Heat loss ventilation advice required

[ruggers]

I'm unsure if this is best posted in Boffins corner or MVHR being ventilation heat loss related.

 

When planning my self-build heating and ventilation system, I've learned a lot of things, but some are a little out of my grasp to fully understand. It's very theoretical and I'm unsure how some of it plays out in the real world. So, I'll post what I have, and see if anyone with a good understanding of it all can advise me or correct anything I'm wrong with. You can over size or undersize a heating system, but undersizing is a more of a concern to me, especially with under floor heating pipe spacings.

 

The property will be 200m2, masonry cavity wall construction. Without going into every sealing detail I've researched, I'll be applying a lot of attention to airtightness and aiming for the lowest score I can.

I'd like to achieve 2ACH @50PA if possible and will have MVHR installed.

 

I've completed a heat loss survey using some popular software, & tried various things to see how it impacts the end result. Being a self-build, I've been lucky enough to obtain the exact U values for all of the construction materials I'll be using, from the manufacturers, but there's still a couple of things that could have a large affect on my total heat loss.

 

My maximum heat loss will be 5.7Kw, this includes me selecting the exposed area tick box to all rooms with an outside wall. I believe this adds 10% on, so if I removed it, the property could be 5.1 Kw

I selected it to air on the side of caution with it being detached and 3 miles from the NW coast in the lake district, where the wind and rain can be heavy at times.

• I was told that this has less impact on properties with good levels of airtightness, should I leave it on or remove it? 

Airtightness: The heat loss software defaults most rooms to 0.5 ACH, except,

Kitchen 1.5 ACH

Toilet 1.5 ACH

Bathrooms 1 ACH
 

• I believe 0.5 ACH derives from the worst case air pressure test result to obtain a pass.
• Building regs lowest pass limit was 10 ACH@50 PA, (Now apparently 8 ACH) which is terrible, I'd expect an average new build to be around 5, higher standard 2 ACH, and passive house 0.6 ACH.
• 10 ACH @50PA  / 20 = 0.5 ACH. Is 20 the correct figure to divide by?
   
• If I achieved 0.2 ACH @50PA / 20 = 0.1 ACH. This lower air change per hour, input to the heat loss software would severely reduce my property heat loss. Without trying it yet, I'd guesstimate to be 3.5Kw which I'd not feel confident was realistic. 
• If I should reduce the ACH from 0.5, how much do you scale the other rooms by with higher values?
• Do I play it safe and leave the ACH at their defaults, and if wrong, at least it's consistent for all rooms.

 

Once the MVHR design is complete, their room by room air flow rates will likely affect this again, which I've glanced at, but one step at a time.

The MVHR designer doesn't factor in heat loss and heat engineers don't factor in MVHR ventilation rates.

Edited October 31 by ruggers

[Conor]

The mvhr doesn't make that much of an impact. And it's only the overall rate they matters. Airtightness is a bigger factor. We designed for 3ACH, and achieved 1.5. don't be confusing building airtightness with mvhr flow rates. You'll be wanting a 7kW ashp.

[ruggers]

29 minutes ago, Conor said:

The mvhr doesn't make that much of an impact. And it's only the overall rate they matters. Airtightness is a bigger factor. We designed for 3ACH, and achieved 1.5. don't be confusing building airtightness with mvhr flow rates. You'll be wanting a 7kW ashp.

Out otf interest, wha construction was your build to achieve 1.5?

 

I'm aware that the building airtightness and MVHR flow rates are separate things, and them flow rates also have to factor in around 90% of the ventilation will have the heat recovered from it. I've attached an MVHR pre design estimate from one company some time ago showing flow rates, it does have a mistake on it for at least one room. If it's so negligible, I'm happy to ignore that part. MVHR is excluded from the heat loss survey.

