Positive Pressure or Balanced Pressure ventilation system?

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bramwell

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Master Geek

#151035 11-Aug-2014 19:05

Hi Geeks

A few weeks ago we received some great advice regarding heating and ventilation and now have another question we hope you can help with.

Our asthmatic daughter, her partner and our wee grandson are renting a very small (60m2) 2 bedroom Wellington property from us.

Our 2 main priorities are to install a system which will ventilate without pushing cold air out of the roof cavity into the flat during winter, and ensuring good air quality for our asthmatic daughter. Following advice and our own research, we had finally settled on having a balanced pressure true heat recovery ventilation system installed.

However today we were advised by a very honest ventilation salesperson that because the heat pump is too small to heat the whole property, the cold air blown in by a PPVS in winter (from the roof cavity) would be no worse than losing possibly up to 3 degrees with the heat exchanger on the balanced pressure system. He was in fact trying to down-sell us to a PPVS from their more expensive balanced pressure system.

The heat pump is currently being run only in the lounge on winter nights with the bedroom doors shut, and they use an oil heater in the baby's bedroom at night. However following advice from here they will be soon trial running the heater with all the doors open when it is easiest to heat - i.e. during the daytime, so that at night the heat pump won't have to run hard to catch up. They cannot afford much electricity.

Now we're back to wondering whether PPVS or balanced pressure?

Balanced pressure systems are more expensive, but if there won't be much benefit perhaps we would be better to look at the cheaper PPVS and put the extra money saved towards a installing a larger heat pump - however ...

Is there a substantial running cost increase from running a 3.2kw heat pump to a 5kw? If we get a larger heat pump which costs them more to run, they may not be able to afford to run it.

We're not in a position to install both a balanced pressure system and a new heat pump.

Thanks again for advice :-)

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Uber Geek

#1106683 11-Aug-2014 19:19

For something that small, wouldn't opening the windows be better for ventilation, that is what EECA recommend as one of the best ventilation methods for house. I wouldn't be too keen on pumping cold dirty air (possibly containing insulation fibres from insulation) from the roof space into my rooms. If I was to setup such a system, I would want to take the air from outside, or to use a heat exchanger so the outside air is heated. The temp difference between the outside and the roofspace air will liekly be very similar.
You can also buy larger heat pumps that will heat that sized space, maybe even a ducted or one with multiple outlets. Installing a wood burner could also be an idea. Most of these though are just bandaid solutions. The real problem will be the house itself, which may not have wall or floor insulation, and probably no double glazing or thermally insulated frames. I would suggest going to the eeca energywise website.

bramwell

120 posts

Master Geek

#1106687 11-Aug-2014 19:37

Thanks Mattwnz
The house is insulated :-) However they don't leave windows open in winter because it's too cold and our daughter suffers some respiratory issues so they need to keep the heat in. Unfortunately the place is so small there is nowhere to fit a woodburner. We agree re the preference to take air from the outside, hence our original plan to install a balanced pressure system - but if the cold air blown in by a PPVS in winter (from the roof cavity) would be no worse than losing possibly up to 3 degrees with the heat exchanger on the balanced pressure system, we're not quite so sure which way to go, given the cost difference. If outdoor air quality is vastly different to roof cavity air however, we'd probably still be inclined to go with the balanced system.

Batman

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#1106706 11-Aug-2014 20:20

must be marketing lingo.

as far as science goes, positive pressure system is when air is pushed into the room.
negative pressure is when air is sucked out of the room.
balanced? what you mean no air movement? or that air is pushed in one end and suck out the other end? what difference does it make surely? suck the air in and blow it back out? great strategy to make money?

