Jump to content

Typical PHPP Values for ventilation.


Donegalsd
 Share

Recommended Posts

Hi,

 

I am in the middle of building a timber-frame house (awaiting first fix electrics and plumbing).

 

The recent hot spell has convinced me to get brise-soliel on the south facing upstairs windows. I have them planned for downstairs already.

 

But I played around with PHPP and although my overheating reduced it didn't disappear.

 

My phpp consultant has no figures inputted for L31 window ventilation air change rate and P59 Window night ventilation (in summer vent sheet)

 

Has anyone got 'typical' figures for these?

 

I have used 0.35 and 0.15 respectively and my overheating drops down to 3.9% (with all southern windows shaded).

 

Apart from fitted brise-soliel there isn't much else I can do as all doors windows are fitted and insulation is in place - so I just want to check if the ventilation figures are reasonable.

 

Thanks

Link to comment
Share on other sites

Ours uses 0.25 for L31 and 0.0 for P59.   What value do you have for L20, I have 0.3?  Also check R22 is checked.

 

WARM suggests stress-testing with:

- minimum user operated summer shading

- MVHR operating in summer at its background rate

- no natural vent during the day

- 0.1ACH due to night ventilation.

 

In my opinion not allowing for any window ventilation air changes may be OK for stress-testing, but is completly unrealistic otherwise. It's bascially saying that in the summer you'll never open any windows or doors!  If I make this value zero in our model then the cooling demand doubles.   Your PHPP consultant should know better than to leave this empty if you want any kind of realistic overheating number.

 

On the otherhand night-time window ventialation in zero, may be a reality if for example you live in a noisy area.

Edited by Dan F
  • Like 1
Link to comment
Share on other sites

Thanks Dan,

 

You gave me the initial figures - I was reluctant to fit brise-soleil upstairs at that stage.

 

The scaffolding shades the downstairs windows pretty well and it was comfortable downstairs over the past week but overheating upstairs.

 

My useless laptop is shut down, I'll look tommorow and report back.

 

 

Link to comment
Share on other sites

12 hours ago, Dan F said:

What value do you have for L20, I have 0.3?  Also check R22 is checked

L20 is 0.39, R22 was not checked - it is now, but it doesn't change the cooling demand or load or frequency of overheating. I guess if the problem is excess glazing I won't be using MHRV to solve it.

 

12 hours ago, Dan F said:

In my opinion not allowing for any window ventilation air changes may be OK for stress-testing, but is completly unrealistic otherwise. It's bascially saying that in the summer you'll never open any windows or doors!  If I make this value zero in our model then the cooling demand doubles.   Your PHPP consultant should know better than to leave this empty if you want any kind of realistic overheating number.

I agree. I guess he essentially has given me a stress test model. If I have a 3kW cooling capacity (through ASHP) then all overheating dissapears and the house is PHPP compliant. It ignores the risk of room specific overheating though.

 

But the underfloor loops will only really work downstairs - Adding Brise-soliel upstairs reduces overheating risk from 8% to 3.9% (b4 active cooling) Assuming I can reasonably use the ventilation figures from 1st post. 

 

12 hours ago, Dan F said:

On the otherhand night-time window ventialation in zero, may be a reality if for example you live in a noisy area.

We are out in the countryside, noise won't be any issue so I can maximise night ventilation when necessary.

Link to comment
Share on other sites

3 hours ago, Nickfromwales said:

Solar reflective glazing is very effective. I was quite impressed by just how much impact that had on nuisance solar ‘intrusion’. 

Hi Nick,

 

I just had a look - There is a company in Ireland doing it. 

 

It's going to a bit of a pain to have fit it retrospectively - we had modeled lower gain glass but we started to run into problems with heating load going above pH levels.

 

I guess if I wasn't going to stick to pH recommendations for glazing proportions then I should have been willing to accept the need to heat the house by a tiny bit more. 

