Posted By ICFHybrid on 21 May 2012 11:56 AM
I'm sorta confused by the "near-passive house" description. Is the OP talking about high mass walls and ceiling at better than R38 and windows up at R8?

Do you shoot for passive house standards and fall short, or how does that come about? Is that typical UK construction or something that is done to utilize a particular heating plant or strategy?

The OP's description was:  "250m2 traditional block house; u-value Walls, roof and floor < 0.15, triple glazing u-value 0.7"

Converting that to US- style number, me U 0.15 metric = 0.15/5.678= 0.0264, for a whole-wall U-factor, which  is >R37.85 whole-wall R (US).  Call it R40-ish, not counting dynamic weighting for the thermal mass.

Current code-max U for new construction walls in the UK is U0.30, which works out to about R19 (US) min whole-wall- more stringent than the R20 (cavity) or R13 +5c.i. code-min requirement in most of the US climate zones 4 & 5, which works out to be ~R15 whole-wall at typical framing factors.

The U-factor for the glazing in US terms would be (0.7/5.678)= U1.23....  or at least sort-of.   The standards by which window U-factors are determined in the EU differ somewhat from US standards, and are measured at a lower delta-T.   Any way you measure it it's a lower-U than most US triple-glazed windows, and indeed comes in at ~R8.

For a UK climate this is pretty damned close to PassiveHouse, in fact it wouldn't surprise me if one could actually hit Passivehouse levels of energy use with a well-tweaked passive solar design in the UK with U-values that low.

I wasn't sure if "block house" referred to a row house or the actual construction method.

I'm sorta turning the solar and passive combination around and around thinking about them. With windows up at R8, you can't be getting too much insolation coming in, can you? And, if you add windows to capture more solar, you are going to put a strain on the passive qualities, but if you reduce glazing is it really a passive solar design any more than any other building with some proper orientation?

The PassiveHouse tools model that pretty well, and most designs that make it in US climate zones 5 and higher have decent amounts of low-gain glazing on the south side, but in more temperate climes (like the UK) that effect is less pronounced. The down-side to going to high gain windows the south facing windows in a home with R40 walls, floor , & roof is the severe overheating during sunny periods leading to significant cooling loads even during cool or cold weather, but that can be offset to some degree with higher interior thermal mass and a variable ventilation rate strategy.

ALL homes are passive solar homes- it's only a matter of degree, and whether anything at all was done at the design phase to optimize the solar gains for heating relative to the losses. The tighter and better insulated they are, the larger the "free" heating fraction, and the less the solar gain is needed from the south facing glass. PassiveHouse has a number for how low the interior surface of the glass can be at design temp and still meet spec, which pretty much eliminates high gain/higher-U glazing, even though tweaking the interior thermal mass to keep the temperature swings bounded even with lossier/higher-solar gain glass works, if you can tolerate the lower surface temp. In a UK climate, if not going for the certification I'd be inclined to go with a more standard higher gain but low-U double pane on the south side of the house rather than go lock-step on PassiveHouse notions of what makes sense. Ultra-low U windows are expensive, and bigger ones are even more expensive.

The preponderance of detached homes in the UK are masonry or stone clad, even when not all structural elements are masonry, so whether "block house" refers to the square-ish fort-like architectural style or a construction method, it's bound to have some substantial thermal mass (at least on the exterior of the insulation.)

ALL homes are passive solar homes- it's only a matter of degree, and whether anything at all was done at the design phase to optimize the solar gains for heating relative to the losses.

Well, that's a good point. More should be done to encourage anyone building a new home to have it reviewed for the most minimal efforts to increase passive solar gain. It would mean literally millions of bbl of oil per year saved here in the US.

a variable ventilation rate strategy.

I've found that is key. Tight, broken up spaces are difficult to work with. It helps to have a high ceiling for the heat to go to and a way to either recycle it or draw it off.

