It feels like a leap of faith. There is a 6 year old thread on this board where someone asks the question and is not unreasonably told that it all depends on insulation and air tightness.

I have visited a house (retro-passive) that has MVHR in 2 zones with post-heaters where there is no contribution to warmth other than the appliances/bathrooms/occupants. Demonstrably that house does what is required, with the option of an electric oil-filled radiator in case of need for a bit of local boost if needed.

I have been told categorically that the problem is not warmth but keeping the house cool in warm weather.

My proposal is a 245m2 4-bed house on 2 floors; timber frame; fabulous insulation and thermal bridging figures, etc. using a MVHR unit to be designed and installed by the timber frame manufacturer, who will also install the windows and doors, validate the thermal performance and calibrate the system before leaving site. I have assurances that the house will pass PHPP with flying colours.

I have a provisional SAP calc. that uses an air permeability design value of 4.0 – despite my insistence that it is going to be under 0.5 – (They say it doesn’t make a significant difference to the result).

The annual space heating fuel requirement comes out at 2639 kWh, with the highest month being January at 511 kWh. The assumed number of occupants is 3.06 – which is likely to be higher than actual for most of the time.

What if the desirable ambient temperature is higher than 21 degrees used for the SAP calc? Could a higher ambient temperature of say, 23 degrees be maintained? Can anyone say with conviction that any house that is good enough to pass PHPP could sustain 23 degrees C without any additional (central) heating?

It would be very nice to save the cost of UFH and associated heat pump if it is going to be superfluous. The converse of regretting not having made adequate provision is unthinkable. Might a couple of post heaters in the MVHR provide enough boost when needed?

There will be a log burner – Chesney “Milan 4 Passive” with a concentric triple wall flue designed for passive houses – but that was always going to be for secondary use and not intended to be lit very often.

TIA

Dare!
If the retro-passive house (Enerphit?) you visited floated your boat, then with new-build you should be more than comfortable. PHPP is a pretty stringent process- I would be asking for a tour of a house or two built by your contractor, with utility bills, blower door test figures, etc available for inspection. Most PH occupants are so feverishly evangelical that this will not be an intrusion!

Your site's location both generally-are you in Devon or Dingwall?- and specifically- is the site shaded and windy?- will greatly affect the design. Your target figure for air tightness should be dialled in to the contract. 4.0 is really poor and above advised MVHR operation levels. MVHR should have a summer bypass function, which in combination with fenestration articulation*, should deal with overheating.

Based on your figures and ignoring any heat generated by appliances, occupants, etc your January space heating bill comes in at about £60 if purely generated by electric post heaters. This feels a little high for PH- others will be here soon with greater empirical data. Your overall energy bill should work out at about a 10th of a conventional new build...

I have a feeling that heat retention will become harder as the desired temperature goes up (exponentially?) but again the wonder that is GBF will spit out some info on this. Certainly the sense of warmth in an airtight well-insulated house, can be had at a lesser temperature than the converse. For instance, with the convection currents and cold downdraughts that an old house serves up, coziness feels at times unattainable. So your desired 23 degrees may prove to be a false target. I visit a well sealed house with UFH which feels toasty at 18 degrees, even for long sedentary periods. But I'm well hard.

I don't want to get too hairshirty but in your enviable position, I would want to feel a tiny bit more involved in the fantastic machine that you can create. Perhaps don't see the stove as so peripheral, that it's lighting on a particularly cold night is in some way an admission of defeat. After all, you are designing it in at this stage.

Must dash, I have a stove to set and light prior to t'other half's return...

Bon voyage and keep us posted!

*window opening

Apologies for the omission: the house is in Gloucestershire and partly shaded by a large tree to the south-east. Mid-day and later sunshine will get through uninterrupted and will be attenuated in summer months by eaves and verges standing out a metre or so from the walls of the house (whose roof has a very low pitch of 5 degrees).

The size and location of windows has been dictated by position and orientation of the house with respect to views rather than trying to stick to the principle of putting the big ones on the south side and the little ones opposite. The big ones are on the west face.

The living rooms and kitchen are above and the bedrooms below, dictated by the desire to get the best views from aloft.