Involuntary autocorrect in operation on mobile device. Apologies in advance.

syousif

41 posts

Geek

#1106737 11-Aug-2014 21:08

depending on the cost difference and how many outlets, you can get inline heating elements for the positive pressure to heat (more like take the chill) of the air from the roof. the heating element is 1kw on the smartvent units.
i used this on a our old house now a rental property, and it worked ok, still needed a heater in colder winter days but didnt need it as much.

also ensure you get good filters for the fan, as getting air from roof with someone that has a respiratory problem may not be best unless proper filters are used.

you can say good buy to the moisture around the windows with a positive pressure unit as well, but also helps to have extractor fans in the bathroom/shower area, and kept on for few more mins after having hot steamy showers.

timmmay

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#1106740 11-Aug-2014 21:12

Balanced means there's both air being forced in and taken out, the ducts are positioned so the air is drawn toward a central point then the stale air taken out. In a heat recovery balanced system the heat from the waste air is used to preheat the incoming air. The air in the ceiling cavity can have all kinds of things in it - fiberglass dust, rat droppings, chemicals from the wood treatment, etc, etc. Outside air can have smoke this time of year, but cold air is generally dry - cold air has a much reduced capacity to hold water compared with warm air. That's why you get condensation.

In my opinion you should get the balanced system. The PPV system will only ever put cold air in in winter, which means if it's 16 degrees inside (which is far too cold) the input could be 2 degrees from the roof. From a heat recovery system it might be 14 degrees, which is a huge win. A heat exchanger is far more cost effective than running a 1kw heater, which is approximately 1/2 as effective as a plug in fan heater.

Don't mess about going from a 3.2 to 5kw heat pump, the incremental cost to go to a 7 or 8kw pump will be minimal and the running cost will be lower. I know the CoP is higher for smaller heat pumps, but not if it's running at maximum all the time.

IMHO you need to do both - heat and ventilate. I would heat first as windows are acceptable (just) for ventilation. We ventilate our house during the day, something like 11am to 3pm with a cheap old PPV system, then it's shut up the rest of the time. Works fine here but we have a pretty large air volume in the house.

If you post your questions to existing threads people who previously contributed get notified, it's preferable to starting a new thread. This thread should probably be merged back in to that one, maybe a mod will do that.

Hammerer

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#1106744 11-Aug-2014 21:19

bramwell:
Is there a substantial running cost increase from running a 3.2kw heat pump to a 5kw? If we get a larger heat pump which costs them more to run, they may not be able to afford to run it.

To produce the same amount of heat you will generally use about the same amount of electricity for heat pumps whose capacity isn't a magnitude different. You can check the technical specs for each heat pump model and there are performance reviews on the web. The important thing is to make sure that it is an inverter heat pump so it can run continuously at the output level required rather than regularly shutting down then starting up again like the old non-inverter heat pumps.

What changes the equation is any situation where the heat pump has to run close to capacity, ie flat out, where its efficiency (the coefficient of performance, COP) is lower. This is much more likely to happen to the smaller heat pump which then increases its relative operating cost. The following conditions increase the likelihood of this happening:

The closer to zero C the outside temperature then the harder the heat pump has to work.
The greater the temperature difference between inside and outside.
The greater the temperature difference inside between the actual and target temperature.
Setting the fan speed so it is too low to transfer the heat efficiently.
Failing to clean and maintain the system so it can function optimally.

Read this heat pump primer or have a search yourself.

bramwell:
However today we were advised by a very honest ventilation salesperson that because the heat pump is too small to heat the whole property, the cold air blown in by a PPVS in winter (from the roof cavity) would be no worse than losing possibly up to 3 degrees with the heat exchanger on the balanced pressure system. He was in fact trying to down-sell us to a PPVS from their more expensive balanced pressure system.

I'm unconvinced. First, I'm sceptical about some claims for PPVS systems. Second, I've had sales staff down-selling because they had to meet their quota for specific groups of products.

If you are comparing a heat recovery ventilation system that has a heat exchanger then you get 50-90% heat recovery. Let's take the worst case of 50% heat recovery then you only have to heat half the air that you pump in. That air should be drier which means that there is less heat required to maintain the interior temperature.