While the average passive house might have so low an aux heat load that 2-4kw meets it, we don't know much about the OP's near passive house load or how much he is counting on passive solar (or where he is in the UK or whether he is building high mass or low mass.) He does say that he needs aux heat for sunless days so I repeat my caution about treating aux heat as "topping up" in climates that aren't overly suited to passive solar/high mass (again, with no idea what the OP proposes.) When the sun disappears for days, aux heat bettter be capable of a good bit more than topping up. When the sun disappears for weeks, it is prudent to have a means of heating mass directly in a high mass house

Peak loads of typical code-min houses in the UK are usually under 10kw- I have confidence that an energy-conscious design would come in at less than half that. The average Passive House has less than 10watts per square meter (if it's over that, it doesn't get the certifcation), and most are well below that, so a 250m house would be only 2.5kwh-peak. He would have to work at it to end up with loads above 6kw at a typical UK design temp of -2C. But a 6kw load is only ~20KBTU/hr, which is on the small to middlin' side for mini-splits- they come much bigger if needed, but that need isn't going to happen in a low-U house. The advantage of going smaller is that smaller units are capable of tracking under very light loads without cycling on/off, leading to higher efficiency and greater comfort.

Note, 6kw is more than half MY heat load, in a house slightly larger than his, but with mere ~R10 whole-wall, ~U0.4 (US) windows, and at a design temp of -15C, a bit more than 10C below typical design temps for the UK, more than 5C below even the coldest parts of Scotland. At -2C (his approximate design temp) my house load is under 6kw.

My uncle lives in a similar sized house (with a lot of windows with more than 2x the U-factor of the OPs), in a climate only slightly lower than the UK (and easily as cloudy) yet manages to heat the place with a mini-split rated for 7kw output @ 47F/8C. It's almost inconceivable to me that a house with very low U-values would need 6kw for heating, but a passive solar house might have that much peak cooling in a UK summer.

The notion that it's necessary to heat the mass if high mass homes directly isn't well founded either. Heating the mass directly has the temporal lag issues to manage, and would result in wider variances in room temp. Ductless mini-splits are "set and forget" modulating systems producing very steady room temps with shifting load, whether those shifts are rapid or slow.

Your uncle needs to move into a high-mass house, Dana, to appreciate the fact that they can run btu deficits and,hence, will run deficits when the power fails, you leave on vacation, equipment fails.... If I am obliged to warm my house by 5 degrees, my 20kw wood stove boiler, plumbed to a radiant slab, represents a time lag, most assuredly, but my 6kw ductless represents a time laaaaaaggggggggg. And we'll set aside the efficiency questions of warming concrete by warming air.

Yes, I have both. I suspect that where you find ductless in the UK, the rationale is the same as mine: a quick and inexpensive way to add AC.

Tod has a valid point on the ductless. I too have radiant floors and ductless both ironically operating from electric. I use the floors even though the electric boiler does not offer the advantage of 25 SEER, COP 3.5 or so. But you can't get a better cooling device for the money.

Comfort is the first thing.

I would not argue against solar, but have noted that no one uses hot water like the Americans. Here in Minneapoiis we can heat about 50% of the domestic hot water for a family of 4 - albeit most of the hot water is made in summer - pay back, about 30 years with natural gas at the current rates.

Morgan: Comfort is of course the first thing, but the house under consideration has a whole-wall R of ~R40, U0.125 windows (the design specs under consideration) in a place where the outside design temp is about +28F, with a likely design-condition heat load of less than 5 BTU/ft. Even if the design condtion heat load is over the PassiveHouse spec (max of ~3 BTU/ft.) the additional comfort of radiant floors are pretty much toast- it would be barely discernable, given the specified R40 floor. Under all conditions the floor temp wouldn't be more than a degree-F or so above room temp, and under average daylight hours conditions it would be at or even slightly below the average room temp due to stratification, even with radiant floors.