But the airtightness of the building which won't be known until it's built and tested, will make a big difference in reducing the total heat loss if the correct ACH are input, rather than the softwares default ACH used which assumes the worst case air tightness result of 10. Sizing a heat pump should be based on an accurate heat loss survey otherwise you face short cycling issues in the shoulder months around 12 to 15C outdoors. The 5 KW heat pump can modulate down lower, Ive had mixed answers of 1.2 and 1.5Kw. 7Kw model is around 1.7Kw

 

Edited October 31 by ruggers

[ruggers]

Just to try it, I've removed the exposed area 10% increase option and halved all of the ACH for each room and it's reduced the total heat loss from 5.7Kw to 4.22Kw. 

It's such a difference and if wrong left with a cold house or an over sized heating system.
 

One of my larger UFH rooms has more glazing, so it's at the higher end of scale for maximum W/m2 floor output required at low flow temperature. I can't add more floor insulation, so it requires better pipe spacings, reduced heat loss or higher flow temperature. That's digressing from the OP though.

[Nick Laslett]

21 minutes ago, ruggers said:

“But the airtightness of the building which won't be known until it's built and tested...”

 

“…Sizing a heat pump should be based on an accurate heat loss survey…”

You have hit the nail on the head. You face an unsolvable conundrum. You don’t know your airtightness until you finish the build, therefore you don’t know your heat loss, so how can you size your heat pump. And for low temp heating in an insulated foundation you have to put in your UFH pipe loops at the very beginning of the build process. 
 

You essentially have to take a huge leap of faith that it will all work, and commit the cost and materials on that promise. Insulated foundation, MVHR ducting, ASHP flow and return. These all shaped my house more than any other factors. 
 

I struggled with this dilemma very soon after finding BuildHub. The curse of the more you know, the less sure you are. Ignorance is bliss!
 

Most of my build decisions have been predicated on how to achieve good airtightness, how to reduce ASHP short cycling and how to stop the house overheating. 
 

PPHP and a good consultant will solve this, but like every other aspect of the build, if only you could click your fingers and have people you can trust work for you. I only recently discovered that the MBC guys that did my foundation only put in 2/3rds of the UFH piping that was on my LoopCAD drawings. This destroys all of my heatloss calculations and ASHP decisions. How can I go back and undo this, when I can’t rip up a concrete floor and put back the missing UFH pipes?

 

You just have to live with the mistakes. 

Edited October 31 by Nick Laslett

[Conor]

A few basic things in reality Vs model.

 

Your heat load is worst case, constant over 24hrs. If it's 5kW, and you've a 5kW heatpump, it won't be enough. Purely because the sCOP (and other inefficiencies, needing to heat water etc) will be less than the label, and you won't be putting enough energy in to the building. You'll need more like 6 or 7.

 

You'll likely run the building as a single zone (or one per floor, like we do) adjusting individual room temps be tweaking flow temps.

 

You might add a buffer. Your mvhr will modulate down when really cold, to protect against low humidity.

 

You probably won't want to run your heating 24x7 (which these models assume). We run ours for 7 hours on economy 7.

 

These things are hard to model. And the reality is you have to just go with close enough, and stop worrying about decimal places.

 

Our peak heat load is something like 5.6kW. but we've no curtains on the big glazed areas, so its probably higher.

 

I went for a 9kW heat pump that modulates down to 2.5kW. it's perfect. When it's really cold, I just adjust the timer so it's on 20 hrs a day, rather than 7. Room stat and weather comp takes care of the rest.

 

If you end up with too big a heatpump, you can easily compensate with behavioural changes. E.g, fewer zones, running for shorter periods of time, buffer tank, and still hit good performance efficiencies. 

 

If you end up with an undersized heatpump, there is literally nothing you can do to compensate in those challenging periods. Only option is supplemental heating. Defeating the purpose of an efficient heating system.

 

You need a 7kW heatpump.

[IanR]

1 hour ago, ruggers said:

• Building regs lowest pass limit was 10 ACH@50 PA, (Now apparently 8 ACH) which is terrible, I'd expect an average new build to be around 5, higher standard 2 ACH, and passive house 0.6 ACH.

