MVHR & combined ASHP for 240m2 new build

[UncleQ]

We're about to build a 5 bedroom near passiv haus home of 240m2 over 2 floors using 450mm ICF and a raft foundation giving us 0.10Wm2 U values with triple glazed windows in an airtight configuration. I've been trying to narrow down our central heating and DHW solution and have narrowed it down somewhat. Our thermal modeling suggest we need a peak central heating requirement of 7Kw and history with 4 kids suggests 250+ litres of DHW. I appreciate lots of you have UFH but we're trying to avoid complications with a wet system with embedded pipes, so were thinking of a purely air-to-air solution for central heating. I can't seem to find the obvious 'nirvana' solution combining an external large volume ASHP with an MVHR system to distribute the warm air. The closest I can find are the Genvex type MVHR units with a built-in ASHP, which whilst they appear to be a nice and neat integrated package, suffer from their inherent restriction on the volume of air they can ingest, thus topping out at the 4-5Kw mark and risking being noisy at full blast trying to reach that.

 

My current thinking was to use a Genvex Premium 3 for the majority of our space heating and cooling needs supplemented by a wood burning stove for the colder winter days, but am concerned that this still might not be enough or could be excessively noisy so may need to be supplemented with pre or post heating or another solution. For DHW I really like the idea of the SunAmp heat batteries but am a little wary after reading about the 50% re-charge issue with their latest models discussed on this forum at length.

 

So my questions are as follows:

1. are there better options we should consider for our space heating needs using an air-to-air solution? e.g. a standard MVHR connected to an ASHP using a duct reheater? Can this cool?
2. is it a practical idea to supplement using a more conventional external ASHP to 'top up' the Genvex unit through the use of a water duct pre or reheater? I understand the theory of both pre and post duct heating using for example a VEAB CWW unit, but would be concerned whether or not the Genvex controller could incorporate them seamlessly without requiring a custom solution?

 

I can't imagine that this question hasn't been asked/debated before, apologies if I haven't spotted all posts and many thanks for any help you can give.

[dimpsy]

I would also have a look at the Nilan Compact P series https://www.nilan.dk/en-gb/frontpage/solutions/domestic-solutions/compact-solutions/compact-p which was around £6000, depends a bit on the exchange rate.

[Jeremy Harris]

We have an air-to-air heating/cooling system built in to our MVHR (a Genvex Premium 1L).  It has the capacity to heat and cool the house without any problems.

 

Some like the feel of warm air heating, but we don't, so have never used it, instead we use UFH fitted into the ground floor passive slab.  It's purely a personal preference thing, though, as at least one other member here heats their passive house using an active MVHR unit (a Genvex, different model to ours, but same working principle) and likes it.

 

 

[ProDave]

I am surprised that with U values of 0.1 your heating demannd is as high as 7KW

 

Our U values are only 0.14 any while our house is smaller than yours at 150 square metres, the peak heating demand when it is -10 outside and +20 inside is only a little over 2Kw  Even if you doubles the size of our house that would only be 4Kw so with your lower U values I certainly would not expect it to reach 7Kw

[Jeremy Harris]

I'd missed that 7kW figure, too.  Looks very high to me.  Our house has roof and floor U values of fractionally under 0.1 W/m².K, walls at about 0.12 W/m².K and triple glazed windows that are around 0.7 W/m².K.  Airtightness is about 0.43 ACH @ 50Pa.  The floor area is 130m² and the worst case heating requirement, with no incidental heat gains at all, is only about 1.6 kW.  In practice the house never seems to need more than a few hundreds watts of heating, and often needs no heating at all, even in winter.

[Gone West]

58 minutes ago, UncleQ said:

I appreciate lots of you have UFH but we're trying to avoid complications with a wet system with embedded pipes, so were thinking of a purely air-to-air solution for central heating.