Assuming the same rate of air replacement then means that the positive pressure system must get air that is at a temperature halfway between the outside air and inside air e.g. 6C outside, 18C interior so must be above 12C in the roof space. You can check what you've actually got now by putting a thermometer up there. I'd be really surprised if it is closer to the interior temperature than the exterior temperature but if it is I'd want to know how it is staying warmer - I've seen houses with obvious sources such as no ceiling/insulation above the hot water cylinder. Is it because there is not enough ceiling insulation? Is the interior air rising into the roof space? In that case you're pumping more humid air back into the house. It will also be a limited resource that can only maintain heat by getting it from the interior of the flat. If it can maintain a higher temperature then you need to look at more insulation in the roof space. I'd always look first at putting more insulation in because I know that I'll get a positive economic return, long term. In your case you were talking about a few years so it could work out more economic for you to forgo it in the short term provided you don't hold onto the flat long-term.

You say the house is insulated but how it is insulated is really important. For example, we have old batts which were poorly installed (they don't cope with being squashed or being soaking wet if your roof leaks). In colder rooms the wooden framework became visible through the ceiling and walls because it was noticeably colder. We've now installed a thick layer of blanket insulation above the ceiling and we can tell the difference because there is only one cold spot remaining at the southern end of the house. We could put another layer and I bet we'd notice a further difference because it cuts down on air flow (drafts) and reduces the likelihood of developing larger, faster convection flows, i.e. where the wind whistles through the attic.

Anyway, back to the comparison. Now if we substitute better figures for the heat exchanger, that is 80-90% heat recovery, then I can't see any way for the in-ceiling system to compete. For example, if the outside temperature is 6C and the interior is 18C then the roof space has to be at 16C to compete. I think that its got no chance.

The 3C the salesman mentioned doesn't sound reliable but it would mean, say, 6C outside, 18C inside, 15C in ceiling space. That is equivalent to 75% heat recovery but your heat exchanger should top that - check the specs to be sure.

I've used 18C because of the small heat pump. 20C is a lot more comfortable but it would not be achievable in 60m2 with a 3.2KW heat pump. Our 6.4KW heats 40m2 comfortably but would struggle all night at 6C.

bramwell

120 posts

Master Geek

#1106748 11-Aug-2014 21:36

Thanks yet again Hammerer for your advice.

Your explanations have reassured us that we should go ahead with the balanced pressure system as planned. The heat recovery system is said to recover 80-90% so we can't go wrong really.

We had the place looked at by a reputable builder (he's also a friend of ours) before we bought it and he said the insulation was all good and fairly new. We'll see how things go heating-wise and if absolutely necessary after the ventilation system is installed we might consider putting in a larger heat pump ... $$$! :-D At least we could sell the existing heat pump for a few dollars.

Once again, many thanks

bramwell

120 posts

Master Geek

#1106756 11-Aug-2014 21:53

Hello again - one further question

The property has a decramastic style roof, with no surfeits/soffits (sic?) So for the balanced pressure system we need to install roof kits for intake and exhaust. We were told that in summer the heat from the decramastic roof cladding will radiate up into the intake cowling and push very hot air into the property making for a very hot uncomfortable internal temp.

Has anyone had a similar situation, and if so, did you find this to be the case?

Thanks again

Hammerer

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#1106795 11-Aug-2014 22:39

We looked at putting a heat exchanger in but we balked at the cost. It was not economic but it would have been great for comfort and convience. At the time, 10 years ago, the controller would switch off the fans if it was too hot or too cold and we could have different limits by choosing, from memory, one of two programmes. If the system you're looking at does the same then you should have no problem.

In winter it is an advantage because you want all the heat you can get.

We've got colour steel and the heat radiates downwards into the roof cavity. Any ducting should be well insulated on top of the insulation in the ducting itself - that applies even with intakes under the eaves. Our roof space will have me profusely sweating within 30 seconds.

mattwnz

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#1106801 11-Aug-2014 22:57

bramwell: Thanks Mattwnz
The house is insulated :-) However they don't leave windows open in winter because it's too cold and our daughter suffers some respiratory issues so they need to keep the heat in. Unfortunately the place is so small there is nowhere to fit a woodburner. We agree re the preference to take air from the outside, hence our original plan to install a balanced pressure system - but if the cold air blown in by a PPVS in winter (from the roof cavity) would be no worse than losing possibly up to 3 degrees with the heat exchanger on the balanced pressure system, we're not quite so sure which way to go, given the cost difference. If outdoor air quality is vastly different to roof cavity air however, we'd probably still be inclined to go with the balanced system.