 

It's actually 8m³/m².h@50Pa rather than ACH. Each house has a different relationship between the two so you'd need to calculate the ratio for your house. (PH is 0.6 ACH as you say)

 

Supposedly, the Building Regs change due 2025 (if Labour continues with the plan) for the future homes standard will drop to 5m³/m².h@50Pa.

 

1 hour ago, ruggers said:

• 10 ACH @50PA  / 20 = 0.5 ACH. Is 20 the correct figure to divide by?

 

20 is used for an unexposed area, 14 tends to be used for an exposed area.

 

1 hour ago, ruggers said:

• I was told that this has less impact on properties with good levels of airtightness, should I leave it on or remove it? 

 

I'd keep it, but use the "rule of 14".

 

But, infiltration losses should be based upon: (air permeability (m³/m².h@50Pa) / 14) + (MVHR flow rate (m³/m².h@50Pa) * 10%)

 

The higher insulation and air permeability you target the greater the proportion of losses due to thermal bridging. Have you attempted to account for thermal bridge losses for your construction type? Most heat loss calculators include a "standard" psi value for typical construction methods, if you are mitigating thermal bridges you should adjust for this.

[G and J]

4 minutes ago, Conor said:

You need a 7kW heatpump.

Or how about a 5kW heat pump and a woodburner?   ……….runs for cover while giggling!

[G and J]

21 minutes ago, Nick Laslett said:

I only recently discovered that the MBC guys that did my foundation only put in 2/3rds of the UFH piping

How the bloody hell did you discover that?

[Conor]

18 minutes ago, G and J said:

Or how about a 5kW heat pump and a woodburner?   ……….runs for cover while giggling!

I just Invite a few friends round for dinner. Few adults and a load of kids running around the house easily adds a kilowatt or so.

[G and J]

1 minute ago, Conor said:

a load of kids running around the house

OK I give in, I’ll buy a bigger ASHP so I don’t have to put up with the noise.

[ProDave]

What software are you modeling your heat loss with?

 

There is a simple heat loss spread sheet on this forum written by a well respected former member.  I used this to predict the heat loss of my new house, and it's prediction of a worst case heat loss of just under 2.5kW turned out the be absolutely spot on.

 

As built air tightness is all down to attention to detail,  all build methods can achieve good air tightness if you detail things properly.  There is no such thing as a design air tightness. You don't deliberately design to have poor air tightness, except in rare cases where for some strange reason you don't want to fit an mvhr unit so you deliberately make sure your house is full of leaks so building control don't insist on you installing mvhr.

 

In short you are approaching this wrong.  Plan to build an air tight house, plan to detail all aspects well to achieve this, and plan to install mvhr.

[JohnMo]

Basically as @ProDave says use the spreadsheet on here.

 

5.7kW sound massive, which would indicate you have something very wrong, for that sized house. Mine is about 3kW at -9. And heat meter shows that to be true, and matches the spreadsheet almost spot on. I don't have the best MVHR, best airtightness, a rubbish form factor, and loads of glass.

 

2m³/m² @ 50 Pa, is at a very elevated pressure, compared to real life (high winds from every direction at the same time).  Using SAP calculation that becomes an infiltration rate of 0.1358. Will come back to that later 

 

So your MVHR will have an air change rate of 0.5ACH. the MVHR unit will have a 80 to 90% efficiency.  So effective air change rate drops to 0.075, plus 0.1358 infiltration rate. So 0.2. Use that figure in your heat loss calculation for ventilation rate and you won't be far off. But once built you are likely to reduce the MVHR flow rated to about 0.3 other wise you end up over ventilating the house, so the figures are likely to reduce a little.

1 hour ago, Nick Laslett said:

don’t know your heat loss, so how can you size your heat pump. And for low temp heating in an insulated foundation you have to put in your UFH pipe loops at the very beginning of the build process.

You do the calcs, in Scotland you have to do it as part of the planning process, they even want my UFH design and supporting calculations.