For exactly that reason we decided to use a Genvex Combi 185LS in conjunction with electric towel rails in the three bathrooms. When the Genvex is in air heating mode it turns the supply fan speed up to maximum for around twenty seconds and then turns the fan down to around 70%. We can hear the fan on maximum but when it slows down it isn't audible. We have a small house, around 306m3, and the system works well for us. If the house were any larger the warm air supply wouldn't be sufficient in cold weather.

[UncleQ]

35 minutes ago, ProDave said:

I am surprised that with U values of 0.1 your heating demannd is as high as 7KW

 

Our U values are only 0.14 any while our house is smaller than yours at 150 square metres, the peak heating demand when it is -10 outside and +20 inside is only a little over 2Kw  Even if you doubles the size of our house that would only be 4Kw so with your lower U values I certainly would not expect it to reach 7Kw

 

Thanks for your reply. It's all theory at this stage of course, but 7Kw is what our architects thermal modelling software suggests. Our raft foundation of 141m2 is specced for between 0.09 to 0.13W/m2K U value, the 450mm ICF for the walls is 0.11 and the roof to 0.13. That said, we also have a significant amount of triple glazing at nearly 90m2 which will push the overall figure up a bit. It would be great if the total heating demand ends up being less, that would be a bonus!

Edited March 10, 2019 by UncleQ

[UncleQ]

26 minutes ago, JSHarris said:

I'd missed that 7kW figure, too.  Looks very high to me.  Our house has roof and floor U values of fractionally under 0.1 W/m².K, walls at about 0.12 W/m².K and triple glazed windows that are around 0.7 W/m².K.  Airtightness is about 0.43 ACH @ 50Pa.  The floor area is 130m² and the worst case heating requirement, with no incidental heat gains at all, is only about 1.6 kW.  In practice the house never seems to need more than a few hundreds watts of heating, and often needs no heating at all, even in winter.

 

Thanks for you reply. Wow, those figures are seriously low, quite amazing and something we could only hope to emulate. I guess that if it is a possibility that our actual heating demand could be considerably less, then perhaps we should start with a Genvex Premium 3 or equivalent, and see how it works? We could always then add an additional heat source later on, particularly if we build in a water duct reheater into the MVHR fabric but not connected to start with?

[Alexphd1]

There is very little extra cost in running UFH pipes in to a insulated raft foundation. Even if you don't use them it's not a major. Gives you a second heating and cooling option at a later date. 

[Nick1c]

If you are having 90m2 of glass have you thought about overheating? We will be having roughly half that glazing and I am more concerned about being too hot than too cold. It may be worth running the design through PHPP to get an idea. In our case we have opted for external blinds on some windows and UFH (with the potential  of using it for cooling). 

[Jeremy Harris]

34 minutes ago, UncleQ said:

 

Thanks for you reply. Wow, those figures are seriously low, quite amazing and something we could only hope to emulate. I guess that if it is a possibility that our actual heating demand could be considerably less, then perhaps we should start with a Genvex Premium 3 or equivalent, and see how it works? We could always then add an additional heat source later on, particularly if we build in a water duct reheater into the MVHR fabric but not connected to start with?

 

Have you modelled the passive house with PHPP to get the heating requirement?  If so, then I'm surprised that you've ended up with a figure of 7 kW, as that sounds to be more like the sort of figure for a reasonably average house, rather than one built to passive house performance standards.

 

If you haven't yet modelled it with PHPP, but want to do a "quick and dirty" estimate of heat loss, then I wrote a spreadsheet a few years ago that simplifies things a lot.  It's nowhere near as refined as PHPP, but is a lot simpler, and people here who have used it have suggested that it gets within around +/- 10% of the real heat loss, which is usually good enough for sizing a heating system:  Heat loss calculator - Master.xls

 

 

[jack]

My house is 289m², 0.11 raft floor, 0.12 cellulose walls and 0.10 cellulose roof. Airtightness just below 0.6 ACH. We're in the South East.

 

We have a 5kW ASHP and it spends most of its time in winter ticking over very slowly. It has no difficulty at all keeping the house warm and heating our hot water as required.

