MVHR for 148m2 house

[Moira Niedzwiecka]

I am technically challenged when it comes to this, not having any real understanding of the technology or the alternatives.

We are building a 148m2 chalet style house, well insulated & airtight.The house will be occupied by just myself & my husband. I have attached the plans.

Our architect has suggested a Genvex combi.

The unit would fit in the utility room.

Has anyone any experience of this system?

Would we be better having separate MVHR & hot water system?

I have read most of the posts & blogs on both buildhub & ebuild & am now completely boggled.

Any suggestions would be very gratefully received.

 

 

 

 

BRegs 10 rev C.pdf

BRegs_11_rev_B.pdf

[Nickfromwales]

One with some good points / pros & cons. 

 

Genvex Combi 185 Ls Ec

 

 

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21 Oct 2015

Our designer proposes one of these as being the only heating & ventilation necessary in our new-build.

As I understand it, this is an ASHP and a MVHR in one unit.

Is my understanding right? Does it also heat & circulate the UFH liquid? The house is small (about 90sq m) and well insulated (can't quote the figures), and the designer is properly qualified and experienced in passive house building, and a registered / accredited for SAP / EPC work.

 

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jsharris21 Oct 2015

We have the Genvex Premium 1L, which also has an air to air heat pump. Ours is, in theory, perfectly capable of heating our passive house, with a heat output of up to about 1.5 kW. However, there is no way that I would use it as the only heat source, for a few reasons.

The first is the air quality. When it is heating it blows warm, dry air through the fresh air vents and my personal view is that this is a bit too dry and doesn't feel that comfortable. It's perfectly effective, but tends to result in the air being warmer than surfaces like the floor, so subjectively it isn't as nice feeling as under floor heating.

The second is noise. When heating (or cooling) the ventilation rate increases to boost speed, and this makes significantly more noise than the normal background ventilation speed. For cooling this is fine, as that's required during the day and the slight noise isn't at all noticeable over the higher background noise level in the house at that time. I definitely wouldn't want it on at night, though, as I think the noise on boost speed would be intrusive, especially as passive houses tend to be very quiet internally.

Finally, the 185 isn't wonderful when heating hot water, as the heat output is limited and if reasonably hot water is needed the efficiency drops a lot. It also relies on extracting heat from the house to heat hot water, which tends to result in high ventilation rates as it boosts in order to get enough air through the heat pump to heat the water up. Again, I'd not want it boosting at night to do this ready for morning showers, but that's really just my own feelings about noise and I'll admit to liking things to be quieter than most.

On the plus side, Genvex build quality and general performance is very high, we are very impressed with our Premium 1L active MVHR (which is similar to the 185 but without the hot water capability). For the price, the Genvex is pretty good, especially the cooling capability, which is very useful indeed in summer. The 185 is about the same price as the Premium 1L, roughly £4200 inc VAT, plus carriage (I've just checked!) so seems to be better value, but I think I'd be concerned about the factors mentioned above and the hot water capacity (it's got a 185 litre cylinder inside it). A couple of showers are going to quickly use that much warm water (and it will be warm, rather than hot, unless the boost capability is used, which is, essentially, an immersion heater).

We use under floor heating as a heating system, and never use the Genvex air heating for the first two reasons above. Very little heating is needed, but our very low temperature (typically 22 to 23 deg C) UFH does provide high comfort levels. My wife has been oiling doors today in bare feet on stone floors and reckons it feels perfectly comfortable. This may well just be a personal thing, as people in North America have been using warm air heating systems for decades and they don't seem to have a problem with the principle.

For reference, our house is a fairly big two bedroom passive house, 130m², with a higher than average volume because of the high vaulted ceilings on the first floor. I've found that the hot water system has caused me a significant amount of work to get right, as has the very minimal heating system to a lesser extent. It isn't at all easy to get these systems to work effectively and efficiently in a smaller passive house, especially if also looking for systems that aren't that expensive to install. In many ways, direct electric underfloor heating is probably the cheapest way of heating the house, and the running costs wouldn't be high. The running costs would also be paid for for the first few years just from the saving in installation cost! However, I couldn't bring myself to use electricity like this, so fitted wet underfloor heating in the ground floor passive slab and provide a tiny bit of heat every now and again from the smallest ASHP I could buy. The capital cost of this system was high, around £3,000 at a guess, so it will never, ever pay for itself in terms of energy saving before it needs replacing, but it did salve my conscience a bit (and got a much better SAP EER).