 

Good thing with UFH and well insulated is that wide or narrow pipe spacing makes very little difference to flow temperature. If I installed 100mm centres or as I did 300mm centres, it adds about 2 degs to flow temp. 300mm centres is just little less responsive, but heating is always on low, so who cares. When you get around the calcs you will end up with a 5kW heat pump. 

2 hours ago, ruggers said:

The MVHR designer doesn't factor in heat loss and heat engineers don't factor in MVHR ventilation rates.

No need for MVHR to factor in heat loss he is just adding ventilation.

 

Heat engineer that doesn't factor in  MVHR heat recovery, should just be sacked and told to get off the project. He is not fit for purpose.

1 hour ago, ruggers said:

One of my larger UFH rooms has more glazing, so it's at the higher end of scale for maximum W/m2 floor output required at low flow temperature. I can't add more floor insulation, so it requires better pipe spacings, reduced heat loss or higher flow temperature. That's digressing from the OP though

Just design it with loopcad. Our living room has 7m wide and 6m tall at the peak of glazing at one end, it's the warmest room of the house on 300mm centres, because its designed to be the warmest. It's designed to be 21, when bedrooms areas are designed for 17/18 at the same flow temp.

 

Get the basics in order

Proper heat loss calculations use the one in boffins corner.

Once you have your head in that headspace everything else starts to slot in to place.

[ruggers]

3 hours ago, Nick Laslett said:

I struggled with this dilemma very soon after finding BuildHub. The curse of the more you know, the less sure you are. Ignorance is bliss!
 

Very true this.

I know that my air test will be under 3 thats a start, and doubt it will be under 2, if it is its a bonus. 

The UFH can be installed once fully weather tight since it's going on top of the pir on the beam and block floor. I'm going to try loop CAD but have found a company who are willing to design snail pattern to match room by room heat loss and consider floor finishes, pipe spacings and usable floor space including weather comp and maximum MWT.

I'll be installing everything except the plant room.

 

@Conor a 5Kw heat pump can output 6.3Kw so it's more than whats required, but slower reheating water. I'm focussed more on the days when its warmer outdoors and requires minimum modulation, but do want the heat survey to be accurate because I don't want to size my radiators too small.

 

 

[ruggers]

3 hours ago, IanR said:

I'd keep it, but use the "rule of 14".

 

But, infiltration losses should be based upon: (air permeability (m³/m².h@50Pa) / 14) + (MVHR flow rate (m³/m².h@50Pa) * 10%)

 

The higher insulation and air permeability you target the greater the proportion of losses due to thermal bridging. Have you attempted to account for thermal bridge losses for your construction type? Most heat loss calculators include a "standard" psi value for typical construction methods, if you are mitigating thermal bridges you should adjust for this.

Thanks for the answers.

Using the rule of 14, at the old air permeability lowest pass rate of 10m3/m2.h would mean that the ACH would be 0.7, which would have increased the property heat loss compared to 0.5 ACH.
If I achieve 2m3/m2.h@50PA / 14 it would mean using a value of 0.14 ACH instead of 0.5 default.

Which rooms can I apply 0.14 ACH to, the wet rooms (Kitchen bathroom toilets) all require higher ACH of 1 or 1.5, as listed in my OP

infiltration losses should be based upon: (air permeability (m³/m².h@50Pa) / 14) + (MVHR flow rate (m³/m².h@50Pa) * 10%)

Do you have a example for this formula using my predicted air permeability score of 2 for the house, and also, what's the *10% for at the end?
The heat loss survey has 1.5 ACH for the kitchen, so I need to know how to change this result?

[ruggers]

3 hours ago, ProDave said:

What software are you modeling your heat loss with?

Heat engineer

3 hours ago, ProDave said:

 

There is a simple heat loss spread sheet on this forum written by a well respected former member.  I used this to predict the heat loss of my new house, and it's prediction of a worst case heat loss of just under 2.5kW turned out the be absolutely spot on.

I'll have to look through my private messages, I think I was sent it a log time ago but couldn't get my head around it, the software seemed easier to use from memory.

