good match to my house, air to water + fancoils (which have many similarities - they heat air quickly, shift it around causing dust issues, and make a noise) have specific advantages for heating, which nothing else matches.  In particular they can adjust the amount of energy they deliver without adjusting the flow temperature, so for example can respond quickly and deliver more energy than 'normal' without raising the flow temperature , by turning up the fan speed.  If you want a perception of fast recovery after a night time set back, this is a distinct plus. 

 

I have a hunch that, for the common pattern described by @Crofter above (in during the evening, happy to cool down during the night, want a quick blast around breakfast, out during the day) they might be the ideal emitter if, as many do, you want to economise a bit when not in, whilst keeping overall efficiency high and maintaining comfort.   Fancoils in bedrooms are particularly unpopular, but consider a fancoil which has sufficient emission without the fan to meet night time requirements, then blasts on (but with a slow start up) just before the alarm goes to warm up the air in the bedroom and bathroom for the couple of hours (if that) before everyone goes out to work/school.  

 

Our UK aversion to air-borne heating is understandable, but perhaps not entirely justifiable.  I sense that there is more in this hot air heating thing than first meets the eye.

 

Edited October 10, 2023 by JamesPa

[Beelbeebub]

9 hours ago, JamesPa said:

Slightly off-topic (but only slightly) I've been giving some thought to a related topic recently. 

 

Although A2A is not a good match to my house, air to water + fancoils (which have many similarities - they heat air quickly, shift it around causing dust issues, and make a noise) have specific advantages for heating, which nothing else matches.  In particular they can adjust the amount of energy they deliver without adjusting the flow temperature, so for example can respond quickly and deliver more energy than 'normal' without raising the flow temperature , by turning up the fan speed.  If you want a perception of fast recovery after a night time set back, this is a distinct plus. 

 

I have a hunch that, for the common pattern described by @Crofter above (in during the evening, happy to cool down during the night, want a quick blast around breakfast, out during the day) they might be the ideal emitter if, as many do, you want to economise a bit when not in, whilst keeping overall efficiency high and maintaining comfort.   Fancoils in bedrooms are particularly unpopular, but consider a fancoil which has sufficient emission without the fan to meet night time requirements, then blasts on (but with a slow start up) just before the alarm goes to warm up the air in the bedroom and bathroom for the couple of hours (if that) before everyone goes out to work/school.  

 

Our UK aversion to air-borne heating is understandable, but perhaps not entirely justifiable.  I sense that there is more in this hot air heating thing than first meets the eye.

 

But if you are going the fan coil route, why not ditch the middle man and go a2a multisplit? The head units for those are far more developed and mass manufactured than water based fan coils. 

 

 

[JamesPa]

47 minutes ago, Beelbeebub said:

But if you are going the fan coil route, why not ditch the middle man and go a2a multisplit? The head units for those are far more developed and mass manufactured than water based fan coils. 

 

 

It's a fair question.  The answer is flexibility.   A2W allows a combination of fancoil and conventional radiators.  Also I don't know of a multispectral which handles >6 outlets.

[JamesPa]

6 hours ago, JamesPa said:

multispectral

That was meant to read multisplit.   The joys of predictive text!

[DanDee]

On 06/10/2023 at 04:03, Crofter said:

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...

 

What will be the energy loss under the floor?

[DanDee]

7 hours ago, JamesPa said:

It's a fair question.  The answer is flexibility.   A2W allows a combination of fancoil and conventional radiators.  Also I don't know of a multispectral which handles >6 outlets.

There's multisplit and then there's mini vrf

 

mini vrf units.pdfmini vrf.pdf

[DanDee]

17 hours ago, JamesPa said:

Slightly off-topic (but only slightly) I've been giving some thought to a related topic recently. 

 

Although A2A is not a good match to my house, air to water + fancoils (which have many similarities - they heat air quickly, shift it around causing dust issues, and make a noise) have specific advantages for heating, which nothing else matches.  In particular they can adjust the amount of energy they deliver without adjusting the flow temperature, so for example can respond quickly and deliver more energy than 'normal' without raising the flow temperature , by turning up the fan speed.  If you want a perception of fast recovery after a night time set back, this is a distinct plus. 

