Impact of MVHR heat recovery on heat demand cals

[Babak]

13 hours ago, SteamyTea said:

Has DHW been taken into accont woth the ASHP sizing?

 

Being a bit oversize does not do any harm.

 

What will the actual percentage heat recovery be in the temperature difference ranges between 10K and 22K, extremes only last hours and can be a bit misleading.

 

MVHR does not replace 'built in' air leakage, it adds to it, but recovers some power.

I would possibly have gone for the bigger output, but the outside unit is double the size as it jumps to two fans. But is also a point of principle. There is a design with certain calculations and also a largish built-in margin for errors, inefficiencies, etc. which was being ignored.

 

unfortunately it became a point of principle when he decided to ignore a big part of the calcs for a very weak reason. He might have thought something in the lines of “what if the calcs are not right? What if the MVHR ends up recovering only half the heat? Etc etc”. I would have resisted those less on the basis of an SME going for gut feeling. But that was not the case….

 

 

 

[Babak]

11 hours ago, Nick Laslett said:

I feel your pain. Until the house has been finished and you have gone through a full year’s weather, it is very hard to model in advance how it will perform. Even the HeatGeek YouTuber casually mentions in one of his detailed ASHP videos that you will have to adjust the weather compensation when the cold weather arrives to get it right for your dwelling, if you install the ASHP in summer. 
 

After reading many threads here on the topic, I feel that short cycling is the main factor you want to engineer our with good design. I’m sure the heat loss tools are good, but you will find a big difference between the PHPP model and the defaults in SAP. It is a lot of effort to model the window junction heat losses accurately. 
 

My understanding is that constant short cycling will wear you ASHP quickest. 
 

I wonder where your installer stands on the Legionnaires weekly boost program?  

so in the end I have just signed the paperwork for the smaller unit. The larger unit modulates down to 4kw and the one I went for down to 2kw. demand at -1 is 6kw with the unit maxing out at 6.8kw. I may still get a low loss header for defrost purposes though.

 

HW is a potential issue. The unit I am getting is a Hitachi Yukati S Combi with a pre plumbed tank all within the same enclosure as the heat exchanger for the internal unit. The original decision was to go for a 260 litre version. However, as of June they have improved the model by changing the capacity down to 220litres. This is probably still OK for my family of 4 plus the occasional guests. The new version’s tank is a heat reserve, so hot water is heated on demand and cyclical heating to 55c is not necessary. I guess there is also slightly less available hot water due to exchange efficiency losses. On the other hand the unit is ultra super insulated and so if I time the water heating well, I could actually save money. 
 

I have about 4kw of solar PV’s (inverter size, actual PV’s are higher rated), so paradoxically enough may be able to use direct (not pumped) electric heating on the coldest days as they tend to be clear and cloudy days slightly warmer. With some tinkering I could potentially use immersion heating only when it is coldest so the heat pump just warms the house, assumption being the water stays hot until the next morning, ignoring intermediate use.

 

thank you for your note about your 5kw ASHP. I was beginning to have my doubts about the heating specs given the entire planet and their dog wanted to upsize the heat pump and the UFH. It is good to read persistence does pay and that low energy designs actually work.

Edited August 11, 2022 by Babak

[Adsibob]

@Babaki think you’ve gone for the right choice. The reality is that your house is well insulated AND airtight. The airtightness will make a big difference as with no draghts, it will take much longer for the house to lose heat once it’s warm. Your avoiding short cycling, which is good. Worst case scenario it takes a bit longer to heat up, but you can compensate for that with a good timer system/smart thermostat.

[Babak]

14 hours ago, SteamyTea said:

Has DHW been taken into accont woth the ASHP sizing?

 

Being a bit oversize does not do any harm.

 

What will the actual percentage heat recovery be in the temperature difference ranges between 10K and 22K, extremes only last hours and can be a bit misleading.

 

MVHR does not replace 'built in' air leakage, it adds to it, but recovers some power.

Hot water might become an issue on colder days and nights if I were to use the heat pump for it.  But, on the other hand, even with today’s prices I reckon using immersion heater for a week or so every year to supplement the pump will be cheaper and faster than getting a bigger heat pump that will cost around £1,000 upfront and likely will fail earlier due to more frequent cycling.

 

in theory at leat I should also be able to use subsidised immersion heating for HW on coldest days as I have PVs. Just a thought though.

 

[jfb]

On 10/08/2022 at 21:15, Babak said:

MCS reply:

What you are asking is to do with design guidance, and unfortunately our technical team are unable to provide this. So we would recommend that you speak to at least three contractors and see what their response is

What the ….  is the point of the MCS if they aren’t able to provide sensible guidance when needed? And then the fact that none of the MCS installers know what is going on. 
Madness.