MVHR, solar & heat pumps

[Cognis0]

I'm not sure if this is the right forum, but here goes...

 

I'm planning a Passivhaus and I'm giving thought to DHW and (occasional) space heating which even super-insulated houses will need in the depths of winter.

 

Because the space heating requirements of passive houses should be quite small it would seem to me most sensible to deliver this via warm air through the MVHR system. There are proprietary "Heat Pump Ventilation" systems such as Genvex and Ecocent which appear to use the warm ventilation exhaust as the heat source for the heat pump. These seem to also offer provision for DHW, but I have not found much information about them on-line, and I am sceptical about this as a solution.

 

Because we will install PV anyway, a better heat source to my mind would be solar thermal via PV-T combined panels. On most days this would produce adequate quantities of hot water for DH, and with a buffer tank this should also provide a source for a heat pump, if such a thing as a water to air heat pump is available?

 

Thanks for reading and comments welcome, particularly from those who have explored solutions along these lines...

[SteamyTea]

Quite simply you have to run the numbers, including prices, to see what is worth while.

Remember that solar thermal, once it has heated the DHW, then does nothing.

If combined with a PV module, it may well use  lower efficiency one.

@Ed Davies has a bit on his website about it (though it could do with a bit of updating).

As a rule, keep DHW and space heating separate. They do different things, at different times, and at different temperatures.

 

[Jeremy Harris]

Welcome.

 

There are several here with passive houses, and at least one has found that heating with MVHR works OK , @PeterStarck. 

 

We have a Genvex MVHR that can heat or cool, but we've chosen to not use it for heating, as we prefer the feel of UFH embedded in the concrete passive slab foundation.  I did try using the Genvex for heating, and it works well, in terms of providing enough heat, but we did find the air to be a bit dry, something that seemed to give me a bit of a stuffy nose.  We now use the UFH for both heating and cooling, from a small ASHP, and only use the Genvex MVHR in cooling mode.  It's OK for cooling, but not that powerful.  The unit we have is the GE Premium 1L, which has a heating/cooling capacity of around 1.5 kW.  The reality is that solar gain, even though mitigated as best we could, with solar reflective film on the outside of the glazing and deep roof overhangs, can significantly exceed the cooling capacity of the MVHR, plus, in order to deliver the rated power the MVHR has to be in boost mode, just to shift enough air around.   We ended up installing an air-to-air heat pump upstairs to provide additional cooling.  We have no heating upstairs, and the roof construction is reasonably well insulated (just under 0.1 W/m².K), with a long decrement delay, yet enough heat was rising up from below in hot weather to make the bedrooms a bit too warm.

 

Heating is really a non-issue we've found, cooling is far and away the area that's caused most concern, and led to changes to the house after construction.  Our house needs at most about 1.6 kW of heating when it's -10°C outside to maintain it at 21°C, but probably needs two or three times that level of cooling in spring and autumn, when the low angle of the sun allows it to penetrate more deeply into the house.  The best solution would have been to fit external shutters, as these work well, but the planners (or rather the Conservation Officer) weren't keen on us having them.  If we'd had the budget, I'd also have looked at fitting Sage glass, as that seems to work very well to reduce solar gain.

 

 

 

 

[SteamyTea]

33 minutes ago, Jeremy Harris said:

I did try using the Genvex for heating, and it works well, in terms of providing enough heat, but we did find the air to be a bit dry, something that seemed to give me a bit of a stuffy nose. 

You could have fitted a humidifier, but I find that after a few days of dry air my nose gets used to it.

If designing from scratch, it would be possible to put in larger ducts to increase the energy flows.

Solar gain from windows is a problem as we tend to like large windows. May be worth having a rethink on how practical they are really. Would save money by not having 60% of one side of a house as triple glazed, low U-Value frames.

 

[Ed Davies]

As has been said, the need for space heating in a passive-like house is small, small enough that it's not worth doing anything complicated particularly if you're on mains electricity (if you're off grid it's a bit more involved).

