Pre-heater or no pre-heater....

[djcdan]

I am torn. 

 

We'll be installing the Zehnder Q350, but unsure whether to purchase the pre-heater with it or not.

 

We are based on the west coast of Scotland, right on the coast. We very rarely get snowfall or frosts (although it was frosty for a few days shortly after New Year) and winter temps very rarely drop below 1-3 deg - even overnight - as a result of the gulf stream affect.

 

However... despite all this, it is still Scotland and who know what the weather can throw.

 

My gut is saying no, but head is saying yes. Any thoughts?

[TonyT]

I would say yes, simply cause I think it’s easier and more cost effective to fit it now.

 

and I live in Scotland too, so know that it’s going to come in handy?

 

 

[ProDave]

I am on the much colder (but dryer ? ) side of Scotland and have no problem without a heater on the MVHR

[PeterW]

Haven’t fitted a pre heater but have fitted a post heater as I’m not convinced that heating the cold air coming in will extract as much heat from the outgoing air. 
 

Out of interest does anyone else have sensors in their intake / extracts and can log the temperatures..? 

[joe90]

Our bedrooms are a little cool at this time of year and I did consider a heater in the MVHR vent into the main bedroom (the others are rarely used so a little oil radiator for occasional visits would suffice). However I did toy with the idea of a wet heater running off the DHW tank (as it’s very near the MVHR unit) and could run on thermo syphon with simple gate valves to turn it off in summer. When sorting out my workshop I found a wet radiator (no idea where it came from but brand new) it has 15mm copper tails so plumbing would be easy and I could make an airtight box to mount it in, like I made my own manifold and silencer. (Sorry @djcdan if I hijacked your thread ?).

[TonyT]

My point in having a heater was as a back up heater, I’m always keen to ensure a plan B is in place.

 

other may have wood burning stoves etc

indeed I have 2 wood burners as back up heaters and also a bit of atmosphere!

[A_L]

2 hours ago, djcdan said:

We are based on the west coast of Scotland, right on the coast. We very rarely get snowfall or frosts (although it was frosty for a few days shortly after New Year) and winter temps very rarely drop below 1-3 deg - even overnight - as a result of the gulf stream affect.

 

To confirm we are using the same terminology, a pre-heater heats incoming air and the vast majority of this energy is transferred to the outgoing air in the heat exchanger. (Another way of looking at it is, the incoming air is heated to be warmer than the outgoing air to prevent it cooling the outgoing air, at least at the 'outside end' of the heat exchanger)

The purpose of this is to prevent ice forming in the heat exchanger passages as a result of water vapour in the outgoing air condensing and then being cooled by below 0°C air in the incoming side. There is an alternative to this strategy. Replacing the standard heat exchanger with an enthalpy heat exchanger will prevent the water in the outgoing air freezing, until the perhaps falls as low as at least -8°C, see bottom of this link :-

 

https://www.paulheatrecovery.co.uk/components/moisture-heat-exchanger/

 

It achieves this by transferring water vapour from the outgoing air to the incoming air, preventing condensing/freezing in the outgoing air.

Edited January 15, 2021 by A_L
to add (Another......exchanger)

[MJNewton]

4 hours ago, PeterW said:

Out of interest does anyone else have sensors in their intake / extracts and can log the temperatures..? 

 

Yes, we do - intake, exhaust, supply and extract. There's also a humidity sensor in the extract to trigger boost for cooking/showering.

 

Regarding heaters, we don't have one - and I don't think we really need one either. Even if the supplied air is a little cooler than the room temperature I haven't noticed it cooling the room down, perhaps due to the relatively low air volume and limited heat capacity of air. I'm also mindful of the higher cost of electricity compared to the mains gas we've got - perhaps if we were all-electric I'd be more open about using an electric inline heater more as a case of 'why not?'. 

Edited January 15, 2021 by MJNewton

[PeterW]

That’s helpful,  do you know the flow rates ..? And what heat transfer you are getting from intake to supply and the extract temperatures ..?

[MJNewton]

29 minutes ago, PeterW said:

That’s helpful,  do you know the flow rates ..?

 

I don't - yet - as I haven't finished the installation (still have a few runs to add) and so haven't done any balancing. All I've done is set the flow and extract to be similar (based on power consumption rather than flow rates) and made sure they're sufficient to convince me it's working.

 

Quote

And what heat transfer you are getting from intake to supply and the extract temperatures ..?

 

'Heat transfer' as in efficiency? If so, that seems to be pretty good at an average of between 80% and nearly 100% depending on the two methods described at the Paul Heat Recovery page on the subject:

 

 

The negative 'blips' appear to align with going into boost as the humidity and power graphs demonstrate, and I guess would make sense as the air is moving faster hence less time for transfer:

 

 

(I have to laugh: when zoomed in you can even distinguish between my wife taking a shower and I! )

 

 

In terms of heat rise between intake and supply temperatures I am therefore seeing pretty much all of what I'm extracting out:

 

 

We don't tend to have the heating massively high (lounge stat set to 20C but bedroom TRVs tweaked down a little), and this is a retrofit to a 2007-built Persimmon house so far from a perfect environment I'm sure. The pipes are running within the ceilings so whilst they are within the heated envelope of the house I doubt the temperatures there are the same as the rooms and will be being cooled by air leaks here and there. The unit itself, manifolds and termination of all the pipework is in the cold loft so there must be losses there too. Still getting air delivered 17C when it's 0C outside so perfectly acceptable I think - even without a heater as I say, and particularly when compared to what would've been coming through the window trickle vents which I have not foamed up!

Edited January 15, 2021 by MJNewton

[PeterW]

Thanks for that - interesting that on the Lossnay the recovery efficiency seems to increase with flow rate which is a bit odd as it’s supposed to decrease ..!! I’ll download the data from the Shelley’s that are on the ducts as it’s looking like for a fairly average install the efficiency is 78.8-82.3% which isn’t far off the actual numbers quoted by the manufacturer. This may be affected by the long run in an unheated but insulated service cavity for the en-suite (17m) and bathroom (10m) extract ducts. Interestingly the extract temperature increases on boost which makes me think some heat is lost in this duct. 

[MJNewton]

7 hours ago, PeterW said:

Interestingly the extract temperature increases on boost which makes me think some heat is lost in this duct. 

 

That's a good point, and I think mine does the same. 

Edited January 16, 2021 by MJNewton

[Dan F]

Other option, depending on size of house and number of occupants, is to use an enthalpy exchanger.  Freezing of condensation in extract is not a problem, at least in UK climate, with an enthalpy heat exchanger as humidity is recovered