 

I'll be parge coating blockwork, setting windows and doors back, air tight tape, blower paint, minimising thermal bridging with as many places as possible, not having a door letterbox, masonry hangers, internal thumb lock doors etc.

But my insulation won't be to the levels you'll have in yours in Scotland, the budgets too tight now too.
 

I'm not sure how I'm approaching it wrong? I've detailed I aim to build to an airtight standard, minimise thermal bridging, Insulation is the most I can get in with adding excessive amounts of cost for small gains or screwing up the thermal mass adding internal insulated plasterboards. I'm just wanting to ensure my heat loss results are based on the correct ventilation info, and then I can obtain all of my quotes for the everything heat related.

[ruggers]

2 hours ago, JohnMo said:

2m³/m² @ 50 Pa, is at a very elevated pressure, compared to real life (high winds from every direction at the same time).  Using SAP calculation that becomes an infiltration rate of 0.1358. Will come back to that later 

 

So your MVHR will have an air change rate of 0.5ACH. the MVHR unit will have a 80 to 90% efficiency.  So effective air change rate drops to 0.075, plus 0.1358 infiltration rate. So 0.2. Use that figure in your heat loss calculation for ventilation rate and you won't be far off. But once built you are likely to reduce the MVHR flow rated to about 0.3 other wise you end up over ventilating the house, so the figures are likely to reduce a little

Theres too many figures and a bit too confusing for me to calculate. MVHR ACH, Heat loss survey ACH which very depending on the room use from 0.5, 1, 1.5 ACH. The lower the number the more it reduces the heat loss for the room. then theres the air permeability score too.

 

Are you saying 2m³/m² @ 50 Pa is much higher than a natural windy day? If so, why are they testing at such a rate?

The MVHR I'll be installing, but it will be designed by ADM and commissioned by them. They calculate it all to comply with English regs and satisfy B.C

2 hours ago, JohnMo said:

Heat engineer that doesn't factor in  MVHR heat recovery, should just be sacked and told to get off the project. He is not fit for purpose.

MVHR isn't to be considered in any heat loss calculation I've been told by a lot of heat engineers and read it online. Reasoning being, the building has to be be sized to heat from cold when theres no heat to recover. Including MVHR in the calculations would result in smaller heat emitters and it would take much longer to reach desired temperature from cold.

 

2 hours ago, JohnMo said:

5.7kW sound massive, which would indicate you have something very wrong, for that sized house. Mine is about 3kW at -9.

It's 5.1Kw without exposed area, I can't see it being lower than this. You have massive amounts of insulation compared to what I'm putting in. 

Mine will be: 100mm T&G PIR in the cavity, taped and self installed. 0.17 W/m2k

400mm loft insulation 0.11 W/m2k

170mm PIR floor 0.12 W/m2k

And intermediate floors and stud walls too which I've added, but it makes no difference when all rooms are sized the same for 21C at -4.4C with a MWT of 37.5C

 

A friends family had an identical house size & layout, built in 2006, it never had MVHR, will have had 50mm PIR vs my 100mm, and probably poor air tightness, they always had the heating on, but said it cost a fortune to run.

[JohnMo]

6 hours ago, ruggers said:

ACH which very depending on the room use from 0.5, 1, 1.5 ACH.

This very much is for a normal leaky house without MVHR.

 

6 hours ago, ruggers said:

Are you saying 2m³/m² @ 50 Pa is much higher than a natural windy day? If so, why are they testing at such a rate?

Possibly test at 50Pa to get a meaningful repeatable result for all houses.  Any lower may not get anything meaningful any higher not needed.

 

6 hours ago, ruggers said:

MVHR isn't to be considered in any heat loss calculation I've been told by a lot of heat engineers and read it online. Reasoning being, the building has to be be sized to heat from cold when theres no heat to recover. Including MVHR in the calculations would result in smaller heat emitters and it would take much longer to reach desired temperature from cold.

