Part L fail with A2W heat pump and MHRV

s00bf06e

Hi Everybody,

We have just got planning and have just got our Part L assessment done for the commencement order. We are planning to have an air to water heat pump and MHRV system but with this spec the building failed the Part L, our assessor then ran it with just the heat pump and it passed even thought the overall energy use for the building is higher by something like 30%. Has anybody ever come across this before and what have you done. My other half is German and is adamant we are not having natural ventilation. The house came it at A2 for the BER. The spec is 125mm cavitiy full fill, 150mm in the slab, 400mm rockwool in attic and triple glazing. Can anybody shed some light on this bizarre result.

Thanks

BryanF

Tell your assessor to run it again

Add pv as oppose to removing Hrv

Calahonda52

What ever about failing part L
whats the energy calculation basis that arrived at using
A2W heat pump and MHRV

Condenser

s00bf06e wrote: »

Hi Everybody,

We have just got planning and have just got our Part L assessment done for the commencement order. We are planning to have an air to water heat pump and MHRV system but with this spec the building failed the Part L, our assessor then ran it with just the heat pump and it passed even thought the overall energy use for the building is higher by something like 30%. Has anybody ever come across this before and what have you done. My other half is German and is adamant we are not having natural ventilation. The house came it at A2 for the BER. The spec is 125mm cavitiy full fill, 150mm in the slab, 400mm rockwool in attic and triple glazing. Can anybody shed some light on this bizarre result.

Thanks

This is one of many anomalies in the deap software. The more energy efficient your house is the harder it is to pass regulations. Removing hrv increases your heat load in the software allowing the heat pump contribute more renewable energy to the overall load.

Efficiency on a2w units is generally very low in hot water mode so when this makes up the most of your load it makes meeting the renewable contribution the software requests very difficult to meet.

Some brands are better than others and will get you over the line as will most ground source units.

sydthebeat

s00bf06e wrote: »

Hi Everybody,

We have just got planning and have just got our Part L assessment done for the commencement order. We are planning to have an air to water heat pump and MHRV system but with this spec the building failed the Part L, our assessor then ran it with just the heat pump and it passed even thought the overall energy use for the building is higher by something like 30%. Has anybody ever come across this before and what have you done. My other half is German and is adamant we are not having natural ventilation. The house came it at A2 for the BER. The spec is 125mm cavitiy full fill, 150mm in the slab, 400mm rockwool in attic and triple glazing. Can anybody shed some light on this bizarre result.

Thanks

make sure the assessor gives the xml file to a specific heat pump manufacturer or specifier.

inputting heat pump details (by extrapolation from certification) a highly convoluted process, so its always better to get the inputs directly from the manufacturers themselves.

after this, try the MHRV inputs again.

yes, the anomaly as condenser mentioned above exists and is completely nonsensical.. however it could be possible to "play" with the air tightness figures to help it comply.

personally, if i do a preliminary DEAP assessment and im having trouble making a HP with MHRV comply, ill push the air tightness result out a bit.....

if the air tightness result comes in at a better value (which it almost invariable does) then i personally do not consider that to make the build non compliant.

i would quite happily stand up in court and argue this point to its end.
the anomoly in DEAP is ridiculous and its an embarrassment that SEAI havent resolved the algorithims to resolve the issue.

greenfit

+ 1 on sydthebeat comment

since the SEAI have introduced the their heat pump tool (excel workbook for calculating efficiency of heat pumps), I find that the problem with DEAP assessments not meeting the renewable contribution target occurs a lot less. Have your assessor go through the heat pump tool again or have them sent it to the supplier.

If that doesn't work, you will have to try and make your fabric (on paper) less efficient. I usually just use the default thermal bridging factor of 0.15 instead of 0.08.

Failing this, maybe contact your local building control and see if they will accept your specification as Part L compliant as is. They should be aware of the anomalies in the DEAP methodology. I would avoid putting up PV panels just to get passed an obvious error in the calculation method.

kieranhr

Also +1 to Sydthebeat's comment.

This has been an issue for many years, and I can't believe SEAI still hasn't addressed it.

If you have MHRV, you're probably assuming an airtightness target of around 1 m3/m2/hr. If it's failing the renewables check at that, increase it to 3 or 5. The max allowable is 7.

If a building control officer was to challenge this (and I can't see it happening, they know about the bug in the methodology too), I would crack open a window, and do the airtightness test again. There you go, the house complies.

FixdePitchmark

sydthebeat wrote: »

make sure the assessor gives the xml file to a specific heat pump manufacturer or specifier.

inputting heat pump details (by extrapolation from certification) a highly convoluted process, so its always better to get the inputs directly from the manufacturers themselves.

after this, try the MHRV inputs again.

yes, the anomaly as condenser mentioned above exists and is completely nonsensical.. however it could be possible to "play" with the air tightness figures to help it comply.

personally, if i do a preliminary DEAP assessment and im having trouble making a HP with MHRV comply, ill push the air tightness result out a bit.....

if the air tightness result comes in at a better value (which it almost invariable does) then i personally do not consider that to make the build non compliant.

i would quite happily stand up in court and argue this point to its end.
the anomoly in DEAP is ridiculous and its an embarrassment that SEAI havent resolved the algorithims to resolve the issue.

Syd - would you mind checking your pms.