Green Building Forum - Help with ASHP and a 5 year nightmare...

Help with ASHP and a 5 year nightmare...

CWatters — Fri, 11 Nov 2011 22:51:37 +0000

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.

Help with ASHP and a 5 year nightmare...

daysleeper — Sat, 12 Nov 2011 07:20:00 +0000

But as that is about the best the unit can do... does that not imply an overall COP of less than 1 ?

Help with ASHP and a 5 year nightmare...

daysleeper — Sat, 12 Nov 2011 09:37:56 +0000

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.

Help with ASHP and a 5 year nightmare...

davidfreeborough — Sat, 12 Nov 2011 11:11:40 +0000

Posted By: JSHarrisModelling this whole system is the challenge, particularly the time-dependent factors through the day (heating demand being out of phase with solar gain, for example), but with an adequately sized thermal store and a careful bit of design and construction it should be perfectly possible to use an exhaust air heat pump very efficiently.

I don't want to undermine any of the good work you're doing; it's all good research for your build. However, the Passivhaus people have put a lot of work into simulating low energy houses & they have years of experience of adjusting their simulations to match reality. They 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.

Have you studied the Silver & Gold level CarbonLite standards on the AECB website? They set quite aggressive targets on Primary Energy Consumption for a Passivhaus in a UK context (with carbon intensive electricity supply). The Passivhaus Planning Package takes into account all the heat sources you mentioned & mandates heat recovery ventilation, but they still specify an additional space heating requirement of 15kWh/m2 per annum. If you are not using heat recovery ventilation then this requirement can only be higher.

To put some numbers on it: for my current plot/build 15kWh/m2 per annum works out as 3150kWh per annum. The MVHR heat exchanger recovers 9590kWh per annum, while the fans consume 525kWh per annum & the electrical resistance pre-heater 250kWh per annum. If I was using electrical resistance heating, this would imply a total energy consumption of 3925kWh per annum excluding DHW, lighting, appliances, etc.

If I was to bypass the MVHR heat exchanger & just use the fans to provide continuous supply/extract then the total energy consumption would jump to 13265kWh excluding DHW, lighting, appliances, etc.

This is the starting point for exhaust air heat pumps which don't involve an MVHR heat exchanger. All the air extracted needs to be replaced with cold air, so assuming the same air flow rates, the 9,590kWh needs to provided by the heat pump. If we assume the space heating is provided by electrical resistance heating, the heat pump just heats the air drawn in & that the heat pump has a CoP of 3 then the heat pump will consume 3196kWh & the total energy consumption would be 6871kWh excluding DHW, lighting, appliances, etc.

If the heat pump has sufficient capacity & it can maintain its COP of 3 while taking the exhaust air temperature down below outside air temperature then the space heating might also be provided by the heat pump. In this case the total energy consumption would be 3196+(3150/3)=4771kWh excluding DHW, lighting, appliances, etc. This is still 20% higher than the Passivhaus case & in reality the heat pump is unlikely to be able to provide the additional capacity with the air flow available.

My advice would be assume an MVHR unit & then look at options for heating the house which bring heat in from outside. Could you use an over-sized solar thermal array? Have you looked at CO2 heat pumps? They are not cheap, but they avoid the need to pay for back-up log burners, chimneys, etc.

David

Help with ASHP and a 5 year nightmare...

davidfreeborough — Sat, 12 Nov 2011 11:27:24 +0000

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

Help with ASHP and a 5 year nightmare...

JSHarris — Sat, 12 Nov 2011 11:45:01 +0000

Posted By: davidfreeborough...........the Passivhaus people have put a lot of work into simulating low energy houses & they have years of experience of adjusting their simulations to match reality. They have mandated heat recovery ventilation because it is the most energy efficient way of ventilating an airtight building.

