Can you solve this mystery?

digannio

Ignite said:

A question for those of you who are using a PIV.  How quiet are they?  Do you hear any fan noise?

Pretty quiet is the answer. Our diffuser is on the ceiling outside the main bedroom. We leave the bedroom door open and you can't really hear fan noise.

Gerry1

Out of interest, what was the cost of the PIV, including installation?

digannio

Sorry, I can't help with that. It was installed in 2018 and we only moved here 9 months ago.

Petriix

Gerry1 said:

Presumably the answer would be to switch off (or reduce) the PIV's heater and mount it on the landing above a GCH radiator?  The rising warm air should prevent draughts (and you don't sit in an armchair on the landing all evening), so the heat loss would be met by the GCH at much lower rates and the extra cost would be lost in the noise.

It doesn't particularly matter where the air is coming in, it's about the temperature of the air being forced out. If you've paid to heat that air to 20 degrees and you're replacing it with air that's at 5 degrees then you're losing that energy. I agree that the extra cost will be lost in the noise insofar as the OP seems to be able to completely deny it's happening, but that doesn't mean it's not happening. 

digannio

We'll take it then that you've never lived with a PIV. 

Petriix

digannio said:

We'll take it then that you've never lived with a PIV. 

That's absolutely beside the point.

Put it another way: did you notice that the house was significantly warmer or that the heating was working noticeably less hard while the 400W heating element was switched on?

I doubt it. But adding 400W of heat to the air will have directly reduced that demand from your heating. Removing that energy source will either reduce the temperature or increase the demand on your heating by an equivalent amount. 

That's objective fact. Basic physics. Not opinion or anecdote.

It's totally fair enough if you're happy to spend that additional energy on having a well ventilated and condensation-free house. But it's nonsense to pretend that there is no cost. This is money saving website so it's important to consider these things from the economic angle.

If there's some flaw in my logic then please point it out. The wasted energy is the only reason why I have chosen not to install a PIV system myself.

QrizB

Petriix said:

If there's some flaw in my logic then please point it out. The wasted energy is the only reason why I have chosen not to install a PIV system myself.

The only thing that comes immediately to mind is that we've not allowed any offset for the energy that would otherwise be lost through other ventilation means. Eg. if you've got a bathroom or kitchen extractor to combat condensation in those rooms, or you need to leave a bedroom/landing window open, you might be able to substitute those with PIV. And, at least in theory, your loft space is warmer than "outdoors" so these's less heating required for air drawn from the loft than would be the case for air drawn in from the wild.

The obvious alternative to PIV would be MVHR but that is likely to have higher installation costs.

Petriix

QrizB said:

Petriix said:

If there's some flaw in my logic then please point it out. The wasted energy is the only reason why I have chosen not to install a PIV system myself.

The only thing that comes immediately to mind is that we've not allowed any offset for the energy that would otherwise be lost through other ventilation means. Eg. if you've got a bathroom or kitchen extractor to combat condensation in those rooms, or you need to leave a bedroom/landing window open, you might be able to substitute those with PIV. And, at least in theory, your loft space is warmer than "outdoors" so these's less heating required for air drawn from the loft than would be the case for air drawn in from the wild.

The obvious alternative to PIV would be MVHR but that is likely to have higher installation costs.

That's reasonable. Not easy to quantify though. Yes, MVHR systems are much more efficient, but more expensive like you say. I wonder about a dehumidifying air-con unit which also does heating. At least that way the heat would stay in the house.

digannio

I don't think this can usefully go much further. You're convinced that my heating usage must be going up. I tell you it isn't and have the daily usage charts that show it but you don't believe it. You've never had a PIV so don't have direct, real-life experience of one.  I use one everyday and my heating usage is fine. Obviously there are a lot of other variables to heating, heat loss and ventilation in individual homes  Probably best to leave it there. Enjoy your New Year.

BobT36

Petriix said:

digannio said:

Petriix said:

digannio said:

It's just not true to say that it's like leaving a window open 24/7. I don't know if you've lived with a PIV but within six months I've gone from living without one (lots of condensation everwhere even with bathroom and kitchen doors closed) to living with one (virtually nil condensation most of the time). I know which I would choose. And it's just not true to say you need to keep the heating on because of it. Now I've turned the heater off my leccy costs have normalised.

I don't have an axe to grind, just telling it as it is for me with real life experience of  a PIV. There is hardly any condensation, less dust in the house, the air is fresher (without it being cold) and any smells, like cooking, disperse very quickly.

Your misunderstanding me. Of course the benefits of the PIV system are better than just leaving a window open. But your heating costs will increase as a result - that's an objective fact because you're literally pumping cold air into your house which will require energy to heat. Now that you've turned off the inbuilt heater, your (presumably gas powered) central heating will be picking up the additional demand. Either that or you're living in a colder house.

If the PIV has been properly installed and set up correctly and in the correct place in your home then it shouldn't have a significant impact on your heating. Do we now have to keep the heating on longer? No. Does the heating take a lot longer to reach goal temperature? No. Does the house generally feel cold? No. It would be wrong to suggest that there is a cold blast of air fighting against the room thermostat. The effect of the ventilation with a PIV is more subtle and gentle than the impression you are giving. But maybe you haven't lived with a good one, I don't know.

It's impossible to quantify your anecdotal observations about how long the heating is running for; there are too many variables. I've used significantly more gas in the past 12 months than in the preceding year simply because temperatures have been lower. 10% extra demand could easily be hidden in the variations. 

If you know the flow rate of the PIV system and the average temperature differential between the loft and the house then you could calculate it. Back of an envelope estimate would be ~ 1kWh every 3 hours the system is running for the coldest 4 months of the year. That's ~ 1000kWh additional demand on your heating. Not enormous, but definitely not insignificant.

If you disagree then please share your calculations. 

I'd rather trust the word of a dude who actually owns and uses one, (as long as no abnormalities with setup / house) than people purely theorycrafting.. 
Also not everyone runs their heating on a specific thermostat, either. I simply turn mine on when I'm too cold (after wrapping up) and turn it off when I'm warm enough. Subtle differences wouldn't change anything for me once I'm used to it.