I'm about to start a short series of blog posts on my build blog here on this forum, detailing how I've finally tackled the hot water needs. With luck I should have the first instalment up later this evening, and it may help you work through the issues with trying to be energy efficient, yet not spend a fortune on stuff to achieve it! 
Edited by jsharris, 21 October 2015 - 05:44 PM. 
Reason for edit:: typos

 

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21 Oct 2015

JSH

What would we do without you? Thanks for taking the trouble to clarify things. Effectively, then, you could justify the Genvex just on it's MVHR performance - is that right?

If we went for ASHP - driven UFH, would we be able to save money on the MVHR side by using a lower-spec unit than the Genvex?

Thanks again
Max

 

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jsharris21 Oct 2015

Thanks for the kind words.

My view is that the Genvex (or one of the other MVHR units that has an air-to-air reversible heat pump) is extremely useful, but primarily for it's cooling capability. We've found it very beneficial to have the ability to blow cool air into the house when it gets a bit warm. Personally, I'd justify fitting the basic Genvex (or similar) active MVHR just to get the cooling capability, but I do accept that it's a high premium to pay over a basic MVHR (our Premium 1L cost a bit over £4,000, including delivery, after the VAT reclaim).

It would have been cheaper, with hindsight to fit a normal passive MVHR and add a couple of cheap split air cooling units at around £500 - £600 each. We could have built these in to the house, had we realised the significance of cooling.

My view (and I'd stress this is just my view, based on our experience) is that wet UFH is worth it's weight in gold, not just because it's a nice heat source (which it is), but because it allows the heat to be evened out around the house, by moving heat from the warm areas to the cooler areas, which is a tremendous benefit. We have our UFH pump (only about 20 watts) running all day, every day, even with no heat input. It just pumps water around the pipes, moving heat around and maintaining an even temperature. This is remarkably effective, as yesterday and the day before were warm and sunny, which heated the house up, and today was cold and wet, yet the floor was a nice, even 20.4 deg C, all through the house, even in the rooms on the North side that never see any sun.

We do have a small ASHP to heat (or cool) the UFH in the floor slab, but it's questionable as to whether this is cost effective for heating. It cost me £1700 (which is cheap, as I bought it as surplus stock) and it saves maybe £150 to £200 a year on electricity, so will take a long time to pay for itself, maybe longer than the life of the unit. It does get a much better SAP EPC, though, and is probably a reasonable choice on environmental grounds. We also use the ASHP to pre-heat the hot water, and that is a useful benefit, one that's maybe worth another £50 to £100 a year in electricity cost at a guess. Additionally, we can use the ASHP to cool the floor slab a bit in very hot weather, and this is very effective, if a bit of a luxury, so overall I'd say it's worth having.

The alternative to the ASHP, would be a simple (and cheap) electric water heater to run the UFH. This would work well for the small amount of heating needed, and the heat distribution using pumped water around the UFH would still work as well, but the cooling capability would be lost and the SAP EPC would suffer.

All these things are compromises, unfortunately, and there's no clear and obvious answer as to which is best. What is best for me, may not be best for someone else, as I place a value on some of the long-term environmental aspects, and am prepared to pay a bit more in order to use less resources, in my case from the grid. 
Edited by jsharris, 21 October 2015 - 09:03 PM.

 

[Moira Niedzwiecka]

Thank you Nick.

It was reading Mr Harris's post of 21/10/15 that initially raised doubts about the suitability of the Genvex combi.

We are looking to fit a 6kw PV array & I have been thinking that a separate thermal store using the PV with an immersion back up may be the way to go.

We could then have a separate MVHR.

Is it usual to have the MVHR on all year round or can you switch it off in the summer & just have the windows open?

I know some have a summer bypass to switch off the heat recovery element.

I am also confused about having an additional heating element fitted to the MVHR & an air conditioning element as well.

Does anyone know if it is possible to have differing amounts of heat to certain rooms using MVHR or is the same throughout?

We would have electric heated towel rails in both bathrooms & have been considering electric under floor heating in the large vaulted landing/study area as a back up.