 

That said, you seem to have an extraordinary amount of glazing for the size of the house. I did a rough calculation and I think we're around 50-55 M². That's with floor to ceiling windows in several rooms (including a 5 x 2m slider and three 1 x 2m floor to ceiling windows in the kitchen alone) and large windows even on northern elevations for bathrooms. 

 

Given this amount of glazing, I'd be more concerned about overheating than how much heat you'll need to stay warm.

 

Also, the larger the house, the greater the importance of airtightness to energy consumption. ICF generally does a good job of this, but it's worth having a clear plan to ensure you maximize airtightness during the build.

 

Oh, and I personally can't stand air heating. It provides a very dry and unpleasant form of heat that I'd personally avoid at any cost.

[UncleQ]

20 minutes ago, Nick1c said:

If you are having 90m2 of glass have you thought about overheating? We will be having roughly half that glazing and I am more concerned about being too hot than too cold. It may be worth running the design through PHPP to get an idea. In our case we have opted for external blinds on some windows and UFH (with the potential  of using it for cooling). 

Yes, it's our architect's biggest concern. We've cut it down significantly to get to 90m2 but have had to add overhangs and bris soleil along with window panels from Bruag to compensate.

[TerryE]

2 hours ago, UncleQ said:

I appreciate lots of you have UFH but we're trying to avoid complications with a wet system with embedded pipes

 

Of all of the complexities in building our house, this but was one of the most trouble-free, so I can't follow this logic, to be honest.  There's about half a dozen blog examples of this done here.  Why not pick a few and visit them; talk to the self-builders about what went well and works well and what doesn't.

 

A raft with UFH is a huge storage heater and is simple to heat. Our house is smaller, about 180m², but we currently heat the house with a 3kW immersion coil in a Willis Heater through the UHF loops and this comes on at the moment for about 4 or 5 hours on overnight E7, so there is something way off with your 7kW estimate. If you build the house to spec, then your daily average should peak at maybe a quarter of this.

[Jeremy Harris]

I agree with @TerryE here.  Fitting our UFH pipes into the passive slab took less than half a day and cost a few hundred pounds.  Even if I'd chosen to not connect them up I wouldn't have begrudged that cost.  It was so quick, easy and affordable that it seemed daft not to just include the pipes in the slab.

 

The big bonus for us was that when I found that our house overheats (solar gain though ~ 9.3m² of shaded, south-facing glazing, plus ~3.84m² of unshaded east-facing glazing) it turned out to be pretty easy to just cool the floor slab down by reversing the ASHP.  I was very surprised at just how effective floor cooling like this is; it surpassed my expectations and is now an intrinsic part of the house temperature control system.

[TerryE]

3 hours ago, UncleQ said:

supplemented by a wood burning stove for the colder winter days

 

And forget this one. Just forget it as you will never use it. Even the smallest efficient wood stove will generate at least two kilowatts.  Do the math, let's say 3 kilowatts; putting this into a single room in a passive class house will get the room temperature up to about 40 degrees within a few hours. I remember one guy who had a passive house with a wood stove telling us that the only time that he used it was one Christmas when the family was round; after 2 hours they had to evacuate the living room and open all the doors to let the house cool down.

 

My wife @JanetE originally wanted a woodstove in our house, but she soon saw the sense in my argument that we would never dream of lighting a stove in the middle of the summer, but a passive house internally has that summery feel every month of the year, so we would never use it.  We have never regretted this.

 

So UFH scores about 1 or 2 on the 1:10 complexity scale, trying to implement usable wood stove maybe 8 - 9, ditto large acres of glass like you propose.  As @jack says, the main issue in a passivhaus isn't working out how to add the extra heat when you need it, it's how to dump the excess heat when you don't need it.

