@editor They return arround 85% of the heat back into the house. A standard extractor fan removes 100% of the heat as the air removed is replaced by cold air form outside.

Ventellation is the forgotten eliphant in the room, it is not insulation as portraid by 99% if so-called experts.

In a new build proeprty, between 75 and 85% of the heating bill is created by the regulated ventellation in a property to pervent condensation and air quality.

It is easy to improve and reduce your energy consumption when you are throwing away 85% of your energy bill, adding more insulatiuon will do nothing.

We have solid stone buildings, with no insulation on the books with lower energy bills than other new builds of the same scize.

@heacol Exactly! I did some outline heat loss calculations based on what I know of the property I am buying and over 40% of the heat loss is due to ventilation when assuming “typical” air-changes per hour for the various rooms. (1.5/h for main living areas, 2/h for kitchen and bathrooms and 1/h for bedrooms)

A way of significantly reducing that could save a lot of money at current energy costs.

Hi @heacol

It would appear that you would require two of these units at £400 each. Is that correct?

@derek-m I only have 1 in my house and have no condensation problems. 2 would be better and they will pay for themselves in less than a year. No brainer!!!

@heacol how do you figure they pay for themselves that quick? https://renewableheatinghub.co.uk/forums/other-renewables/heat-recovery-systems suggested the numbers didn't add up for MHRV.

@mjr It depends what numbers you put in your calculator. If you do a heat loss calculation on a new build, you will find that 70-85% of the heat loss is ventellation.

Therefore if the average heating bill is now cloce to £2000.00 per annum and a heat recovery system is 85% efficient, you are saving over £1200.00 per year. I have seen this in real life many times. We have a noumber examples of houses that are over 300m2 with heating bills, at current prices of under £1000.00 per year.

You would not buy a bucket with a hole in it, but we, as a scoiety are quite perpaired to buy a house that leaks like a sieve. For heat recovery to work properly, the house must be as air tight as possable (less than 1 m3/m2/hr), or do not bother.

Posted by: @heacol

@mjr It depends what numbers you put in your calculator. If you do a heat loss calculation on a new build, you will find that 70-85% of the heat loss is ventellation.

Therefore if the average heating bill is now cloce to £2000.00 per annum and a heat recovery system is 85% efficient, you are saving over £1200.00 per year. I have seen this in real life many times. We have a noumber examples of houses that are over 300m2 with heating bills, at current prices of under £1000.00 per year.

You would not buy a bucket with a hole in it, but we, as a scoiety are quite perpaired to buy a house that leaks like a sieve. For heat recovery to work properly, the house must be as air tight as possable (less than 1 m3/m2/hr), or do not bother.

@Heacol, I was the one who came up with the calculations in the thread @mjr mentioned. I was also the one who started the thread since I'm actually keen to see heat recovery technology working given it appears to be a solid argument. In short, I'm open to be convinced, but the calculations on an actual scenario with accurate measurements proved pretty damning.

If you can provide a specific example scenario with measurements and numbers that demonstrate the MHVR is paying for itself in a reasonable timeframe, I'd go out and buy one. I have a bathroom that is struggling with condensation even with an extractor fan, so I have a place to put the MHVR immediately. If, of course, it can justify itself.

For reference, I know what I was spending on heating when we were on oil and I know what we're spending now using an ASHP. I know what we're spending on electricity and where it's being consumed. I know what we're producing from solar PV and how much is being consumed vs exported. I know our house's various dimensions and obviously the detail held within its EPC. I do not know to any accuracy how air tight it is. With all those pieces of information, we should easily be able to apply to my situation any conclusions from your example scenario and therefore demonstrate whether or not the MHVR would be justifiable in our home.

@majordennisbloodnok As you have no idea what the most important figure in the calculation is "I do not know to any accuracy how airtight it is", any calculation you do is wrong. To put is simply, a bucket with a hole unit does not hold water, a leaky house, does not hold heat.

Installing an MVHR fan in the bathroom will make some difference, just like blocking 1 hole out of 10 in the bucket.

If you are not prepared to do it properly, do not bother. We no longer install MVHR systems as builders cannot understand or comprehend air tightness, however we have some near passive houses that cost no more than a conventional house to build on our books. If you install any form of PIR (kingspan, celetex ect), do not expect your building to be airtight or suitable for and MVHR system.