Were this anything like a code-min or 1.5x code-R house in a US zone 5 or higher climate I'd be inclined to go with hydronic floors though. At more than 2x code on R values and window performance, maybe not. The O.P.s winter climate is comparable to the warm edge of US zone 4/cool edge of zone 3 (Atlanta GA has a lower design temp!) and has building performance factors that are more than 2x code-min in US zone 6 (with the exception of the roof-R, which is comparable to code-min for zone 6.)

I'd hazard that your windows are more than 2x as lossy, and walls more than 3x, with average midwinter heat loads on the order 2x his design condition load.

toddm: My uncle doesn't need to move- he's happy right where he is, in a great house with views of glaciated volcanos overlooking beautiful waterfront in the foreground, and is downright THRILLED with the comfort factor boost he gets from heating with a mini-split compared to the (now-idle) condensing propane hot-air furnace he had been living with. (Not to mention that its cut his heating bills by something like 80%.) Living in the temperate climate first-world (like the original poster) his power grid is pretty reliable too. So even if he DID live in a high mass house rather than that slummy low-mass timber-framed shack on the bluff overlooking Puget Sound, he's not likely to have to raise & lower the temps in 5F steps very often, or at least not often enough to rationalize spending another 5-10 grand or more on hydronic heating. There's no indication that the O.P. was intending to go high-mass on the interior, but doing the passive solar simulation to include sufficient mass to limit overheating would be wise.

+28F !, we break ice and swim in that weather! ehehheee

Posted By BadgerBoilerMN on 23 May 2012 12:09 PM
+28F !, we break ice and swim in that weather! ehehheee

Me too!

I would too if I could swim

I am not asking anything of your uncle except to reserve his role as exhibit A for cases in which we know enough facts to judge his worthiness. But in the interest of moving on, a ductless is the right answer....

IF the house is low-mass or the hp is sufficiently oversized to accomplish reasonable recovery.

IF forced-air hvac is not a significant resale penalty in the UK, or the OP doesn't care. (Saw a study that forced air accounts for 2 percent of residential hvac in Europe.)

IF he prizes economy over comfort.

Now you may have divined these answers, in which case, kindlly tell us which horse wins the Belmont.

Ductless isn't the same as forced air, and any super-insulated house is already well outside the normal resale home realm in the UK (or anywhere else), and would not be assessed by potential buyers in the same way.

There is no comfort penalty to ductless in an ultra-low heat-load house.  Radiant floors would make less than a 1F difference in average mid-winter floor temp- you literally wouldn't know whether the heating system was on or not. With 1-1.5 ton ductless the blower would be either idle or running on low during any waking hours, about as noisy as a refrigerator, so you'd know if it were running if you're in the same room as the head, but otherwise not. The HRV system would likely make as-much or more sound.

There is a significant uptick to comfort when displacing/replacing US-style ducted air heating systems with ductless.

But OK, have it your way, don't use my uncle as exhibit-A (despite the climate similarity.)  If you get a chance, take a house-tour on a PassiveHouse sometime- preferably on a mid-winter day, and see if you can tell if the heating system is on (or even what it is.)  There's a house 5 crow miles from mine heated via ductless (often being cooled by ductless, even in winter- the kids have too many game machines plugged into big-screen TVs.)  This place is pretty damned cozy and radiant floor heating would be ridiculous.

You don't have to hit true PassiveHouse levels to get to the point where radiant heating is pointless.  And having the ability to cool would be as important as heating, since the heating/cooling balance point of a house at the OP/s U-values would likely be around 50-55F, in a climate with a  mean winter temp of about 40F. (much warmer than my climate.)

Gotta go with the OP's label of near passive, as in not passive; among the many things we don't know is how much glass he wants. Yes, ductless doesn't necessarily mean forced air, but air/water doesn't necessarily mean inexpensive any more either.

How is it helpful, when we don't have sufficient data, to introduce another house, your proximate passive house, that may or may not be a legitimate Exhibit B?

Dana is right.

"you literally wouldn't know whether the heating system was on or not"

This is true! though it hurts me to say it. Perhaps I will soon be obsolete? But "ridiculous" now that hurts.

