Heat loss calculation check

[JohnMo]

On 21/12/2024 at 00:31, Gordo said:

As air is blown in it is sucked out at a measured / adjusted rate to be left "slightly" pressurise in the house

Sorry this is incorrect. The MVHR in and out flows, are balanced - same is taken out as brought in, so house is neither pressurised or depressurised by MVHR. 

 

MVHR also slowly decreases house temp (assuming colder outside) as the efficiency of the heat exchanger within the MVHR unit will never be 100%. Ventilation heat loss will be based on approximately 10 to 20% of the overall flow rate of the MVHR unit, plus natural ventilation heat losses based on house airtightness. So airtight house, ventilation losses are almost all losses produced by MVHR, leaky house plus MVHR, are MVHR losses plus natural air leakage losses.

[Russdl]

6 hours ago, JohnMo said:

The MVHR in and out flows, are balanced

Our supply is always slightly higher than extract, so theoretically we are slightly over pressured. 
 

[G and J]

53 minutes ago, Russdl said:

Our supply is always slightly higher than extract, so theoretically we are slightly over pressured. 
 

And what percentage accuracy are the air volume measurements do you think?  

[JohnMo]

2 hours ago, Russdl said:

Our supply is always slightly higher than extract, so theoretically we are slightly over pressured. 
 

Why? You will always get less heat recovery!

[Russdl]

6 hours ago, G and J said:

And what percentage accuracy are the air volume measurements do you think?

Absolutely no idea, it’s what the Brink is reporting and I’ve no reason to question the accuracy thus far. 
 

5 hours ago, JohnMo said:

Why? You will always get less heat recovery!

As for the “why” that’s the way it operates and always has but being slightly ‘over pressure’ seems like a good thing to me. Isn’t it? We’re quite airtight but not a sealed box. 
 

I haven’t given this a huge amount of thought until now but surely that slight over pressure will act as an air barrier to any small air leaks we have  - ensuring the air leaks out and not in - so we’d be giving away a little bit of pre warmed cold air instead of welcoming in the little bit of cold air that hadn’t been pre warmed. 

[G and J]

Ummm, what’s wrong with it being balanced? Less leakage either way unless I guess it’s blowing a hooley. 
 

I would look at those readings and, unless I knew their accuracy was better than +/-1%, I’d conclude that the difference is too close to call. 

[Russdl]

7 minutes ago, G and J said:

Ummm, what’s wrong with it being balanced?

Until I gave it a bit of thought a few minutes ago I would have said “nothing” but now having given it a bit of thought I’d say “air curtain”. 
 

Like I say, it’s always been like that, the supply has always been slightly higher than the extract. I have no reason to doubt the data the machine is giving me so perhaps the manufacturer thinks it’s a good thing?

 

Standing by for the contrary views. 

[joth]

Well if the difference is 5m3/h that's about 30 watts for deltaT of 20 ° so not exactly killing ya.  And that's probably just biasing the direction of flow of the natural air changes due to imperfect airtightness. The (pessimistic) MCS method has our house natural air changes at 200 m3/h for comparison 

[Gordo]

On 22/12/2024 at 09:19, JohnMo said:

Sorry this is incorrect. The MVHR in and out flows, are balanced - same is taken out as brought in, so house is neither pressurised or depressurised by MVHR. 

.

Sorry to bust your bubble. But it is indeed correct. Look at your commissioning / design fow rates measurements. If it has been set up / balance properly 

Edited Monday at 15:16 by Gordo

[Gordo]

15 hours ago, G and J said:

Ummm, what’s wrong with it being balanced? Less leakage either way unless I guess it’s blowing a hooley. 
 

House should be slightly pressured. Especially if a open flue heating appliance is fitted. 

Edited Monday at 15:18 by Gordo

[JohnMo]

3 minutes ago, Gordo said:

Sorry to bust your bubble. But it is indeed correct. Look at your commissioning / design

flow rates measurements 

Sorry - no bubble been burst here. Mine is balanced, will stay balanced.