Posted by: @heacol

@majordennisbloodnok As you have no idea what the most important figure in the calculation is "I do not know to any accuracy how airtight it is", any calculation you do is wrong.

Actually, all my calculation was doing was the simple physics of how much energy is required to heat a room full of air from one temperature to another (i.e. replacing the heat lost from an extractor fan) vs the energy saved by recovering 85% of that heat. If you can show me where that's wrong I'd be grateful. Even more, I'd be interested how that calculation becomes incorrect in a leaky vs airtight building.

I'll reiterate what I said before; I would genuinely like to be convinced that an MHVR is capable of saving enough energy that it can justify its cost in a reasonable amount of time.

@majordennisbloodnok Look at it another way, How much water do you need to continuously pour in to a bucket with a hole in it to keep it full?

Without knowing the size of the hole, the temperature of the water (viscosity) and the altitude you are at (pressure or gravity pushing the water through the hole), it is impossible to calculate, so any assumptions are irrelevant. It can range from a pinprick hole or a drop an hour or the whole bottom of the bucket which could be many hundreds of M3/hr, or anything in between. Both are a hole in the bucket.

I hope this illustrates why it is critical to seal a house up properly if you want to install an MVHR system, hence "passive house standard" was developed to force builders to do their job properly.

I understand the analogy, @heacol. However....

If we use your example, the bucket already has several holes in it and our aim is to keep the water at a constant temperature.

Now we add a tap to the side of it so we can turn it on and off. This equates to an extractor fan. If we turn it on, we lose water and heat. That's extra loss of heat than if the tap was turned off, and we know exactly how much water (and therefore heat) is lost if we leave the tap on for a minute. It doesn't matter how many other holes the bucket has, we can see exactly how much water (or heat) is being lost specifically through the tap.

Now let's change the tap for a fancy one that takes 85% of the heat it's losing through the water and transfers that heat to the water being added to the bucket again. You lose the same amount of water as the original tap if it runs for a minute, but only 15% of the heat. But it's only saving 85% of your heat for the water that travels through the tap, not 85% of the total heat in the bucket.

As far as I can see, an extractor fan is an extra hole, irrespective of how many holes exist already. an MHVR extractor will be at its most efficient if you don't turn it on at all, so the savings you get from it are based on the energy used to heat the air it extracts, and that's independent from any other holes in the house. If I need to remove 10m3 air from my bathroom to combat condensation, and it costs £1 to heat that air from outside temperature to internal temperature, an MHVR will save me 85p of that whether I'm losing 1m3 throughout the rest of my house or 100m3.

Obviously, if you have a house that's completely airtight and perfectly insulated, it'll lose no heat, but the occupants will suffocate fairly quickly. Ventilation is necessary, but moving air in and out of that container is an easy way to lose heat unless it's reclaimed. I get that, and it's the perfect argument for MHVR as a way of dealing with heat loss from mandatory ventilation. However, an extractor fan in a bathroom or kitchen is an extra bit of voluntary ventilation, and I have yet to be shown that the amount of air it moves whilst doing the job it's needed for contains enough reclaimable energy to make the MHVR alternative pay for itself in anything like a reasonable timeframe. I am still open to being convinced, but that will take some kind of practical scenario and figures and measurements that can demonstrate it explicitly.

@majordennisbloodnok You have misunderstood me. The tap is your heating system. Your house is full of holes (the bucket is full of holes). Currently, your heating system (the tap) is maintaining the temperature at say 20 Deg C (the bucket half full). When you switch on your extractor fan, you are removing the bottom of the bucket. You are pumping hot air out the house at a rate of 30 litres / second, which is replaced by cold air. To maintain the 20 Deg C in the property, the heating system has to significantly up its gain. If you have a heat pump, you do not stand a chance of maintaining a steady internal temperature. 

In reality, the building regulations state that the pass for an air test is 10 m3 /m2/hr. At 0 degree outside, with a room temperature of 20 Deg C, in a 100 m2 house, your heat loss will be (at an RH of about 60%) around 6.6 Kw / hour for these ventilation losses. This does not take into account when the wind blows, or you switch on your extractor fan.

What amasses me is you are force to install high quality double-glazed windows, and then you drill a big hole in the frame to allow air infiltration.