When design HVAC systems we often run into triple-glazed windows, R-40 walls, etc. and do not push our beloved radiant floor, walls or ceilings on the uninformed. It does happen that we get down to single digit heat loads in basement remodels where we pulled up concrete floors and framed out wall with high density foam including rim joists. The low design water temperatures 1-2°F over ambient make for great efficiency for our condensing boilers.

If pays to keep in mind that the definition of comfort is the lack of discomfort. Thank God for old houses or I would be back to plumbing for a living...

The "ridiculous" part is about doubling or tripling the system cost with no perceptible change in comfort, and a possible shortfall on cooling potential that would require additional mechanical systems.  If it were cost-neutral, and had sufficient cooling capacity (which a chilled floor might have, in that relatively low dew-point climate) or produced a commensurate boost in heat pump efficiency that had a financial rationale, I'd be all over it.

At such low total heating energy use the money is better spent on rooftop photovoltaics than a more expensive & complicated heating scheme, an approach that would reduce annual site use of energy by considerably more than the nominal efficiency boost for an ASHP from very low-temp radiant floors.  With either hydronic or mini-split ASHP, he'd be hitting in the 4s for annual system COP not 5s.

Todd, we have plenty of data on the O.P.s objectives and climate, even if his "near PassiveHouse" falls well short of PassiveHouse spec. To meet PassiveHouse the heat load at design condition has to be under 10W/meter, which is (34.12/10.76= ) 3.17 BTU/ft at design condition.  If he hits 5 BTU/ft, (58% higher than PassiveHouse) by going with too much glass it doesn't change the average floor temp enough to matter, but it would mean the air-condition loads would increase substantially.  Hell, call it 2x Passivehouse (which would be possible to do by leaving windows open at night, I s'pose), you'd be able to tell the floor was warm for a few pre-dawn hours on the coldest days of the winter, but not during 99% of winter waking hours. 

I picked 6kw (20KBTU/hr) as the likely sweet-spot for a heating "system", assuming the possibility of a 5kw heat load (= 17KBTU/hr, or 6.3 BTU/ft for a 250m house), but it's likely that it'll be under 4kw (13.6KBTU/hr which comes out to the 5BTU/ft number for his 250meter house.)  In the US a high heating efficiency mini-split with a 6kw nominal output runs about 4-4.5 grand, installed. A 2-head 6kw multi-split runs about $5.5-6k, installed.  Any hydronic output system + radiation costs are going to be at least 2x that number after installation, unless people in the UK have decided to work for free.

PassiveHouse designs have quite a bit of glazed area already, but are selective about orientation to control gains for the not-too-hot, not-too-cold averages. If he undersizes the glazed area he'd have an easy time meeting PassiveHouse spec W/m  peak heating load, but could still blow the annual heating BTU limit by insufficient passive solar gain.  Google for images of "PassiveHouse UK", you'll see that they are not dark little hovels devoid of daylight & view by any stretch. And doubling the glazed area might bring the design-condition heat load up to 5BTU/ft, but probably not 6.  The wall and roof U-values of actual UK designs are comparable to what the O.P. was talking about (maybe a bit more on the roof, a bit less under the floor, but about the same for walls.)  This one took it a step further and went for Net Zero as well as Passive House. 

The design condition heat load (at +5F)  of the Beaton house near me is 4BTU/ft, which exceeds the 10w/meter requirement spelled out for Europe (maybe that spec has been relaxed for the PassiveHouse US certification they're going for?), so I consider it a very good comp for a UK home with near-PassiveHouse design goals, despite the ~50% higher R required for the outside design temp that is more than 20F colder.  The BTU/ft number is what makes or breaks radiant floor cush-factor, and it's right in there with what I'd anticipate for a realistic max for a house with his design goals.  Whether its 3, 4, or even 6 BTU/ft at design condition, it won't mean squat for comfort at the winter average load.  Were it more like 10BTU/ft & up @ design temp, radiant floors would actually offer a comfort premium.