 

I have a hunch that, for the common pattern described by @Crofter above (in during the evening, happy to cool down during the night, want a quick blast around breakfast, out during the day) they might be the ideal emitter if, as many do, you want to economise a bit when not in, whilst keeping overall efficiency high and maintaining comfort.   Fancoils in bedrooms are particularly unpopular, but consider a fancoil which has sufficient emission without the fan to meet night time requirements, then blasts on (but with a slow start up) just before the alarm goes to warm up the air in the bedroom and bathroom for the couple of hours (if that) before everyone goes out to work/school.  

 

Our UK aversion to air-borne heating is understandable, but perhaps not entirely justifiable.  I sense that there is more in this hot air heating thing than first meets the eye.

 

 

There is someone out there installing A/W that is looking to make a better fancoil than what is on the market now, that would give a better result than having to oversize radiators.

There is room for better everywhere. 

[Crofter]

1 hour ago, DanDee said:

 

What will be the energy loss under the floor?

Good question, I'd imagine it would be similar to the other option which is running it in the cold loft.

[JamesPa]

2 hours ago, DanDee said:

 

There is someone out there installing A/W that is looking to make a better fancoil than what is on the market now, that would give a better result than having to oversize radiators.

There is room for better everywhere. 

Hopefully there are a few doing this.  The range is currently limited and the price is currently high, other than for very crude models.  It's difficult to believe that it's not possible to do better.

[Beelbeebub]

5 minutes ago, JamesPa said:

Hopefully there are a few doing this.  The range is currently limited and the price is currently high, other than for very crude models.  It's difficult to believe that it's not possible to do better.

Yeah, this is the problem. 

 

Wet fan coils are very bulky and industrial and often very expensive 

 

Refrigerant fan coils are cheaper, better looking and have better performance 

 

There is some physics behind refridgerant fan coils being able to be smaller and more compact and the economics of scale are pretty compelling when it comes to price and choice. 

 

Discreet fan assisted conector radiators like the stelrad version (basically a bunch of PC fans strapped to a conventional radiator) might have some merit. 

[Beelbeebub]

If I'm honest, air to air looks like a better route from a technical POV. 

 

If we could make the VFR layout the default so you just need to pipe up from emitter to emitter it light be a winner. 

 

It would make sense from a national perspective. 

 

A2A systems could be installed without touching the existing gas/oil system and run in tandem. 

 

This negates the "throwing out a perfectly good boiler" argument. It also puts the users mind at rest about "what if it costs more or we have a super cold snap" 

 

The user could then choose whichever system they found best for their comfort, use and cost perspective.

 

It wouldn't necessarily do DHW, but again that might be a plus. There wouldn't be the cost and space issues of fitting a tank. The existing system can trundle on as is and maybe replaced at a later date with a new tank if/when the user is convinced. 

 

The installs are likely to be cheaper and easier too. 

[JohnMo]

6 minutes ago, Beelbeebub said:

better route from a technical POV

Having lived with one (a few) for a couple years, I found them annoying and the wife hated them. As much as the blurb says they don't blow at you, they always seemed to be aimed at someone. A2W gets my vote (radiators or better UFH), A2A doesn't get my vote.

 

14 minutes ago, Beelbeebub said:

A2A systems could be installed without touching the existing gas/oil system and run in tandem

If you want a hybrid system, have a hybrid system, keep the old boiler, most if not all A2W, can be run in hybrid mode, boiler just gets used when it's really cold. Especially useful for a combi.

[Crofter]

1 hour ago, JohnMo said:

Having lived with one (a few) for a couple years, I found them annoying and the wife hated them. As much as the blurb says they don't blow at you, they always seemed to be aimed at someone. A2W gets my vote (radiators or better UFH), A2A doesn't get my vote.

Was this the conventional split unit with a wall mounted emitter?

I wonder if I can find out any information about flow rate/speed to compare with my ducted plan.

[JohnMo]

30 minutes ago, Crofter said:

Was this the conventional split unit with a wall mounted emitter?

Your bog standard multi split with wall units

[Crofter]

2 hours ago, JohnMo said:

Your bog standard multi split with wall units

Most people seem to be singing their praises, it's interesting to hear a dissenting voice. Do you remember what specific unit it was, and was it running at full speed most of the time?