The concept is the opposite approach from cropthorne house in that it is a very low thermal mass structure.

I take your point about the feel of the environment in an MVHR environment, the retrofit house I visited near Oxford was very comfortable to my mind at 20 deg. C but I don't mind when the quilt gets nicked. Whereas she who nicks it has other ideas.

Done it, got the tea shirt, living there. <tonyshouse.info>

Are you basing 23C on a non airtight house? Even a slight draughty makes a difference.

You might find UFH pipes useful to provide cooling. Not active cooling; just the pump running to distribute heat through the floor.

I am overwhelmed by the number and depth of responses, thank you all.

There is mains gas in the road outside the property but the connection cost doesn't make the cost/benefit calculation persuasive. Oil and LPG are eliminated on grounds of space for tanks and inconvenience of supply logistics. Being entirely dependent on electricity is a risk but other energy sources require some electrical control or pumping, so a small auto-backup generator was the way I was thinking of keeping the place running in an emergency. The log-burner would provide the proverbial belt and braces for cold weather power cuts.

I would like to think that Cerisy's experience with a log burner might be indicative of our situation, except that having the living rooms (largely open plan) and the main bedroom upstairs, the natural heat-rising would not work against us. The Poujoulat "Efficience" chimney system appears to address ringi's concerns about the log burner acting as a heat-sink when not in use.

I like the Viking House principle of circulating cold water from a ground loop in the ground floor slab at roughly 10 Deg. C in summer to counteract any tendency for the building to overheat but at this stage I am not even planning to have a slab. I want to build using ground beams/ring beam spanned with prestressed joists and decked with OSB over the above-formation-level insulation. In using Fermacell for internal walls I can virtually eliminate all wet trades from the site, except for Alumasc rendering on carrier boards outside.

Nobody has either confirmed or rejected the idea that post heaters in the MVHR could manage to raise/hold the temperature and I think I need to do some more research on air-to-air heaters as suggested by Ed. There is a psychological reluctance to using oil-filled electric radiators because it infers an admission that whatever heating system is built into the house is inadequate. However, the cost saving should assuage that.

At the end of the day, the objective is to have a simple house that doesn't need a 100 page instruction manual for the owner to keep handy all the time. I think JS Harris' house is terrific but the objective in this case is very different. It has to be as near to fit-and-forget as possible.

gp,

You seem to be going in the right direction, but a couple of points (apologies if they have been raised already?)

.....an air-permeability difference from 4m3/m2h to 0.5m3/m2h is MASSIVE, who told you it doesn't make much difference is talking rubbish!

Are you sure a TF co is the best choice for designing and installing MVHR?

Same above for the window/doors installation?

Is the PH certification valuable/important to you? or part of a contract/warranty/guarantee?

If mains gas is available, you will regret not getting it brought into the dwelling, even if just for cooking? make your design as future-proof as you can?

Good luck...

@ Going Passive:

Housing - New Build: Dare to design a house with MVHR and no central heating?

Would you mind if I change the title a bit:

viz.
"Old Build: Dare To Remove Central Heating"

In effect, this is what we've done - viz. put the radiant electric slab into retirement.

Bioclimatic house built 1983. After adding insulation and new windows, we started carting electric radiators to the municipal tip. In Another Leap Of Faith, last year we had the electric meter derated, so the floor won't even work now...

So have now halved the bill, and more to come - feels nice

We retained a 2 kW ACOVA in the basement lobby, but it is little-used, and next year (given the understair radiant electric I plan on installing), ACOVA will just be for show and "feel-good" effect

gg

Posted By: goingpassiveThere is mains gas in the road outside the property but the connection cost doesn't make the cost/benefit calculation persuasive

Have you had a quote? Our gas connection only cost a few hundred whereas water and elec were 1000's. Unless there's major work on your side of the boundary it's normally cheap.

Posted By: DarylPIf mains gas is available, you will regret not getting it brought into the dwelling, even if just for cooking? make your design as future-proof as you can?

I don't miss a gas hob - induction is so much more controllable.

Posted By: GreenfishMrs G also misses having a hot spot for warm clothes and towels (even if she is actually warm it seems to be psychological).