Just to be clear, I'm not proposing using exhaust heat recovery as the only means of ventilation and heat recovery. This wouldn't be wise because of the relatively short time that the exhaust heat recovery system would work during low heat demand periods - it would be out of phase with the ventilation needs in all probability. It all comes down to the point I was making about the time dependent factors that affect the house ventilation and heating needs

My plan is to use the Ecocent system, which includes an MVHR unit and heat pump together, with a valve that ducts air as required, so the MVHR is effectively bypassed when the heat pump is drawing air from the house, and the cold air inlets to the house are bypassed when the heat pump is off and the MVHR is working.

Help with ASHP and a 5 year nightmare...

davidfreeborough — Sat, 12 Nov 2011 12:25:33 +0000

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

Help with ASHP and a 5 year nightmare...

JSHarris — Sat, 12 Nov 2011 12:45:42 +0000

Posted By: davidfreeboroughOK, 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

Yes, we've been through this before. You're assuming no other energy input to the house (as I've mentioned before) when in reality you have to consider the house as a semi-closed system, with heat inputs coming from multiple sources. If you have a house that is around passivhaus standard, then the heating demand is low. Solar gain, occupant heat output and the waste heat from all the electrical loads can easily exceed the total house heating demand for part of the day. Using a heat pump to take that excess low grade heat and store it in a thermal store as higher grade heat allows that heat to be phase shifted to later in the day when it is required. If you like, it's an intelligent way of managing the house temperature to cope with varying external and internal heat sources that cannot otherwise be controlled.

The passivehaus people have done a lot of work using massive thermal storage structures in the house, to absorb excess heat from solar gain and occupant activity etc and give it back (very slowly) when the house temperature drops, which is a valid approach but one that lacks control. All I'm proposing (as I've mentioned before) is a way of doing the same thing but with better control and the ability to use excess heat from solar gain etc to supply DHW, with the added benefit of cooling the house down whilst doing so.

Help with ASHP and a 5 year nightmare...

CWatters — Sat, 12 Nov 2011 18:42:51 +0000

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?

Help with ASHP and a 5 year nightmare...

daysleeper — Sat, 12 Nov 2011 20:14:07 +0000

Posted By: CWattersDaysleeper.. 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?

Afraid not, doors are cut high over wood floors but mainly stay ajar anyhow. We do have blinds iso curtains, but experiments with leaving them up don't make much if any difference.

Help with ASHP and a 5 year nightmare...

tony — Sat, 12 Nov 2011 21:06:56 +0000

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

Help with ASHP and a 5 year nightmare...

daysleeper — Sat, 12 Nov 2011 21:13:16 +0000

Windows are double glazed 4mm glass, 18mm spacer then 4mm thermo glass with argon fill. exact U value I'm not sure. I think 1.3 ish but could be less.

Help with ASHP and a 5 year nightmare...

GreenPaddy — Sun, 13 Nov 2011 22:49:27 +0000

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 Â£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 Â£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 Â£3000?"

Sorry - didn't mean to write so much.

GreenPaddy.

Help with ASHP and a 5 year nightmare...

GreenPaddy — Sun, 13 Nov 2011 22:54:27 +0000

Oopps - don't start a new "threaT" - just a new "threaD"

Help with ASHP and a 5 year nightmare...

djh — Tue, 15 Nov 2011 14:17:29 +0000

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?)

Help with ASHP and a 5 year nightmare...

roserambler — Wed, 16 Nov 2011 21:34:13 +0000

Just to go back to the issue about the rate you pay for electricity, I thought I might float this:

I have an ashp 14kw air to water, with a ST panel - servign 210 sq m house over two floors, 310 lt DHW tank and two wee ones so use lots of water etc. Seems to work ok so far - fingers crossed. 8 months in, I have used 5,000kw total energy, but have the worst / coldest 4 months to come. Everything is electric - as we are in the sticks. Just about to install 2 x country 4KW woodburners to take strain off ASHP when its cold. I am guessing I will use at least double in the next 4 months what used over first 8, but that would put me somewhere around 15,000kw per annumtotal consumton for all energy bills.