 

As @SteamyTea mentioned I did look into the trade offs of solar thermal vs PV quite a few years ago. For heating to DHW temperatures in weak sunlight (e.g., low sun in winter) my conclusion was that PV is actually more cost effective than solar thermal. That's not to say that solar thermal isn't cheaper for space heating or cheaper than DHW when you have bright sunshine but if you're designing a system to work well in the worst conditions then the best bet is PV. The prices of solar thermal have come down a bit since I wrote that page [¹] but not as much as for PV.

 

That's purely on a watts/£ of actual panels basis and there are other considerations as well: inverters and pumps ought to be taken into account but they'll probably cancel out roughly speaking but, more importantly, there's the question of excess energy in summer: extra hot water is a nuisance to get rid of and actually potentially dangerous whereas excess electricity can be left in the panel or used for better purposes (e.g., charging a car).

 

For an on-grid house now I'd go for a simple MVHR with no actual heating or cooling beyond the heat exchanger with water UFH. Heating would be either all direct electric (optimized to use the output of the PV panels where that made sense) or from an ASHP. It's right on the edge for the ASHP on cost (against direct electric) purely for heating so it really depends on the cooling requirements. PV panels would be steeply mounted (much steeper than a typical 35 or 40° house roof) to optimise winter generation accepting that that reduces summer generation a bit.

 

If you're badly shaded that would change things of course but I'm assuming the OP isn't.

 

[¹] E.g., the 20-tube Navitron evacuated tube panels now have a VAT-inclusive price about the same as what it was VAT exclusive then.

Edited November 24, 2019 by Ed Davies
Typo

[Gone West]

@Cognis0  Hi and welcome to the forum. As indicated we have a Genvex Combi 185LS which provides us with DHW, ventilation and warm air heating. It is used in conjunction with towel rails in each of the bathrooms and works well for us. This is because our house is small, just under 130m2, and because there are only two occupants. I think it would struggle if the house were much larger. Our house has walls and roof U factors of 0.09 W/m2K and as a result the heating requirement is very low. As Jeremy has already said designing out overheating is the most important element with PH design . We considered PV-T and visited a house with PV-T fitted but were told it wasn't worth the added complexity. Our PH was designed with simplicity of running and maintenance a major factor.

[Cognis0]

Thanks for your comments gentlemen, particularly useful @Jeremy Harristo learn about Spring/Autumn overheating and Sage Glass, although we are not planning to have large expanses of glass which could well be a short-lived fashion. This is clearly a complex subject which requires much thought and research.

 

At this early stage my wall/roof build-up is showing about 0.105 W/(m2K).

 

Agree that all panels should be steeply sloped for winter sun angle. Whilst solar thermal is attractive on paper, particularly in PV-T format which provides more efficient PV, I accept it is flawed (my brother who works for Honeywell was called to a property where the pump had failed and steam was being emitted from several locations!).

 

DHW and space heating could be supplied from a single source, and I know of systems where the hot flows through the DH cylinder to the central heating rads, so DHW is always prioritised. Potentially this could work with low temperature hot, it just needs to be designed properly.

 

I'm not convinced about UFH in a passive house, which seems like a sledgehammer to crack a nut when a couple of low temperature radiators in strategic locations should be adequate? (also I'm not planning to have a concrete floor slab).

 

I also quite like the sound of Magic Box thermodynamic "solar" panels, which I suppose are just simple heat pumps with some solar capture. Simplicity seems to be the byword for this product, with little maintenance requirement. It seems these are only used for DHW, but I wonder if they could be used for space heating when the demand will be small. But presumably they don't offer a reverse cycle mode to deal with cooling requirements. 

 

More research needed!

[SteamyTea]

8 minutes ago, Cognis0 said:

Whilst solar thermal is attractive on paper, particularly in PV-T format which provides more efficient PV

Not sure it does offer better PV efficiency. 

