Cold Air

[Fiona Brick]

We have an MVHR unit installed in a new build - 220 sq. m, A rated house with an air tightness result of 0.882. 

 

The house is designed like two bungalows with a central hall. One section of the house is emitting cold air as low as 9 or 10 degrees when at the very same time the air being emitted by the vents in the rest of the house is 19 or 20. This happens when the machine is in standby due to reaching the required temp, when the system is in water heating and when the external temp drops below 8 or 9. The air being emitted in one section never really changes much while in one large section we are observing huge temperature differentials. 

 

It is a new system. We have gone back to the supplier and have been told this is normal, we should have expected this. 

 

What are your thoughts?

 

 

 

 

[Mr Punter]

What is the make and model of the system?  I have not heard of a MVHR that anything to do with water heating.  Is there a single unit for the whole house?

[JohnMo]

Do you have one or two MVHR units?

 

Do you have a post heater on all outlets or just some?

 

Assume you don't mean the MVHR goes into standby just the heat source?

 

Is this the only heating you have or do you have normal heating as well?

[Fiona Brick]

One MVHR.

No post heaters.

Yes, just air heating in standby when set temp is reached, ventilation still operating. 

This is currently our only heat source. We always knew we would need supplementary heating and we are looking into these options. 

However my concern is that in current circumstances the supplementary heating won't be kicking in and the bedrooms were emitting 9 degree air when going to bed last night. 

Were trying to figure out what we need to do. 

Thanks for your reply. 

[JohnMo]

Bit confused

 

  On 29/03/2025 at 09:29, Fiona Brick said:

No post heaters.

Yes, just air heating

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What is doing the air heating and how is it being done?

 

Is MVHR outlet temperature you are discussing or something else.

 

What is your actual heat source - gas, heat pump or direct electric?

[Fiona Brick]

The MVHR unit has a counter flow heat exchanger and an integral 2kw heat pump. The integral heat pump heats the water and the supply air.

 

Obviously as a 2kw system, we never expected it to meet out full heat demand albeit it wouldn't be far off.

 

However the air supply temperature differentials in one part of the house v's the other is what is causing concern. Any additional heat sources will struggle when operating against an often constant flow of cold air. This is the issue we are trying to address and solve. 

[Redbeard]

  On 29/03/2025 at 09:06, Fiona Brick said:

One section of the house is emitting cold air as low as 9 or 10 degrees when at the very same time the air being emitted by the vents in the rest of the house is 19 or 20.

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  On 29/03/2025 at 09:06, Fiona Brick said:

We have gone back to the supplier and have been told this is normal, we should have expected this. 

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Surely some sort of balancing issue? I would be asking the supplier exactly why they believe  that a 10 degree difference in supply air temp is to be 'expected'. I would then expect them to point to design specifics if they are to explain why.

[MikeSharp01]

So are there two branches off the MVHR and are there two heat batteries, the things the water flows through and the air passes over on on its way to the rooms - one on each branch? The counter flow heat exchanger deals with the heat recovery bit of the job. If there is only one branch / heat battery then the problem must be a flow rate / balancing issue because the difference cannot be explained any other way I can see and even then its a long shot so something odd at the very least.

[Fiona Brick]

I should have explained that the run from the machine to the areas where we are experiencing cold air is a relatively long run and passes through a cold attic. My guess is inadequate/poor insulation on the pipes and manifolds in the attic and/or a miscalculation at design stage of the heat loss per linear m. 

 

This is what I am requesting the supplier/installer to investigate and remedy. 

 

I guess what I'm trying to ascertain is, is it normal or to be expected in anyone's experience that there would be such a large temperature differential?

[Mike]

  On 29/03/2025 at 12:11, Fiona Brick said:

is it normal or to be expected in anyone's experience that there would be such a large temperature differential?

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If there's a long run of poorly insulated duct through a cold attic, then that would certainly be the expected result.

[JohnMo]

  On 29/03/2025 at 12:11, Fiona Brick said:

My guess is inadequate/poor insulation on the pipes and manifolds in the attic

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Think you have answered your own question. The only way two different areas can have two different supply temps is if the heat is being sucked away on one run compared to another.

 

Your installer has done a rubbish job of duct insulation.

 

Our longest duct is around 20m as the crow flies, going through our roof insulation and there is about 200mm+ all around with no issues.

[Redbeard]

Oh yes, poor insulation and a cold void will do it! (a) The supplier should come back and remedy that (unless insulation of ducting was specifically 'by others') and (b) What did they say/promise? I'm trying to establish whether they in effect gave you a 'performance specification'. If so, the 'actual' has not met that spec. You say that you did not expect the MVHR to provide for your full heat load, but not too far off. What did they tell you it *would* provide?

[Fiona Brick]

Thanks for that John. This is exactly what I was thinking having observed and monitored the system in different operating states and external weather conditions. Thankfully the house is performing well so just need to get this aspect of things addressed by the supplier/installer. 

[Fiona Brick]

  On 29/03/2025 at 13:35, Redbeard said:

The supplier should come back and remedy that (unless insulation of ducting was specifically 'by others')

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Ducting is by the same person, our supplier/installer. 

[Fiona Brick]

  On 29/03/2025 at 13:35, Redbeard said:

You say that you did not expect the MVHR to provide for your full heat load, but not too far off. What did they tell you it *would* provide?

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Our estimated heat demand at the outset was expected to be 5kw during the coldest winter months. This was based on an airtightness test of 3.0 and a room temp of 21. We achieved an air tightness of 0.882, are A rated in terms of Insulation (ICF house) and took thermal bridging measures throughout the build. We have a vaulted space in the living room which was pointed out as being an area of concern. However, this isn't the area that is causing the issue. This area called for very slight additional heat during the cold snap we had but still maintained a steady temp of a min of 17. We are very open to putting an additional heat source here. The system itself has a 2kw integrated heat pump so in current weather conditions, it should be meeting our needs. 

 

However the area we are having issues with was never considered to be an area of concern by the supplier/installer. We were certainly never expecting it to be blowing cold air so frequently. My fear is that if it is an installation issue surrounding ducting or a mis calculation at design stage, supplementary heating will not address the issues in this section of the house as any supplementary heating will be counter acted by the constant flow of cold air and will end up costing an arm and a leg. 

[Fiona Brick]

  On 29/03/2025 at 12:42, Mike said:

If there's a long run of poorly insulated duct through a cold attic, then that would certainly be the expected result.

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Effectively an installation issue then as the supplier/installer was the person doing all the duct work too?

[Mike]

  On 29/03/2025 at 14:28, Fiona Brick said:

Effectively an installation issue then as the supplier/installer was the person doing all the duct work too?

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If inadequate insulation is indeed the problem, then it's either a specification or installation issue (unless the design required someone else to do something - for example to bury the ducts in the loft insulation).