 

With reduced solar gain in summer my cooling load would have reduced and the total energy use would have been similar.

 

Oh well.

 

I'll get brackets for brise-soleil fitted and I can decided next year if I need the canopy itself.

 

The brackets are black against black cladding so shouldn't be seen even if no canopy fitted.

 

Pic below of downstairs brackets 

IMG_20210720_212354_412.jpg

IMG_20210719_211516_241.jpg

  • Like 1
Link to comment
Share on other sites

We have some large west facing windows on a passive house in Berkshire so can get quite a blast of summer heat. East and south glazing all have external motorised blinds.

 

We're learning to manage it on those very hot days- key is

  • cooling house overnight with stack ventilation (have velux in atrium roof) and closing windows to keep cool air in.
  • keeping east blinds down until sun goes overhead
  • pulling over some internal linen drapes on the west windows when the sun is on that side
  • once its early evening, can relax window opening and drapes

We considered a bris soleil or pergola to give more western shade and remove the need to use drapes but also thinking on solar reflecting film.

 

Link to comment
Share on other sites

3 hours ago, Donegalsd said:

But the underfloor loops will only really work downstairs

 

The MVHR will help distribute temperature around the house but the first floor will likely always be slightly warmer on the first floor.  Most people live with this using night-time ventilation and with the bedrooms slightly warmer, but depending on the numbers in your stress model and what you comfort requirements are, it may make sense to consider either fancoils or MVHR cooling for the first floor for future-proofing and/or as an insurance policy. 

 

Typically cooling via MVHR isn't advisable or practical as the cooling power it can distribute is very low but, if you have i) external shading to protect from majority of solar gains ii) a trust-worthy PHPP model to give you a fairly accurate idea of what your cooling demand is, I would argue that it's easier than adding fancoils to each bedroom and plumbing cold water to them.

 

Link to comment
Share on other sites

29 minutes ago, Dan F said:

Typically cooling via MVHR isn't advisable or practical as the cooling power it can distribute is very low but, if you have i) external shading to protect from majority of solar gains ii) a trust-worthy PHPP model to give you a fairly accurate idea of what your cooling demand is, I would argue that it's easier than adding fancoils to each bedroom and plumbing cold water to them.

Yes, I was chatting to the builder today.

 

I could fairly easily retrospectively fit ducts from a fan coil unit to 3 of my bedrooms and my upstairs stair landing (with some false ceilings at the entrance lobby/doorway to bedrooms).

 

I will run a chilled water loop up to the mhrv room and then I can fit a duct cooler and or fancoil unit if necessary.

 

Can one fan coil unit be ducted to supply 3 or 4 rooms?

 

If using MHRV (and duct cooler unit) would I need to insulate to MHRV ducts? (I'm too late to do that now).

 

Or would the low heat capacity of the air in MHRV not really cause condensation problems?

 

For both, I guess I would need to specify a condensate drain to be fitted nearby?

 

 

Link to comment
Share on other sites

5 hours ago, Bitpipe said:

cooling house overnight with stack ventilation (have velux in atrium roof) and closing windows to keep cool air in.

Hi Bitpipe,

 

I am hoping this will work for me.

 

I have no Velux or atrium but I have a two large tilt only upstairs on the North aspect- one at the head of the stairs. And a few more smaller casement windows in the bedrooms - so hopefully I can still get a breeze running through.

 

Thanks for suggestions.

Link to comment
Share on other sites

2 hours ago, Donegalsd said:

If using MHRV (and duct cooler unit) would I need to insulate to MHRV ducts? (I'm too late to do that now).

Yes, but only on the first-floor if you are only using it for first floor, also only for supply ducts not extract ducts.  Not sure how uneffective it would/wouldn't be without insulation.  It's not only about avoiding condensation, but also ensuring the right place (the rooms) get cooled and not e.g. loft space.  The duct cooler unit needs a condensate drain yes.