 

Not sure where you are getting your information from, but maybe helpful if you actually start quoting building regs, because I cannot find anything that supports your thought process.

 

2 minutes ago, Gordo said:

House should be slightly pressured. Especially if a open flue appliance is fitted. 

Should you have an open flue appliance in a house with MVHR? Does that comply with common sense, let alone build regs? Open flue equals a requirement to permanently open ventilation when better than 5.0m³/m² at 50 Pa!

 

 

[Gordo]

On 21/12/2024 at 22:58, SBMS said:

Only if airtight and using MVHr. Otherwise It certainly can be. If a house is well insulated and poorly ventilated, then ventilation can easily account for the majority of the heat loss.

Sorry that was a typo error on my part. Ventilation is indeed a huge % of the heat loss in a well insulated house now, not like years ago

[Gordo]

11 minutes ago, JohnMo said:

start quoting building regs

The guidance documents don't cover such detail simply that it should be commissioned against design flow rates. CIBSE will give the design parameters.

[Gordo]

15 minutes ago, JohnMo said:

Should you have an open flue appliance in a house with MVHR?

I wouldn't suggest an open flue appliance with MVHR is a good idea at all. But some people do it. You can do it with a room sealed unit but it needs to be a positive pressure MVHR

[JohnMo]

40 minutes ago, Gordo said:

You can do it with a room sealed unit but it needs to be a positive pressure MVHR

Sorry what is your source information.

 

Or are just making this up as you go?

[Gordo]

55 minutes ago, JohnMo said:

Sorry what is your source information.

 

Or are just making this up as you go?

Do you dispute something it said? What do you dispute and what are your sources for disputing it?

Edited Monday at 17:47 by Gordo

[JohnMo]

5 minutes ago, Gordo said:

Do you dispute something it said?

Yep

 

1 hour ago, Gordo said:

but it needs to be a positive pressure MVHR

Where does this requirement come from?

 

[Gordo]

18 minutes ago, JohnMo said:

Yep

 

Where does this requirement come from?

 

You will find many references to this. Some not so obvious. If there is a negative pressure in the room there would obviously be risk to reduced draft in the flue particularly with open flued. HETAS has identifed concerns. Only a "flue draft interference test" can confirm safe operation and even then it would be unwise not to have positive room pressure. Do you disagree 

Edited Monday at 18:17 by Gordo

[SBMS]

49 minutes ago, Gordo said:

You will find many references to this. Some not so obvious. If there is a negative pressure in the room there would obviously be risk to reduced draft in the flue particularly with open flued. HETAS has identifed concerns. Only a "flue draft interference test" can confirm safe operation and even then it would be unwise not to have positive room pressure. Do you disagree 

ChatGPT disagrees:

 

“When building an airtight house, it is generally recommended to maintain neutral or slightly negative air pressure in a room with a multi-fuel stove. Here’s why and what to consider:

1. Neutral or Slightly Negative Pressure

• Neutral pressure is ideal to prevent backdrafts (where exhaust gases flow back into the room) and ensure safe operation of the stove.
• Slightly negative pressure ensures that combustion gases from the stove are safely drawn up the chimney or vent. However, too much negative pressure can cause drafting issues or make it harder for the stove to operate properly.

2. Key Considerations

• Air Supply: A multi-fuel stove requires a dedicated supply of fresh air for combustion. In an airtight house, this is typically achieved by installing an external air intake directly connected to the stove.
• Ventilation System: If the house has a mechanical ventilation system (e.g., an HRV or ERV), ensure it is balanced to avoid creating excessive positive or negative pressure in the room.
• Chimney Design: The chimney should be properly designed to create a consistent draft, which helps with safe exhaust of combustion gases.
• Room Sealing: The room containing the stove should be sealed from other areas of the house to prevent pressure imbalances caused by other appliances or ventilation systems.

