Considering PIV

[MortarThePoint]

11 hours ago, PeterW said:

So given you need to do all this, why not use MVHR and get all the benefits ..??

 

I need to install extractor fans anyway so the trickle overhead is small there. That's a lot less involved than ducting to a centralised unit.

 

I don't want this to be anti MVHR as I think it suits many well.

Personally:

 - I don't like all the ducting of MVHR.

 - I don't like the expensive single point of failure of MVHR

 - For my situation I don't believe the economics of MVHR work. Across 6months of heating season the average dT is about 13C and so the PIV annual running cost is £172 electricity. MVHR only about halves that due to its fan having higher power consumption. Even at £172pa the payback time would be looking like >15 years, based on a modest £2500 outlay. I suspect there are other annual costs that raise MVHR to costing over £100pa to run, taking the payback beyond 30 years.

 

MVHR is largely about the heat recovery. What would you do if you ignored the heat recovery part of it? Sell me on MVHR without mentioning heat recovery ?

[saveasteading]

Bitpipe, interesting points.

 

Your choice obviously, although £8k sounds very expensive for MVHR. The entire system for my 400m2 cost £4k.

I was using a previously mentioned sum, and don't know the costs. We are looking at 400m2 too, a lot of which will be high ceilings, so the volume will be very large.

also, that is 350m2 of footprint , with little upper floor, so the ducts will be up to 45m long for a single system,

Do you mind mentioning your supplier?

 

In my opinion, MVHR's primary function is to deliver continuous controlled ventilation to every room, the heat recovery capability means it does so efficiently.

I can see the advantage of that. It must be tricky to balance a large system where the tendency will be for the air to travel along the shortest duct. Are there chokes to balance this?

If there are rooms not in much use, then they could be over-ventilated.

 

In an airtight house it is essential but as yours is a conversion, you may not reach the airtightness levels to really take advantage of the efficiency anyway.

Intending to construct a sealed box inside the original structure, so airtightness should be very good. that is, of course, sealed except for trickle vents and fan ducts, which get taped over for an air-test, then opened again. With en-suites where possible there will be a lot of air movement without sucking it out 24/7 too.

 

In a sufficiently airtight house you can leave the front door open and there will be no draught in or out as there is nowhere for the air to go.

I don't mean draught, but that when someone opens the door there is an automatic exchange of air from in to out. A porch limits this to a small volume and in an area where this is less of  problem.

 

I am still dubious that mvhr is effective in my situation. 

very good insulation, close to building regs. for new.

underfloor heating

air source heat

solar panels and battery

good airtightness.

people open doors and windows.

high ceilings*

 

* changes the concept, as there is a lot more air to breathe and to dilute exhaled air. Also must change the dynamics of how much air needs to be changed.

I am used to building to air tests at around 3 thingies per whatsit, including for schools. and the benefit of high ceilings has been remarkable.

 

Obv still interested in being persuaded on cost/benefit, and of any wrong assumptions, and real life test figures. It may be appropriate to the 'normal' size rooms.