I have been puzzling over a question for a page or two. If the peak load in a code min UK house is less than 10kw, or 34kbtu, or slightly less than 3 tons -- as per Dana -- why is the country not ASHP heaven? The answer not surprisingly is the government. As a Kyoto Protocol signee, Britain regulates CO2 rather than btu, and applies a fuel factor to a house's anticipated carbon emissions. The fuel factor for grid based hvac, including heat pumps, is 1.47. The fuel factor for a pellet stove is 1.0. While the OP's near passive house is sure to meet code regardless, the OP, as a green conscious Briton, is also quite likely to embrace the renewable bias in its implementation.

The scuttlebutt in Britain, for a couple of years now, is that gas boilers will be banned in new construction by 2016. I haven't been able to find a definitive regulation yet but most heating engineers I know there are comment on it. ASHPs and GSHPs may yet rule the day.

Posted By toddm on 26 May 2012 09:47 AM
I have been puzzling over a question for a page or two. If the peak load in a code min UK house is less than 10kw, or 34kbtu, or slightly less than 3 tons -- as per Dana -- why is the country not ASHP heaven? The answer not surprisingly is the government. As a Kyoto Protocol signee, Britain regulates CO2 rather than btu, and applies a fuel factor to a house's anticipated carbon emissions. The fuel factor for grid based hvac, including heat pumps, is 1.47. The fuel factor for a pellet stove is 1.0. While the OP's near passive house is sure to meet code regardless, the OP, as a green conscious Briton, is also quite likely to embrace the renewable bias in its implementation.

In fact the UK is indeed ASHP heaven, despite the current carbon factor of the coal-fired grid.  Like most of Europe hydronic output ASHP options in the 3-10kw range abound.  While it's not currently being subsidised for those on the gas-grid, there's a £850 subsidy on ASHP (£1,250 for GSHP) for those heating with propane or oil.  Nothing like expensive fuel and subsidies to get an industry rolling, eh?

But the average performance of the installed base is pretty low compared to mini-splits (see figure B, p. 4)  due to poor system design (mostly higher than optimal water temp requirements, distribution losses, etc.)  To get anything like mini-split performance out of them REQUIRES low-temp hydronic floors, which is a cost-adder.  Unlike hydronic-output ASHP, systems, a mini-split is a "system in a can", with very low design-risk on the performance end, and proven performance in the mid-3s or higher for UK type climates.  A 50% oversized system (for the 99% heat load) would very likely hit a seasonal performance of 4, which is nearly 2x the performance of the existing fleet of ASHP systems in the UK.  This is performance I would expect in the O.P.s house, since even a 1.5 ton unit would be oversized for the likely heat load with a great deal margin, but may be necessary to handle peak cooling loads.

But even the low-performing ASHPs on the UK power grid have about the same carbon footprint as homes on the UK gas grid- see figure H, p.21 in that document.  It only takes a seasonal COP of 2.2 hit parity on CO2 emissions with gas fired heating.  Wind & wave power development is currently on the rise in the UK, though well behind Scandanavia in implementation. The UK grid is getting greener over time. The fact that a near-passivehouse in the UK will have as much or more cooling load as heating load makes a pellet-boiler approach less practical, and even if pellet boilers are currently greener than ASHP on the 2012 version of the grid, the carbon footprint of a mini-split will improve over time. 

Spending the money on rooftop PV instead of low-temp radiation would more than offset the carbon footprint of the cooling load of a near-Passivehouse building (unless they really screw up oversizing of the passive solar gain), and might even make the house lower-carb than with the pellet-boiler / radiant floor solution.

If you haven't already, I recommend David McKay's 2009 (free, if downloaded) book, Sustainable Energy- Without the Hot Air, which (among many things) analyzes the present and anticipated future carbon footprint of heat pumps on the UK grid.  It can also be read on-line a chapter at a time, if you like.