Now I looked into E7 and E10 tariffs and decided they weren't worth it as I didn't have a thermal store. Also the Mitsibisi 140 is an inverter driven unit, thus varies the rate it works dependent on load / external temps. I worked out it is cheaper to use a low gerneral tariff than go for a E7/10. Mainly cos you are so restricted to when use, which seemed to be when I didn't want my heating on. DHW would be the only true bonus, but I have ST and even when not sunny, only seems to take 40mins -50mins to heat whole tank.

So 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?

Any views?

RR

Help with ASHP and a 5 year nightmare...

davidfreeborough — Thu, 17 Nov 2011 08:48:01 +0000

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

Help with ASHP and a 5 year nightmare...

Gavin_A — Thu, 17 Nov 2011 10:44:12 +0000

as I pointed out in the previous thread, from the manufacturers manual for this system and the floor space figures you gave, you have system that's significantly undersized - it's not pushing the limits as you state, it is undersized.

This is the major cause of your condensation problems and your heating problems, there's no great mystery to it, and you simply need to chase up whoever designed your system for a replacement (or their insurers). No amount of tinkering is going to solve the fundamental problem here, and I'm amazed that you didn't sort this out after the last thread when I pointed this out after you finally gave us the make and model of the unit installed.

if you personally designed the system and just paid someone to install it, then you'll have to take it on the chin because it would be your fault, not the installers, although it does sound like they didn't help the situation.

sorry to put it so bluntly, but beating around the bush isn't really going to help anyone here.

Help with ASHP and a 5 year nightmare...

daysleeper — Sun, 18 Dec 2011 15:39:28 +0000

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.

Help with ASHP and a 5 year nightmare...

CWatters — Sun, 18 Dec 2011 17:20:47 +0000

Any kind of new build warranty?

Help with ASHP and a 5 year nightmare...

daysleeper — Sun, 18 Dec 2011 18:20:02 +0000

Zurich

Help with ASHP and a 5 year nightmare...

CWatters — Mon, 19 Dec 2011 10:58:01 +0000

Have you approached them to see if they cover it? This guide appears to suggest it's a possibility...

http://www.zurich.co.uk/NR/rdonlyres/9AFAF3B2-B902-46AA-9C67-D0ED85D6F1C0/0/500021006WebOpt.pdf

Claims Notification....

...provide to us professional reports at your
expense to verify the claim where it relates
to the performance of central heating,
sound insulation, squeaking floors.

Help with ASHP and a 5 year nightmare...

Chris P Bacon — Wed, 21 Dec 2011 10:04:08 +0000

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 Â£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.

Help with ASHP and a 5 year nightmare...

Chris P Bacon — Wed, 21 Dec 2011 10:07:56 +0000

Link for the smoke pen

http://uk.rs-online.com/web/p/leak-flaw-detection/4945963/

Help with ASHP and a 5 year nightmare...

Nick Parsons — Wed, 21 Dec 2011 11:27:41 +0000

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

Help with ASHP and a 5 year nightmare...

davidfreeborough — Wed, 21 Dec 2011 14:06:54 +0000

Posted By: Chris P BaconHere 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.

The NIBE website describes this as an exhaust air heat pump. There's no mention of an MVHR type heat exchanger, but a supply air module is available for heating the supply air.

The only unit I've come across which combines an MVHR heat exchanger with an air to water heat pump is the PAUL Compakt 360 DC.

David

Help with ASHP and a 5 year nightmare...

Chris P Bacon — Wed, 21 Dec 2011 15:15:55 +0000

[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?

Help with ASHP and a 5 year nightmare...

davidfreeborough — Wed, 21 Dec 2011 16:14:08 +0000

It seems to have disappeared, although you can still buy filters for it.

Attached is the brochure I have for it.

David

Help with ASHP and a 5 year nightmare...

daysleeper — Sun, 22 Jan 2012 10:03:09 +0000

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.