 

[warby]

Couple of things to consider:

a. Could use the DHW system to heat your towel rails, rather than electric.

b. Air to air heat pumps, i.e. most air conditioning units, can be reversed to produce either hot or cold air. 

c. You can put heating/cooling units on the MVHR inlet duct to heat and/or cool the incoming air, I think @joe90 recommended these on the other place:  http://www.veab.com/en/duct-water-heat-cool/produkter/kanal-varme-kyla-vatten    

http://www.dealec.com/acatalog/hydronic-duct-heaters.html

[Gone West]

I wouldn’t use a Genvex 185 Combi unless it was in a house built to Passivhaus standards. It is useful in that it is compact and in a small house that can be a consideration. In a PH not requiring underfloor heating, where all the heating requirement can be met by the Combi, there are savings to be made by using a single appliance. If there is not a large hot water requirement, ie one or two occupants or water saving measures are good then the Combi is fine. It is good way of helping to keep the build simple rather than having a lot of different systems.

[VIPMan]

We have a house with similar floor area and there are just two of us with a 5.5kWp PV array - so very similar. We went for a  MVHR with post heater for heating and ventilation.  Sunamp PV Heat batteries for the DHW charged by economy7 surplus PV.   We haven't moved in yet but everything seems to be working fine so far.

[Nickfromwales]

5 hours ago, PeterStarck said:

 

I wouldn’t use a Genvex 185 Combi unless it was in a house built to Passivhaus standards. It is useful in that it is compact and in a small house that can be a consideration. In a PH not requiring underfloor heating, where all the heating requirement can be met by the Combi, there are savings to be made by using a single appliance. If there is not a large hot water requirement, ie one or two occupants or water saving measures are good then the Combi is fine. It is good way of helping to keep the build simple rather than having a lot of different systems.

 

 

Would the CoP be hugely affected if the dhw demand was above average though?

[Gone West]

9 hours ago, Nickfromwales said:

 

Would the CoP be hugely affected if the dhw demand was above average though?

 

Almost certainly that would be the case which is why I stressed it's suitability in a PH only. The proportion of energy used for space heating compared to DHW changes from a conventional house to PH. Reducing the amount of energy used to heat DHW is more important so hot water saving measures become more critical. You would not expect DHW demand to be above average in a PH.

[Nickfromwales]

I meant more about excessive use eg baths more than quick showers etc. PH would allow you to ascertain expected dhw consumption, but it's best to stress that if dhw use rises then the stated / expected efficiency would drop considerably. 

Thats the tipping point where it may be beneficial to go to a different arrangement for heating and hot water. 

[Moira Niedzwiecka]

Thank you for all your replies.

Not sure what we are going to do at present.

I have been looking into alternative MVHR and separating this from the DHW.

Has anyone any experience of Nuaire MVHR systems?

[Nickfromwales]

http://www.ebuild.co.uk

 

Search under Nuaire as there's already a good bit we've already discussed there. 

 

[joe90]

There are some good deals on EBay, I bought a kingspan ASHP 5 Kw new for £700 and a kingspan oversized MVHR for £380. I plan to have UFH ( downstairs) running from the ASHP mainly on E7 and warm ( not hot) water from the same source and stored, I will then top up the warm water to hot for outlets that need it ( bath, shower,kitchen) with a modulating inline water heater, I think other outlets such as basins can be given just warm water. I will wire for an MVHR electric post heater during the build but not install unless it's needed, I decided against using stored warm water for heating the air from the MVHR as I would rather use the water for DHW. I will also use the ASHP to cool the slab should we have hot weather. I  know it's a contentious issue but we are having a small wood stove for the depths of winter so we are covered for heating in very cold weather.

Edited July 29, 2016 by joe90
Typo

[warby]

@joe90 I really like your ideas. Did/have you considered using an air air heat pump, instead of the woodburner, for heating and in the summer for cooling? It would also avoid the need for a post heater. I have found them very efficient and easy to use in both modes of operation.

[joe90]

Hi Warby, the wood stove is a must from swmbo, in fact we both would prefer to watch wood burning than watch ( most) tv. We have a wood supply and want it as part of our country cottage lifestyle out in the sticks, although I realise it's a very contentious issue. Jeremy uses his UFH running in reverse as a very efficient way of cooling the slab or even just pumping the water around to even out floor temperatures and we already have it for heating anyway.

Edited July 31, 2016 by joe90
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