9 minutes ago, Cognis0 said:

I'm not convinced about UFH in a passive house, which seems like a sledgehammer to crack a nut when a couple of low temperature radiators in strategic locations should be adequate?

Does keep your walls free of clutter.

10 minutes ago, Cognis0 said:

(also I'm not planning to have a concrete floor slab)

What sort of floor are you having.

 

10 minutes ago, Cognis0 said:

Magic Box

Should be called a frost box. They tend to ice up.

 

11 minutes ago, Cognis0 said:

More research needed

That is always the case.

[ProDave]

Don't forget you don't have to have a concrete slab to have UFH. This is the second house I have built with UFH on a suspended timber floor.

[SteamyTea]

4 minutes ago, ProDave said:

Don't forget you don't have to have a concrete slab to have UFH. This is the second house I have built with UFH on a suspended timber floor.

How much insulation do you have under it, and what is the U-Value?

[ProDave]

1 minute ago, SteamyTea said:

How much insulation do you have under it, and what is the U-Value?

300mm of Frametherm, U value about 0.14

[Cognis0]

2 hours ago, SteamyTea said:

Not sure it does offer better PV efficiency. 

Does keep your walls free of clutter.

What sort of floor are you having.

 

Should be called a frost box. They tend to ice up.

 

That is always the case.

 

Much research still to do but.. (and of course relying upon the accuracy of material I have found available/advice received)..

PV-T panels run cooler, and therefore PV more efficient

I can cope with just one or two rads in the house

Glass foam granulate raft, possibly with cork/lime screed (UFH could sit on top of this of course, if needed)

This is the first negative I've heard regarding Magic Box/Energie type systems..

 

Thanks for comments

[SteamyTea]

6 minutes ago, Cognis0 said:

PV-T panels run cooler, and therefore PV more efficient

They run at close to the water they are heating temperature, this may not be cooler than the natural airflow they receive in a typical UK setting.

Though there are some that can be incorporated with a heap pump, but PV efficiency figures seem hard to come by.

12 minutes ago, Cognis0 said:

This is the first negative I've heard regarding Magic Box/Energie type systems

There was a large push on these about 12 years ago, there is a reason that we don't see many of them in the UK.

They are basically the working of an ordinary domestic fridge, just with the cold bits placed outside and painted black.

[Jeremy Harris]

I looked long and hard for a few years before starting our build, and was initially keen on both PV-T and the Magic Box thing. 

 

I went so far as to see an installation of "thermodynamic panels" at a supplier in Southampton.  The bottom line with the "thermodynamic panels" system is that it is just an air source heat pump that operates without the benefit of a fan to move air past the heat exchanger.  The system isn't in any way solar, and in reality the panels seem to work best when mounted vertically.  The real problem I found was that neither I, not the supplier I went to see, could get any verified performance data from the Spanish/Portuguese manufacturer at all.  In the end that supplier pulled out of dealing with the company.  I found a few Irish customers who seemed happy with the performance of their "thermodynamic panel" systems, but none could provide any meaningful data on performance.  The inability to provide independently tested data was the killer for me, together with them having been struck off the MCS register a few years ago for falsely representing how the product worked.  We fitted a conventional ASHP, and are very pleased with the performance indeed, at the low flow temperature needed to run UFH in a passive house the COP is significantly higher than quoted in the specification.

 

PV-T doesn't stand up to logical analysis at all.  In hot weather, when the benefit of taking heat out of the PV panels might be seen as an advantage, there will be nowhere to move the heat.  By mid-morning the hot water system will be hot and the panels will then run just as hot as they would without the thermal bit on the back.  The only possible advantage of such a system would be if there was some external heat dump capable of accepting a few tens of kWh of heat per day from the thermal bit, to allow the panels to run at a lower temperature.  Frankly it seems to be a solution looking for a problem, as in reality an ordinary PV system works fine in hot weather, our 6.25 kWp system regularly maxes out in summer, and all we're doing is stuffing loads of power to the grid at a time when the grid doesn't really need it.  We use our PV system to heat our hot water, and that will generally have the hot water system up to temperature and fully charged by mid-morning in summer, just with a 3 kW heating element driven from excess PV generation.  With just the two of us, we need about 6 kWh worth of hot water per day, so a couple of hours with the PV system delivering at around half power is enough.