The thing is that the air temperature of the air coming in through an open windows is pretty much the same temperature that comes in from the roof space, so you would still be pumping in cold air, whether it is from the roofspace, from outside via soffits, or air flowing in though an open window. My parents have a ventilation system, and I can feel the cold air coming in through the vents in the winter, and I switch it off for them. Theoretically it should switch off during the winter when it gets too cold anyway, but that is the time when you need it to be working, to prevent condensation. The roofspace maybe a little warmer if you have downlighters, but it isn't hugely different. I think you will notice a bigger difference with a heat exchanger than just 3 degrees, you maybe best to speak to a company that specialises in heat exchangers, as to what they can do for you. They are pretty basic principle, but these sorts of things in NZ are very expensive compared to Europe.

Just to add, with companies that sell things, some will give you 3 prices, and will often try to sell you the middle option which will often appear far better value than the most expensive one or the cheapestone. It is good marketing, because they are making you compare their own prices, rather than you comparing the prices of different companies. It is all about psychology.

timmmay

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#1106830 12-Aug-2014 07:32

Also don't forget you can turn the ventilation system off at night - I do it using a timer. That means moisture in the air will go up, but it'll be quickly fixed in the morning when it comes back on. Those things have massive fans that change the air multiple times per hour, and the good ones have variable speed fans that slow down at night. None that I know of are made to turn off at night though.

bramwell

120 posts

Master Geek

#1106843 12-Aug-2014 07:49

Thanks again everyone for all your help.

Timmmay, good point re adding on to my previous thread - I'll bear this in mind in the future. If the family find the air coming in at night is too cold we'll look at putting a timer on for night - great solution, thanks.

Mattwnz, do you think the heat exchanger would reduce the inside temp by more than 3 degrees? Or less than 3 degrees? Hopefully less - for winter, anyway :-)

timmmay

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#1106848 12-Aug-2014 08:17

A heat exchanger sits between the outgoing and incoming air. You said yours is 80% efficient. If your indoor air is 20 degrees and the outdoor air is 0 degrees then the incoming air will be around 16 degrees. If the outdoor air is ten degrees the incoming air will be around 18 degrees.

Amosnz

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#1106871 12-Aug-2014 09:31

It's probably not so important for this scenario where everything is close together, but the insulated ducting (R0.8 from memory) loses 1 deg of heat for every 3m of ducting.

I have quite a long house so we have 6m of ducting from the source to the heat exchanger, then another 9m of ducting in the longest branch (4 branches in total). With a wood burner we can easily get the lounge (source room with cathedral ceiling) up to 25+deg. If the outside temp is 10 deg and the heat exchanger is 75% efficient (I know they say up to 90%, but in reality...) then (25-2-10)*0.75 +10 = 19.75 deg after heat exchanger. Subtract another 3deg for the loss of the 9m run and the air going into the far room is 17deg.

I'd love to get some kind of temperature monitoring setup where I can track 6+ zones and actually see what difference the ventilation makes.

On a side note I used to get bad chesty coughs at night. It didnt happen very often (4-5 times a year maybe) but the only way I could stop it was to put a sweatshirt on and try to make my chest as warm as possible. Since we've had the ventilation I haven't had it occur (I actually had to stop and think to remember the last time it happened).

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timmmay

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#1106877 12-Aug-2014 09:47

Amosnz: It's probably not so important for this scenario where everything is close together, but the insulated ducting (R0.8 from memory) loses 1 deg of heat for every 3m of ducting.

This probably needs to be qualified with both the temperature of the duct and the ambient temperature. It's probably a rule of thumb someone made up. You can always add extra insulation to ducting, though it may not be easy or practical.

I'd like a good temperature monitoring solution for multiple zones too.

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