Posted By: daysleeperBy the way the house is about 418 M^3 and apparently this Genvex machine should balance at 120-140 M^3 an hour

So that is 0.28 to 0.33 Air changes an hour across the house. (seems low to me?)

Suppose we said it delivered 140m^3 an hour at 30C (10C above room temperature). That works out at about 500W using specific heat of dry air = 0.0013 J/cm^3.K.

Just to put some more context on this... It was rainy here yesterday followed by clear ish skies overnight. MHRV running at 100%, heat pump had been on for a couple of hours but off overnight. This mornings room readings (from a multi room sensor) Temp and RH...

Outside 8.9c 95%
Occupied bedroom 19c 69% (top of bookshelf)
Utility room 18.3c 70% (top of machinery so 6ft up)
Kitchen 18.4c 67%
Upstairs open plan 16.6 72%

and there is condensation on the insides of the windows in occupied rooms (this is before any showers/cooking etc has taken place)

The MHRV is extracting air at 19c and supplying (at the machine) at 17.4. Air gains about 2c between outside and the entry to the machine.

Posted By: daysleeperJust to put some more context on this... It was rainy here yesterday followed by clear ish skies overnight. MHRV running at 100%, heat pump had been on for a couple of hours but off overnight.

Given the limited capacity, I would a leave the heat pump running 24x7 whenever the extracted air drops below a preset temperature.

If you get an Economy 7 meter then the overnight period wouldn't cost much & you could probably also afford to leave the bathroom underfloor heating running overnight.

David

OK, understood. One last observation from the following Ecocent specification:

http://www.esavep.com/products/domestic-hot-water

The Ecocent requires 450m3 per hour to transfer 2.6kW into the hot water tank. The 450m3 per hour being drawn into the house will require 2.6kW of heating when the outside air temperature is 16 degrees below room temperature. So, ignoring DHW requirements & assuming the Ecocent is dedicated to heating the air required for its own operation, when the outside air temperature is more than 16 degrees below room temperature the net effect will be to cool the house despite consuming around 810W of electricity.

David

Daysleeper.. Do you have bedroom doors shut at night? Are they too well fitting? Is it possible the MRHV can't remove the water vapour because doors are shut preventing air circulating from the incoming to extract vents?

What is the U value of your windows? glass? and what spacer bars? 3g or dg?

Daysleeper,

just ploughed through 3 pages of this very interesting debate. If I can add a couple of thoughts as a passiv-haus (in all but rubber stamp) inhabitant, they might be able to help you a little? My house is very similar to yours in size and insualtion, but windows (mine are 0.7W/m2K), and mine is super airtight. I have MVHR but not your heat pump units.

I'm going to ignore the COP debate, cause I don't think that's your immediate problem.

1. Condensation - When I first moved into my house in mid winter, for about a 3 days, I didn't have the MVHR rumnning, and the windows were wet on the inside, and the air started to become stale. Got my finger out, and commissioned the MVHR - by the end of the first day, no condensation anywhere, and never had since. I've dropped my air changes right down to about 0.2ac/hr or lower.

Based on this, and some points above, I suggest you're not getting the air flow volumes through your ducts. My house is usually around 18oC, sometimes below, and still no condensation (even on door glazing which has a small DG panel). I had an occassion where my air flow rates had dropped, and I discovered my MVHR heat exchanger cartridge had become saturated with water (about 2 litre poured out when I removed it - still waiting to hear back from manufactures as to why the water held up in it) - might be something for you to look at (ie. is your vent unit actually supplying and extracting the expected air volumes - right at the unit outlets (split a few ducts off to check), and then compare at the room outlets - a gross check, but then you appear to have a gross air flow failure.

2. Heating - My calcs show my house needs about 2kW additional heat input to maintain temp when it's (minus 10oC) outside - and that's near enough how it performs, so comments above mentioning say 5kW for your house sound about right (finger in the air stuff - remembering you've got a gross failure, not fine tuning for a few % savings). I think it was suggested to make use of your log burner as a (more effective) heat source, but I got the impression you didn't bite with that.

I think you simply have not enough heat going into your house by way of heat addition (either because the heat input unit is under sized, or the air volume is inadequate (very likely both). My experience, my calcs, and just about every other person commenting above suggest that you will not get enough heat into your house via your ducts. So are you flogging a dead horse to continue to try to do so?

Solutions? (at a sensible cost)

So what about retrofitting a boiler to your log burner (either wrap around or in the flue) and get some hot water you can use for DHW and or heating. Then just switch of the heat-input part of the air heating heat pump, and run it as MVHR. This will necessitate plumbing mods, but I think you have to bite the bullet. Without understanding how your house is currently plumbed (for DHW, cylinder etc) then hard to start to recommend a solution.

3. DHW - I think from above you mentioned £360/year on DHW generation. Is it really worth struggling with a system that takes 20 hrs to heat your tank. Even if you COP were 2, for your unit, you're only saving £180/year versus a straight immersion. Bin it, get solar thermal for summer, or since your PV's are on the way, link them up to a 3 kW immersion in the hot cylinder, and chuck all the excess power you make into it, plus your log boiler's input, as required.

4. Air Infiltration - if you can actually see air gaps (around a french I think you said), how bad do you think the things you can't see will be. Air test is important (as stated above) but it's very, very hard to retro-seal a house, unless your actually renovating at the time - you might see air escaping using a smoke pencil, but where is the real hole when its behind the plasterboard. Worth trying though!