 

My advice would be to keep things simple, and just use an ASHP running UFH for heating and cooling, and use excess PV generation for hot water, perhaps with the ASHP for preheating.

[Cognis0]

Thanks guys, this is great, real life, information now steering me in the right direction ~ exactly why I chose to join the forum!

[Cognis0]

On 24/11/2019 at 14:55, Jeremy Harris said:

I looked long and hard for a few years before starting our build, and was initially keen on both PV-T and the Magic Box thing. 

 

I went so far as to see an installation of "thermodynamic panels" at a supplier in Southampton.  The bottom line with the "thermodynamic panels" system is that it is just an air source heat pump that operates without the benefit of a fan to move air past the heat exchanger.  The system isn't in any way solar, and in reality the panels seem to work best when mounted vertically.  The real problem I found was that neither I, not the supplier I went to see, could get any verified performance data from the Spanish/Portuguese manufacturer at all.  In the end that supplier pulled out of dealing with the company.  I found a few Irish customers who seemed happy with the performance of their "thermodynamic panel" systems, but none could provide any meaningful data on performance.  The inability to provide independently tested data was the killer for me, together with them having been struck off the MCS register a few years ago for falsely representing how the product worked.  We fitted a conventional ASHP, and are very pleased with the performance indeed, at the low flow temperature needed to run UFH in a passive house the COP is significantly higher than quoted in the specification.

 

PV-T doesn't stand up to logical analysis at all.  In hot weather, when the benefit of taking heat out of the PV panels might be seen as an advantage, there will be nowhere to move the heat.  By mid-morning the hot water system will be hot and the panels will then run just as hot as they would without the thermal bit on the back.  The only possible advantage of such a system would be if there was some external heat dump capable of accepting a few tens of kWh of heat per day from the thermal bit, to allow the panels to run at a lower temperature.  Frankly it seems to be a solution looking for a problem, as in reality an ordinary PV system works fine in hot weather, our 6.25 kWp system regularly maxes out in summer, and all we're doing is stuffing loads of power to the grid at a time when the grid doesn't really need it.  We use our PV system to heat our hot water, and that will generally have the hot water system up to temperature and fully charged by mid-morning in summer, just with a 3 kW heating element driven from excess PV generation.  With just the two of us, we need about 6 kWh worth of hot water per day, so a couple of hours with the PV system delivering at around half power is enough.

 

My advice would be to keep things simple, and just use an ASHP running UFH for heating and cooling, and use excess PV generation for hot water, perhaps with the ASHP for preheating.

Hi Jeremy, I'm visiting this one again...

 

I'm still intrigued by the "thermodynamic panel" concept and would like to start engaging with some of the suppliers. Other than asking for their COP rating, what other "meaningful data on performance" would you suggest I seek from them?

 

Many thanks,

 

Tony

[joe90]

@Cognis0 just to let you know that @Jeremy Harris no longer visits this forum but I am sure others will be able to help you with this one.

 

[SteamyTea]

28 minutes ago, Cognis0 said:

Other than asking for their COP rating, what other "meaningful data on performance" would you suggest I seek from them?

The main one is power, it is no good having a really good CoP but only delivering 500W.

One of the problems with heat pumps is that people forget what they do, and how they do it. Then get bogged down in details like thermal stores, flow rates different temperatures for different jobs.

Main thing to remember is that space heating and DHW are done at different temperatures, different times and different flow rates. The 3Ds.

[Cognis0]

Thank you @joe90 and @SteamyTea