"Combination" MVHR/DHW/UFH systems

[M J Harwood]

Have any members had actual running experience of the latest so called "Combination" heating systems that handle MVHR plus DHW and have sufficient capacity to run say an UFH system as well? They can also accept input from solar panels.

They appear to rely on an air to air heat pump plus any spare heat left over from the exhaust air in the MVHR unit.

Is this asking too much of the system? It is hardly the KISS principal!

Some, but not all, need a 3 phase supply which presents difficulties on my project.

Some heating engineers I met last year, at a show in Harrogate, said  on no account start mixing solar panels with UFH systems. Use solar to augment DHW and a small gas boiler "ticking over" for the UFH.

I appreciate that I am putting the cart before the horse on this as I intend to get a PHPP analysis done to determine my energy requirement and may need only some or none of this kit.

[Dave Howorth]

I don't have any experience, so I can't answer your questions, but I can say that there are at least three meanings for 'com-something' so it's important to avoid confusion.

In this country, there is the combi-boiler, which heats water instantaneously as well as providing space heating. I think this can be excluded from your topic!

On the continent, a 'combisystem' is the name for a solar thermal system that provides space heating as well as water heating (DHW). They usually also have another heat source such as a boiler. UFH is often used to distribute the heat.

In the passsivhaus context, a 'compact unit' is the name for an air source heat pump combined with an MVHR to provide DHW and heated air. Since the necessary power is limited by the passivhaus standard, I'm surprised that three-phase is needed. It is also possible to use solar panels, either solar thermal or PV, in conjunction with such a unit.

The level of uptake of these concepts and products in this country is considerably behind that on the continent. Perhaps not everybody you've talked to were talking about the same thing?

[Nick Grant]

In Passivhaus context you are looking at a compact Unit as Dave says. These are arguably complex but not inherently more complex than say a fridge freezer. The attraction is the avoidance of a gas connection or ground loop for GS heat pump.

For Passivhaus the vent rate is limited to what is required for ventilation and heat is supplied directly to this air so is limited by volume and temperature.

However I believe there are non passivhaus versions that manage to deliver more heat by over ventilating. That is the suck more warm air out and recover the heat from it.

This is sort of pulling yourself up by the bootstraps thermodynamically but even cold outdoor air contains heat.

I'm getting out of my depth now as not sure how these units work.

There is some discussion elsewhere on the forum including why an 80% efficient MVHR is more efficient than a 300% 'efficient' heat pump!

If you are at Ecobuild I'm chairing a session on hot water and I'm hoping this will be at least touched on by John and Alan. I expect there will be kit on sale but beware of green claims and loose use of phrases like 'renewable energy' 'free heat' and 400% efficient!

http://www.aecb.net/forum/index.php?topic=2288.msg9156#msg9156

[john cantor]

The energy in ventilation air seems to correlate very well with DHW energy requirements.
This does not mean that it is an ideal match.
1, Could one do better things with that air? - yes probably passive HRV
2, Tiny heat pump on exhaust air will result in very slow DHW heat up time, so cylinder needs to be big. (not necessarily a problem).
3, What happens when DHW satisfied? - probably put heat into rooms.
(This is the basis of the Compact unit)

When ambient is fairly mild, then passive HRV doesnt do a lot, so DHW heating might be worthwhile. 

Some of this technology comes from Sweden where air temperatures are quite different, and CO2 figures for electricity are 1/10 ours. So it needs looking at closely.

Will cover this at the ecobuild session, but no time to do in depth in a short session.

[M J Harwood]

Thanks for all the information.

Dave, the question was specifically with reference to a Passivhaus design.

Nick and John. Sods law in operation here. I will be at Ecobuild only on Tuesday and so will miss your spot on Wednesday.

I would have liked to met the two of you and Alan Clarke.

Will there be presentation notes?

I guess what this all boils down to, for a mere mortal who only wants to live in an energy efficient home, is proceed to with extreme caution.

Later addition: Whoops just read the EcoBuild Fringe programme. At least I can probably get to the Tuesday presentation on Passivhaus uptake. Maybe you will be lurking around the AECB stand on Tuesday?

[Nick Grant]

Presentations from Ecobuild will be available but as John says only covered briefly unless there is someone there asking the question and PPT slides not totally self explanatory.

I'm on the AECB stand Thurs but looking around myself Tuesday.

Nick

[Alan Clarke]

Re the compact unit - I would avoid for now - German research presented at the last PH conference showed that one of these didn't work at all well when monitored - not much better than direct electric heating, and Viessmann no longer sell the unit in question.

I have had good experience of combining solar and ufh - but only with an expensive carefully designed (and very big) thermal store (Consolar) - all UK ones look like they will fail to work well because of the conflict between the need to maintain stratification for solar and the need to move heat up to the dhw coil. UK approach tends to be rely on heating the whole store up with a boiler and a pump to mix the contents. Consolar use clever internal bouyancy driven circulation.

Alan

[Colin Gleeson]

Nick tells me that there is CSU monitoring data for a row of housing in Frankfurt.  Does anyone have any details?  This may provide some clarity on their overall performance