 

I'll let @joth or others comment on the fan-coil based approach.

Link to comment
Share on other sites

18 hours ago, Donegalsd said:

Can one fan coil unit be ducted to supply 3 or 4 rooms?

 

It can, if sized appropriately and with a manifold to distribute to them all. ( I got a bespoke manifold from here at a  reasonable price: https://www.ductstore.co.uk/acatalog/3_Spigot_Plenum_Box.html)

The challenge with this approach is balancing the airflow in the rooms. If they have different degrees of air restriction back to the return plenum of the FCU, the less restricted rooms will "hog" all the air flow and the more sealed off room not get any. An 8mm door undercut is not necessarily enough, and if some rooms like to sleep with door open and others don't, or you want to turn on/off flow to rooms depending on usage or temperature preference, you'll need some way to actively manage the air flow using duct values , air flow monitors, and actuators that quickly add up in cost not to mention time spent tuning it all. (It is of course an problem - countries where "central air [con]" is standard know well how to have a single furnace/heat exchanger and blower drive each room. Tends to be left to the user to adjust the manual outlet vent diffuser per room)

 

For this reason if starting over I'd definitely plan on one FCU per room. They of course could be co-located in a loft void (e.g to simplify the flow and condensate plumbing), which is how I'll go if I do retrofit to this level. They can even share one water circulation pump, just turn on/off or adjust speed of each FCU individually to control flow and hence temp of the associated room.

 

 

 

 

 

Link to comment
Share on other sites

  • 3 weeks later...
On 28/07/2021 at 12:54, joth said:

If they have different degrees of air restriction back to the return plenum of the FCU, the less restricted rooms will "hog" all the air flow and the more sealed off room not get any

@joth

OK, do I understand correctly that in practice and in your experience - it is not necessarily the volume of cooled airflow to the room that will make a difference to the cooling experience but the rate of return of this cooled air that is important?

 

Or are you saying both flow and return are important and it is important not to overlook return?

 

Not sure if this clarification adds any clarity ?

 

Link to comment
Share on other sites

On 17/08/2021 at 17:46, Donegalsd said:

@joth

OK, do I understand correctly that in practice and in your experience - it is not necessarily the volume of cooled airflow to the room that will make a difference to the cooling experience but the rate of return of this cooled air that is important?

 

Or are you saying both flow and return are important and it is important not to overlook return?

 

Not sure if this clarification adds any clarity ?

 

Disclaimer: I'm just an amateur messing about with one fan coil. 

My experience is the smaller of the supply and return flow will define performance.

My main point is if a single FCU feeds two rooms, you need to balance them. Otherwise it's very likely one will get more cooling . A FCU per room makes things simpler. 

Link to comment
Share on other sites

On 17/08/2021 at 17:46, Donegalsd said:

@joth

OK, do I understand correctly that in practice and in your experience - it is not necessarily the volume of cooled airflow to the room that will make a difference to the cooling experience but the rate of return of this cooled air that is important?

 

Or are you saying both flow and return are important and it is important not to overlook return?

 

Not sure if this clarification adds any clarity ?

 

The supply and return flow rates need to be in as close to equilibrium as is practicable, otherwise the rate of air changes will not be optimised. If any hint of cooling is to be realised, this needs close examination and to be executed robustly. Elevated flow rates associated with 'cooling' or 'heating' bring with them a few bugs also, as in the audibility of the system when operating will be higher than that of domestic MVHR.

For this type of install I usually fit 2x 92mm ducts to 2 separate air valves for supply, and then the same for return, with the cooled air at ceiling height and the return as low as is practicable. This promotes quieter operation and equilibrium in the room, which in turn preserves normal function of the MVHR system which would be running in the same room in parallel ( and must stay in balance as per its condition when commissioned ).

The fan that provides air flow for this cooling should be room sealed to these ducts, both on the supply side and the return side to make sure the MVHR balance isn't compromised.

 

  • Like 1
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...