Still a flawed assumption. It assumes you have house at ambient outside temp and its the coldest day and you never recover any heat until until at full temp. As dT increases between outside and inside temps, the heating load increases.  Its when you get to an inside temp of say 21 and outside at say -3 when your heat demand is highest.

 

Your real heat loss is around 3kW, but as said above you will most likely install a 5kW heat pump, anything bigger will be harder to manage.

6 hours ago, ruggers said:

MWT of 37.5C

I would doubt you will actually flow that high, your MWT will be closer to 30/32.  

[IanR]

7 hours ago, ruggers said:

Using the rule of 14, at the old air permeability lowest pass rate of 10m3/m2.h would mean that the ACH would be 0.7, which would have increased the property heat loss compared to 0.5 ACH.

 

It would mean 0.714m³/m².h, rather than ACH, natural ventilation. For total ventilation you'd then need to add the MVHR ventilation (converting your 0.5ACH to m³/h). Or you need to convert the air permeability target units to ACH and add to the 0.5 ACH of the MVHR.

 

7 hours ago, ruggers said:

infiltration losses should be based upon: (air permeability (m³/m².h@50Pa) / 14) + (MVHR flow rate (m³/m².h@50Pa) * 10%)

 

Apologies, I shouldn't have included the "@50PA" on the MVHR figure, it would be just the m³/h.

 

The *10% is to account for the 90% heat recovery of a PH certified MVHR.

 

7 hours ago, ruggers said:

Do you have a example for this formula using my predicted air permeability score of 2 for the house, and also, what's the *10% for at the end?

 

If we're keeping everything in ACH, and the target is 2 ACH, then total house loses from air flow is the 0.2 ACH (from natural ventilation) + (0.5 ACH * 10% (from MVHR)). If the heating engineers don't consider MVHR then then I'd give them this value.

[physical air permeability testing (blower door test) in the UK tends to provide results in the m³/m².h@50Pa units, as that's what building regs uses, so you need to work out how to convert between them and ACH for your property]

 

8 hours ago, ruggers said:

Which rooms can I apply 0.14 ACH to, the wet rooms (Kitchen bathroom toilets) all require higher ACH of 1 or 1.5, as listed in my OP

 

I'd ignore natural ventilation for the MVHR figures. The building regs airflow requirements are a minimum, so base those on MVHR only.

 

I'd only consider natural ventilation figure within the total house losses, when sizing the heating system.

[Mike]

11 hours ago, ruggers said:

I know that my air test will be under 3 thats a start, and doubt it will be under 2, if it is its a bonus. 

If you are building a new house, it's entirely possible to achieve well below 2. As you mentioned in your first post, the Passivhaus standard specifies a maximum of 0.6 ACH and thousands of those have been built. I've seen air test results mentioned below 0.2, though that is exceptional.

 

However achieving that does depend an a high standard of work and attention to detail that you'll only get by using a well trained staff with hawk-eyed supervision (preferably your own). If you're using a single main contractor then you could specify achieving that (0.6 ACH @ 50 Pa) as an essential requirement. If you split it into packages, then you may literally need to take matters into your own hands, and allow enough time for it.

 

3 hours ago, JohnMo said:

10 hours ago, ruggers said:

MVHR isn't to be considered in any heat loss calculation I've been told by a lot of heat engineers and read it online. Reasoning being, the building has to be be sized to heat from cold when theres no heat to recover.

Still a flawed assumption. It assumes you have house at ambient outside temp and its the coldest day and you never recover any heat until until at full temp.

+1.

 

On top of that, your UFCH will result in fairly constant temperature throughout the day, even if you heat it only during off-peak hours. It's not at all like a radiator system where the heating comes on for a few hours in the morning and evening and needs to respond quickly to achieve the required temperatures, before temperatures drop significantly while it's off. It's not sensible to design the system to be sized to heat the building from cold within X hours; instead it need only be sized to maintain normal temperature on the coldest day of the year. That does mean that if you move in in December you may need to plug in some electric radiators for a while, but after that you should be good.