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Author Topic: MVHR - duct heater using water  (Read 19537 times)
knighty
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« Reply #15 on: January 24, 2012, 06:51:09 AM »

if you wanted to do something like that.... you're be better off getting something like the condenser from a commercial/industrial fridge (2nd hand)

they're square so easier to deal with, and have a shed load of fins in them to transfer heat... so you could use a much lower water temp :-)

something a bit like this...
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pmagowan
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« Reply #16 on: January 24, 2012, 01:58:20 PM »

What about wrapping 10mm copper pipe round the ducting and then boxing this in with insulation?  You could use a bit of metal ducting for good heat transfer and even solder a few fins into it.  This would reduce the resistance on the airflow.
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optic
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« Reply #17 on: January 25, 2012, 09:46:41 PM »

in our last house i used an after market car oil cooler as a blown rad in a bathroom, fitted it up with three small computer fans, a transformer with a relay on the boiler and it worked very well, the bathroom was small and did not want a rad on the wall, the connections are bsp and worked with copper pipe and olives. was run off a combi go higher temp but did pump out some hot air. The oil cooler is designed to have a hi airflow rate so can be plonked anywhere in an airflow without ducting etc and will still work, perfect for the small computer fans.

this is a 19 row oil cooler think the one i used was 16 row,


only thing with heater matrix's and those aircon rads is the back pressure/loss of cfm, they put the same design aluminium rads on cars as an upgrade but you can hear the difference the kenlowe type fans work alot harder, older more open type not alot of air passes through would it be too much for the MHRV fan. one wayaround it would be to increase the surface area,

how about a car rad ??



rover rad 30 delivered from ebay if you get some pipe made up to the right dia and use the crimp hose clamps which should not leak as the clamping force is better distributed around the pipe.
« Last Edit: January 25, 2012, 09:51:26 PM by optic » Logged
hiccup
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« Reply #18 on: February 09, 2012, 08:38:32 PM »

So a few posts back I merrily said I had a water heater battery on the incoming air on my HRV primarily to guard against frost.

Well it has been running like that for over 2 years and seemed to be working well.

However, and this is a biggie... over the weekend it sprang a leak! I found out when I just happened to go and make a cuppa and heard water dripping! Checked the tap, nope.  Looked in the lounge... ARRGGGHH! Water dripping from the lights.

Sprinted up into the loft (I live in a bungalow), and quickly determined the source was the HRV ductwork. Took the cover off the pollen filter... Horror the filter box, and therefore the entire 30 feet of inlet duct was more than half full of water.

I isolated the water heater battery from the manifold and some quick work with the aquavac saved the ceiling - no lasting damage.

The carpet is now dry and peace has returned to bungalow.

However, the water heater battery is a goner. It has split several of the pipes that loop the matrix together.

Really bad idea that - having a water heater in the direct path of freezing cold air. Mind you, it survived last year which was much colder, but I suspect it was weakened and just finally let go. And this failure is even more mysterious when I realise the frost stat on the system should have kept it warm. Hey ho!

So, I'm going to use the housing to build an electric heater - using cooker hob elements (5 each) in series to get about 500W of heat.

It will be controlled to only come on when absolutely necessary, and if it's much too cold I'll automatically turn off the HRV.


Soooo, my revised advice is not to use a water heater on the HRV inlet.

I might still experiment with heating and/or cooling on the duct from HRV to rooms, but that is for sometime in the future. (Maybe never!)


Drama over. Nobody got hurt. And the ceiling is remarkably intact.

Hic!
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dhaslam
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« Reply #19 on: February 09, 2012, 10:04:53 PM »

The heating coil should be located after the heat exchanger on the way into the house.  In that position it shouldn't freeze. 
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MarkB
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« Reply #20 on: February 09, 2012, 10:41:31 PM »

The heating coil should be located after the heat exchanger on the way into the house.  In that position it shouldn't freeze. 

If it's there for frost protection, i.e. to stop the heat exchanger freezing up then it needs to be on the "out" side.

If it's there to provide heat to the house, then it needs to be on the "in" side.

I believe this one was was there for frost protection.
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JeffT
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« Reply #21 on: February 14, 2012, 07:19:46 PM »

The heating coil should be located after the heat exchanger on the way into the house.  In that position it shouldn't freeze. 

If it's there for frost protection, i.e. to stop the heat exchanger freezing up then it needs to be on the "out" side.

If it's there to provide heat to the house, then it needs to be on the "in" side.

I believe this one was was there for frost protection.
If the heat exchanger is doing it's job then why would it freeze up? It has outgoing warm air passing through it.
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hiccup
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« Reply #22 on: February 14, 2012, 11:52:37 PM »

Quite right Mark B, it was there for frost protection.

