BadgerBadger Posted March 6 Share Posted March 6 So we are finally getting stuck into first fix and ready to start MVHR duct work. I've had a few quotes from the usual suspects, and even paid for a preliminary design, but to be honest have been a bit underwhelmed by what I've been getting back. So I've been doing calculations myself and I think I'm getting to a solution. Using a mix of 90mm semi-rigid and some flat/oval semi-rigid to run a service void, and have managed to keep the flow velocities all well under 2 m/s. Ventilated internal floor area is approx. 250m2 so I'm working from a max ventilation rate of 275m3/hr (~75l/s). I've done some sums on the pressure drop of my longest duct run which on paper seems to be about 40 Pa (as I've kept the flow velocity low) but also assuming it may also be somewhat higher. I'm going for a Zehnder ComfoAir unit but some quotes have been based on Q450 and others on the Q600. So am unsure which to go for. Initially I'd assumed the Q600 would be quieter/better as I'd be running it further away from is maximum but the ComfoAir brochure has some handy plots, I've added my operating point and now I'm not so sure! Feels like I'm a long way down the curve on the Q600 already and in reality may reduce ventilation rate further if building regs rate is too high. The Q600 sound power also looks higher at ~45-46dB, instead of ~43-44dB on the Q450. Is that right? What do you think? Link to comment Share on other sites More sharing options...
JohnMo Posted March 6 Share Posted March 6 Only thing I would say is once signed off you are likely to reduce flow rates. Because building regs rates tend to be a bit excessive. Even with our rates reduced by 30% we are still dropping to mid 30s relative humidity when it's cold which is getting a bit low. I would be tempted to go with the smaller unit. Link to comment Share on other sites More sharing options...
Mike Posted March 6 Share Posted March 6 (edited) The Passivhaus component database indicates that the Q450 is suitable for a capacity of 345m³/hr (according to their independent testing). As a guide, you'd want to run it at a maximum of 2/3 to 3/4 of full capacity in normal use to provide some headroom, so in the 230 to 260m³/hr range. As you're likely to be able to run it at less than Building Regs suggests, that's likely to be adequate. As for noise, you probably wouldn't notice a 2dB difference; if it's in a plant room anyway then ignore it. On the other hand, if the cost difference isn't much I might go with the upgrade. Edited March 6 by Mike prematurely posted Link to comment Share on other sites More sharing options...
BadgerBadger Posted March 8 Author Share Posted March 8 Thanks - I think I'm leaning to going with the Q450. I can't quite see any justification for the extra money on the Q600. So now I'm into enthalpy exchangers and pre-heaters! I was very tempted by enthalpy, until I realised the efficiency was ~10% lower than the standard. So how essential is the pre-heater? It seems somewhat power hungry so I'd be a bit worried about it running lots. Does anyone know if the Zehnder units protect themselves if frost is a risk? (i.e. shutting down?) We're down south so not expecting hugely extended periods where frost would be an issue. Link to comment Share on other sites More sharing options...
JohnMo Posted March 8 Share Posted March 8 10 minutes ago, BadgerBadger said: now I'm into enthalpy exchangers and pre-heaters! I was very tempted by enthalpy I live in NE Scotland, we have days at a time at -9 without a preheater (never bothered to install and zero issue. Enthalpy exchangers - we don't have them either - our lowest house humidity hit 37% and we average just over 40% in the heating season. Not sure you really need them. 1 Link to comment Share on other sites More sharing options...
MPH243 Posted March 8 Share Posted March 8 We have the Q450 in a 260sqm house in Devon, we are running at about 75% of building regs flow rate. This is normally about 40% humidity which is fine. The unit goes into a frost protection mode when below freezing, it just reduces air supply by a few percent. The unit is in a cupboard in my sons bedroom which has been soundproofed a bit. The supply vent is louder than the unit. 1 Link to comment Share on other sites More sharing options...
Mike Posted March 8 Share Posted March 8 3 minutes ago, BadgerBadger said: how essential is the pre-heater? It seems somewhat power hungry so I'd be a bit worried about it running lots. Does anyone know if the Zehnder units protect themselves if frost is a risk? (i.e. shutting down?) I have a pre-heater on my Zehnder unit in France. Without it, in 'standard' mode the unit would shut down if the incoming air drops below around 5°C (there are 3 other modes, including an 'eco' mode that plays with the ventilation rate to switch it on as late as possible. I'd guess that all Zehnders would do the same. I'd rather pay a little extra on the electricity and have the unit working all the time, but you can probably track down some weather data for your area if you want to evaluate roughly how much it would be. 1 Link to comment Share on other sites More sharing options...
JohnMo Posted March 8 Share Posted March 8 1 minute ago, Mike said: in 'standard' mode the unit would shut down if the incoming air drops below around 5°C That's a rubbish design. My Triton units uses Automatic Frost Protection During very cold weather, Automatic Frost Protection will detect temperatures that could form ice inside the unit. Automatic Frost Protection will reduce the supply ventilation rate to prevent ice build up within the heat cell. Automatic Frost Protection reduces the flow rate of cold air, thus allowing the warmer stale air to raise the temperature within the heat cell to such a level that prevents the formation of ice. As internal temperatures rise Automatic Frost Protection will increase the supply ventilation flow rate back to the commissioned settings. Link to comment Share on other sites More sharing options...
Mike Posted March 8 Share Posted March 8 5 minutes ago, JohnMo said: Automatic Frost Protection will reduce the supply ventilation rate to prevent ice build up within the heat cell. Automatic Frost Protection reduces the flow rate of cold air, thus allowing the warmer stale air to raise the temperature within the heat cell to such a level that prevents the formation of ice. As internal temperatures rise Automatic Frost Protection will increase the supply ventilation flow rate back to the commissioned settings. ...which is exactly what the Zehnder can do in Eco mode, as mentioned rather more briefly above. Link to comment Share on other sites More sharing options...
BadgerBadger Posted March 8 Author Share Posted March 8 25 minutes ago, JohnMo said: I live in NE Scotland, we have days at a time at -9 without a preheater (never bothered to install and zero issue. Enthalpy exchangers - we don't have them either - our lowest house humidity hit 37% and we average just over 40% in the heating season. Not sure you really need them. Thanks, I'm tempted to try without - I can always buy a a pre-heater separately to install in the unit if I have issues. Bit worried about over-ventilating our master bedroom and getting very dry air, it's got quite a large floor area so ventilation rate has ended up a bit above passivhaus guidelines for bedrooms with two people but likewise I can always re-adjust rates if we have problems. Link to comment Share on other sites More sharing options...
MPH243 Posted March 8 Share Posted March 8 The Q450 only has one mode and the frost protection works As @JohnMo said, I looked on the app in the cold weather in January to see what it does. I think about 6 or 7% at -5 but I guess depends on humidity levels. 1 Link to comment Share on other sites More sharing options...
BadgerBadger Posted March 8 Author Share Posted March 8 5 minutes ago, MPH243 said: The Q450 only has one mode and the frost protection works As @JohnMo said, I looked on the app in the cold weather in January to see what it does. I think about 6 or 7% at -5 but I guess depends on humidity levels. Perfect, thank you! I can handle that I'm sure! Link to comment Share on other sites More sharing options...
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