I suggest (and am happy to be told I'm talking via my chair cushion) that you've got a system for a small, well built passiv-haus, but are actually living it a midsized, spec'd close-ish to but not built to passiv-haus dwelling. Accept that it is much better than most houses out there, and if you need 5kW to heat it mid winter, that's a lowish number, but you need to move perhaps to a more "traditional" type wet heat system. Your DHW system should also be replaced as part of the wet heating system upgrade .

This need not be a lot of money, if you can be careful with the design, and creative with the pipe routes (eg. mount all rads on room walls adjacent to the stairwell (core of the house) so pipe runs/disruption is minimsed).

Maybe start a new threat "Here's my current plumbing/floorplan - how do I get 1 rad to each room and a thermal store for less than £3000?"

Sorry - didn't mean to write so much.

GreenPaddy.

Posted By: davidfreeboroughThey have mandated heat recovery ventilation because it is the most energy efficient way of ventilating an airtight building. I suggest using their approach as a starting point & building any DHW/heating system around this.

I may be mistaken, but I don't think they do mandate HRV and certainly not MVHR. I think passive HRV systems are acceptable. The only issue about either HRV or MVHR is whether the building meets the energy specs, and without heat recovery that's pretty much impossible.

They also do certify 'compact units' - MVHR units with an exhaust air heat pump and a DHW cylinder - so they don't rule out that combination. I understand they have found performance not up to spec in some installations, though.

Oh, and regarding

So that is 0.28 to 0.33 Air changes an hour across the house.

- those are plausible numbers but I seem to remember that building regs require a capacity of at least 0.44 ACH (possibly expressed in m3/m2?)

Posted By: roseramblerSo I am paying a blended rate of 10p per KW. If you don't have a thermal store set-up is here a busness case for E7 / E10? Especially if all my other power / etc is electric, and I have my heating when we are up - not in bed?

You need storage to make Economy 7 work, a thermal store &/or a well insulated building with high thermal mass. A masonry/concrete build insulated to Passivhaus standards will only lose a couple of degrees a day, so heating timing ceases to be critical & you can pump heat in at the times which are most economical/convenient.

David

We didn't spec the system it came with the house so to speak. Someone cocked up the design, but it wasn't us. The install may not have been optimum but that wasn't us either...

So some changes in the last month...builder accepts the system is underspec (finally). A couple of days of draught hunting and reduction plus a change to the system fans and operating style has led to... a much improved condensation situation.

Some measurements showing that with new improved fans the "normal" (50%) speed provides 134m^3 extract and 140m^3 input. Pushing it up to max speed and accepting the unbalance it's 170 extract and 190 supply... still short of the design spec of 0.5 ach which would be in the 215 region for both, but much closer. We;ve been running at 100% during the day and lowering to 50% for nights as pushing 190m^3 through 125mm duct is very noisy. Too noisy really, but as a pointer to where our condensation problem is from it has been enlightening.

It is now back with the suppliers to resolve. Suggestions of putting a new machine in have been made, I think re-ducting the house might be needed and that will be very very expensive.

Zurich

Having just read the entire thread, it has been helpful to me and perhaps in return I can make some suggestions.

I'm just about to build a 195m2 dormer bungalow here in the south of Sweden, u values are walls 0.096 floor 0.1 Roof 0.082 windows inc. frame 0.98.

PHPP estimates a space heating demand of 22 kWh/(m2a).

Here the building regs no longer permit the use of air source heat pumps in new houses, as far as HPs go all that is now allowed is ground source or exhaust air.

In order to fulfil this demand NIBE has developed the F750 exhaust air heat pump which combines both a balanced MHRV and a 6kw air to water heat pump, the exhaust air temps from which go to about -20 deg C.

http://www.nibe.eu/Domestic-heatingcooling/Exhaust-air-heat-pumps/Product-range/NIBE-F750/

This unit is a step up from the F470 unit which stones has. To compare the two the F470 is designed to ventilate a house of up to 120m2 floor area and provide 2kW of water heating whilst the F750 can ventilate a house up to 200m2 and provide 4-6kW of water heating.

I have also been looking at the solutions from Genvex but your experience has confirmed what I suspected that the output is too small for my requirements.

If you do look at replacing your units I would suggest you look closely at the F750, here in Sweden it retails for around 64,000SEK inc. VAT of 25% so at today's spot rate about £6,000.

It's the only exhaust air heat pump which I can find that gives these kind of outputs. IVT do resell a unit called the ComfortZone but it does not provide balanced ventilation.

My intention is to distribute the heat using wet UFH downstairs and a 3kW nominal output fancoil situated in the family room at the top of the stairs which has the four bedrooms off it, I am hoping that I won't have to provide heat sources directly in each bedroom.

You could employ a hot water duct heater to distribute some heat via the HRV ducting but my research so far suggests that this is a less than ideal solution.

Another possibility to help fix your current heat distribution problem is to install an air to air heat pump in your upstairs hallway to help boost the temperatures upstairs.

I'd also suggest that in your hunt for air leaks a smoke pen might well be a worthwhile investment.

Many thanks, CPB. I was just about to buy one from elsewhere at over £30. Nick

[edit] Now I'm even confusing myself!

In the spring time Nibe will introduce the SAM 40 supply air module to dock with the F750 which will provide a balanced ventilation system.

BTW Is that Paul unit still available I don't see any sign of it on the Paul website?

Well not much new... another month on and still waiting on Denmark to figure out how they will redo the ducts in the house to allow a minimum of 0.5 air changes an hour.
The MHRV failed again the other day, condesate drain had blocked and the unit filled with water. Fairly easy fix for once.
I've asked the gas company for a quote to connect the house up. So much for "green" living.