3. Avoid Positive Pressure

• Positive pressure in the room can cause combustion gases to leak into the house instead of exiting through the chimney, which poses safety risks, including carbon monoxide exposure.

admission: I know nothing about this topic, just thought I’d ask ChatGPT. Its responses do make logical sense though. 
 

admission #2 ChatGPT could be wrong. It’s sources might even be this forum 😂

[ProDave]

4 minutes ago, SBMS said:

1. Neutral or Slightly Negative Pressure

• Neutral pressure is ideal to prevent backdrafts (where exhaust gases flow back into the room) and ensure safe operation of the stove.
• Slightly negative pressure ensures that combustion gases from the stove are safely drawn up the chimney or vent. However, too much negative pressure can cause drafting issues or make it harder for the stove to operate properly.

 

I disagree with that and think your chosen AI tool has got it wrong.  If the house is at negative pressure with respect to outside, it will draw air in through any leak path, which if anything will draw air DOWN the flue.

 

 

[Gordo]

25 minutes ago, SBMS said:

ChatGPT disagrees:

Yes I’m afraid you must have got a bad result from chatGPT. Never trust chatGPT without researching it yourself. The key is in your question. This is my question to it “Concerns about MVHR and open flue heating appliances ”

 

this is my output from it

Mechanical Ventilation with Heat Recovery (MVHR) systems and open-flue heating appliances can pose safety and operational concerns if not properly designed and coordinated. Below are the primary concerns and considerations:

---

1. Risk of Backdrafting

Open-flue appliances rely on natural ventilation to expel combustion gases (such as carbon monoxide). MVHR systems create a tightly sealed environment, which may interfere with this process by altering air pressure.

• Negative Pressure: If the MVHR system is not balanced (supply and extract airflows are unequal), it could create negative pressure in the home, drawing combustion gases back into the living space.
• Health Risk: Backdrafting of flue gases can lead to carbon monoxide poisoning.

Edited Monday at 19:37 by Gordo

[SBMS]

12 minutes ago, Gordo said:

Yes I’m afraid you must have got a bad result from chatGPT. Never trust chatGPT without researching it yourself. The key is in your question. This is my question to it “Concerns about MVHR and open flue heating appliances ”

 

this is my output from it

Mechanical Ventilation with Heat Recovery (MVHR) systems and open-flue heating appliances can pose safety and operational concerns if not properly designed and coordinated. Below are the primary concerns and considerations:

---

1. Risk of Backdrafting

Open-flue appliances rely on natural ventilation to expel combustion gases (such as carbon monoxide). MVHR systems create a tightly sealed environment, which may interfere with this process by altering air pressure.

• Negative Pressure: If the MVHR system is not balanced (supply and extract airflows are unequal), it could create negative pressure in the home, drawing combustion gases back into the living space.
• Health Risk: Backdrafting of flue gases can lead to carbon monoxide poisoning.

I think ChatGPT is in itself a pretty good research tool as it can provide a balanced list of reasons. I would say that whilst I agree with the point from @ProDave on air drawn down the flue what ChatGPT is actually advocating is neutral pressure which makes sense. I can see an argument why slightly negative is not desirable to @ProDave’s point. However I can’t see many scenarios in which positive pressure (your argument) is desirable and I think this accords with what @JohnMo was stating. 

Edited Monday at 19:42 by SBMS

[Gordo]

Here’s an extract form AD J which in typical fashion skirts over the subject

[Gordo]

4 minutes ago, SBMS said:

ChatGPT is actually advocating is neutral pressure which makes sense. I can see an argument why slightly negative is not desirabl

I think what chatGPT was referring to here is the pressure within the flue rather than the room. As a neutral or negative pressure in a flue is necessary for safe operation ie gasses will not escape

[Gordo]

Here’s an extract form HETAS guidance regarding MVHR. For @JohnMo benefit so he knows I’m not just making it up. There so many reference materials out there. I can’t think of any benefits in opting for overall negative pressure MVHR

Edited Monday at 20:15 by Gordo