I've been missing a radiator this week. Stuff on an airer dries really quickly in a ventilated house but there are just a few occasions when you want somewhere warm to put something - I was trying to dry a pair of ski gloves this week.

Posted By: DarylPAre you sure a TF co is the best choice for designing and installing MVHR?
Same above for the window/doors installation?

I can see the advantage of this. It requires the TF design to properly think about the integration. We did the 3 things separately and self installed the MVHR. There was a lot of work to do between the TF and the window install that I didn't anticipate. I had to make adjustments to the MVHR install because, although the TF co had the design, some of the beam penetrations just didn't work and I had to relocate things.

Further thoughts on mitigating the risk of getting this wrong:

Ed Davies pointed me to air-to-air systems, which turn out to be very modestly priced when compared to compared to an un-filled UFH installation. "Un-filled" because more than one potential provider has suggested to me that you can install a UFH matrix provisionally without the immediate intention of using it. i.e. defer the cost of the heat pump until after you move in and discover whether or not the house is warm enough.

I have gone out for quotations from Mitsubishi partners for a SCM45ZM-S pump to be coupled with 2 internal SRK25ZM-S units, capable of heating or cooling and thereby providing a good degree of control of ambient temperature should it be necessary. Nominally under £1100 (supply only and VAT discountable for new-build), if I can install suitable air-sealed pipe ducting during the build, the installation cost should be modest. I will report back in due course.

Responding to Greenfish with regard to DHW, I am in awe of JS Harris' setup but the objective here is simplicity, ease of use and minimising the number of appliances that can go wrong. I am therefore specifying a Joule 260 litre heatbank cylinder, located in a central position so that runs to hot taps are as short as possible. I acknowledge that the temperature recovery time for such a device is lengthy and therefore the biggest 300 litre alternative might be more pragmatic but I don't have the headroom to install such a tall one together with its ducting. The occupancy of the house is low so there should be ample hot water for normal daily consumption. The price of having visitors staying in the house will be the need to run the immersion heater from time to time. I would not intend to apply extra insulation around this unit as it will be located in a bathroom and any excess heat should be both pleasant in that room as well as useful feed to the MVHR for recovery and redistribution. No warm toes or towel rails but warm fresh air if it works out as intended.

Peter, the virtue of the Joule heatbank is the ducting option. By standing it next to an external wall the heat pump takes in external air and exhausts outside so there should be no interference with the ambient internal environment other than a bit of heat loss into the room.

The preliminary SAP calc. requires no extra point-scoring from added renewables such as PV units, so while I make no attempt to deny the benefits from using them, I regard them as an unnecessary complication in this case. There is a height constraint on the site, which is why the roof has a low pitch of only 5 degrees, which also means that installing panels at the appropriate angle on the roof is not an option.

It's a fair point Peter. I hadn't thought of laying them down. The roof is a standing-seam type so will readily take clipped-on framing for the panels and there is ample area for the extent of array you suggest. I will have to reconsider the cost/benefit and whether or not keeping them clean and the trapping of airborne debris is likely to become an issue.

Having said that, the governing approach is not about cutting running costs to the bone in exchange for adding more technology to the mix. Leaving behind the annual ~£4k cost of energy for a 1960s house which will be sold when this one is ready and moving into a new world of the low hundreds is persuasive enough. I struggle to believe headline COP figures but the marginal cost of the actual COP being a below target is not a constraint in this case.

Thanks
Edward

Posted By: goingpassiveI have gone out for quotations from Mitsubishi partners … I will report back in due course.

Please do, look forward to hearing how you get on.

…and VAT discountable for new-build

Good point - this is a general problem with builds which are somewhat experimental in that you have to declare the house complete at some point and various rules change. It makes it hard to try something, with a fallback plan in case it doesn't work, but instead pushes you in the direction of over engineering from the beginning. There are similar problems with building regs (via SAP and friends) which make you come up with a design which can be documented to work.

Heat pumps are about as reliable as refrigerators

They have efficiencies of between 300 and 400%

They beat economy 7 for water heating providing you cost them over a decent length of time.

Posted By: tonyThey beat economy 7 for water heating providing you cost them over a decent length of time.

What length of time?