And almost right Jeff, the hrv heat exchanger does have the warm moist air from the house going through it, but that heat is lost (or most of it) to the incoming air,  so the air that actually leaves the exhaust port of the HRV is only a couple of degrees warmer than the incoming air. If the incoming air is at -15C (like last week here) then the exhaust air is at -13C so the external side of the HRV  WILL FREEZE!  snow  freeze  snow  freeze

The only problem here was that the frost stat on the system was not set high enough to ensure the water heater battery did not freeze.

Note: Some HRV units have the water heater built in to the inlet port!

Hic!
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16 x Sanyo HIT250E01 into SB4000TL inverter, 2 x 20 x 58mm Navi Tubes on 22deg roof facing SSE, Gledhill Torrent RE Solar 277litre Store, TDC4 Ether Controller, Xpelair Xcell400BP HRV, Stovax Riva 66 Wood Burner
dhaslam
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« Reply #23 on: February 15, 2012, 10:53:21 AM »

I didn't have cold enough conditions to test this but   if you follow the warm air from the house at say  20C starting,   it  meets the cold surface of the heat exchanger and the moisture condenses.  This should reduce the  heat loss a little and the air when it exits the house  may  be much warmer than the incoming air.     The condensate freezes on the ground  where it exits the house and  there is always a lot more  when the weather is cold but no ice on the air exit vent.       
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DHW 250 litre cylinder  60 X 47mm tubes
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A.L.
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« Reply #24 on: February 15, 2012, 11:55:49 AM »

modern high efficiency MVHR units (85%-90%) will shut down the inflow side at -4C or so, becoming extract only, between 0C and -4C they will will reduce inflow gradually or mix in internal air to increase temp if they do not have a pre-heat battery fitted
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knighty
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« Reply #25 on: March 15, 2012, 02:12:21 PM »

anyone get anywhere with this ?

I need to heat the air going through a 6 inch duct at work, so I can feed a machine with pre heated air, instead of letting it electrically heat the air

there's a decent amount of airflow in the duct, so I'm thinking I'll either need a very long heater, or a couple of heaters in series

found these...
http://biggrowhydro.com/index.php?main_page=product_info&cPath=330&products_id=15219

made for cooling air in ducts for hydroponics..... but by the time I have 4 of them shipped to the uk it starts getting expensive... not sure how good they are either.... they look a bit cheap and not very air tight....

or I could use...
http://www.ebay.co.uk/itm/Black-Aluminum-Heat-Exchanger-Radiator-PC-CPU-Water-Cooling-System-/200720168293?pt=Computing_ComputerComponents_Fans_Heatsinks_SR&hash=item2ebbdab565

a few of those in series, and make the duct adapter myself....


I thought there'd be a lot of options for this.... but I can't find much ready off the shelf at all... not for reasonable money anyway... (not once I need 2 or 3 of them)
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acresswell
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« Reply #26 on: March 15, 2012, 07:06:58 PM »

I'm not yet at the MVHR stage... only just poured the foundations but have decided based on this thread that I can omit UFH from the majority of upstairs rooms and just fit a water heater battery to the MVHR instead

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trubble
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« Reply #27 on: March 16, 2012, 04:49:58 PM »

Just been looking at this on ebay - 250700379875 - Stainless Steel Air Heating Coil for Ducting or Process.
200 or best offer so not cheap, but with 1" connections it's ideal.
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knighty
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« Reply #28 on: March 16, 2012, 07:32:22 PM »

the problem is have is, I need to dump a lot of heat into the airflow...

the machine runs at 75'C and has an extractor fan pulling in cold dry air and blowing hot wet air out the back....

not sure how much air it's moving, but I'd guess it's a couple of hundred cf/m easy
(totally guess with my hand in front of the fan, comparing it to other fans)

the combi boiler at work is about 18kw iirc, so I'd really like to be able to dump as much of that as possible into the airflow

I'll probably have to bump up on the temp on the combi boiler.... but it'll still be much cheaper/greener than heating with electric!

(also means I can use waste heat from other processes to heat this)
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knighty
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« Reply #29 on: March 16, 2012, 07:51:31 PM »

p.s. has anyone tried/seen any of these cheap ebay MVHR units ?

http://www.ebay.co.uk/itm/Heat-Recovery-Ventilation-Unit-Whole-House-Extraction-Ventilation-System-/130591379982?pt=UK_Home_Garden_Hearing_Cooling_Air&hash=item1e67da520e

or

http://www.ebay.co.uk/itm/Heat-Recovery-Unit-Fan-Whole-House-Ventilation-System-Bathroom-Shower-Fan-/280832440923?pt=UK_Home_Garden_Hearing_Cooling_Air&var=&hash=item870f158d96

?
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