[UncleQ]

Wow, you guys have given us a lot to think about and we're very grateful for your real world experience, which is exactly what we were after. For a few hundred pounds it really is a no-brainer to at least plan to put the UFH loops in while the slab is being laid, even if we end up not using it, at least it keeps our options open. Our plot has a fair amount of trees which as a fuel source was one of the reasons we were considering a stove, but having just had the discussion with my wife we'd be quite happy to ditch if it just adds complexity we don't really need.

 

So, plenty to think about but after this thread we're leaning more towards UFH for our ground floor heating from an external ASHP that can also feed DHW (tank or SunAMp), and perhaps a Genvex type MVHR with ASHP to supplement heat or be a backup in case of failure, but mainly to help cool if needed.

 

Which external ASHP brands do people recommend? Friends of ours have a Mitsubishi unit powering their UFH/DHW and are very happy with it.

[Dreadnaught]

5 hours ago, Alexphd1 said:

There is very little extra cost in running UFH pipes in to a insulated raft foundation.

&

4 hours ago, JSHarris said:

Fitting our UFH pipes into the passive slab took less than half a day and cost a few hundred pounds. 

 

MBC have quoted me £1,952 for UFH piping (125 m² , no manifold), which does not strike me as cheap. Should I challenge such a quote? Hmm.

[Jeremy Harris]

22 minutes ago, Dreadnaught said:

MBC have quoted me £1,952 for UFH piping (125 m² , no manifold), which does not strike me as cheap. Should I challenge such a quote? Hmm.

 

Our slab was ~75m².  We had ~300m of UFH pipe.  The cost of the pipe would have been about £200, plus maybe £20 worth of cable ties, plus two guys working for around half a day.  At a rough guess, I'd say that came to around £400 to £450 for 75m² back in 2013.

 

Not sure how the length of pipe needed or the time taken scales up for a bigger slab and perhaps more complex floor layout (a fair bit of time was spent marking all the wall, toilet, stairs and kitchen unit positions on to the EPS).  Prices may well have changed in the intervening years too.

 

Do you know how much pipe there is in the slab?  Might be able to scale up the cost by just scaling up the pipe length and the likely layout complexity increase, as both will also increase the time taken.

[Alexphd1]

My raft was DIY so was just material cost so hard to say. A quick calc you would need approx 600m of pipe approx £400 maybe less if you shop around. 

[Oz07]

Forget burning the trees you might be glad if the shading in summer with your glazing 

[jack]

Bear in mind that if MBC does it and something goes wrong, they're responsible for putting it right. If someone else does the UFH in MBC's slab and something goes wrong, you potentially have a fight over who caused the problem. I'd personally consider that worth at least some sort of a premium.

[Gone West]

15 hours ago, Dreadnaught said:

MBC have quoted me £1,952 for UFH piping (125 m² , no manifold), which does not strike me as cheap. Should I challenge such a quote? Hmm.

It's also worth considering whether UFH is the best form of 'wet' heating. There's a lot more to UFH than just the pipework and it is less efficient than radiators or skirting heating. It does have it's advantages but I wouldn't say it was a no brainer.

[Dreadnaught]

29 minutes ago, PeterStarck said:

It's also worth considering whether UFH is the best form of 'wet' heating. There's a lot more to UFH than just the pipework and it is less efficient than radiators or skirting heating. It does have it's advantages but I wouldn't say it was a no brainer.

 

Yes Peter, I tend to agree.

 

I am seriously considering not having UFH, especially given the cost of the piping as quoted, and instead revert to the traditional approach of a few radiators. I will be having a gas boiler for heating so it is also what the person-in-the-street will expect to see and any future buyer will understand it. With the level of insulation I am planning, there won't be many radiators and, as you say, they are more efficient than UFH.

 

When I get to this stage, I will do the costings and I suspect that radiators will win out.

Edited March 11, 2019 by Dreadnaught

[PeterW]

Really surprised at that price tbh as if you have a passive slab with rebar in it, the pipe will be £250 at most. 

 

Have you asked for a breakdown of the price ..?