MVHR - duct heater using water

[acresswell]

Anyone got any experience of adding a duct heater to MVHR?  Especially one that uses hot water from the CH rather than an electric element.

We're fitting UFH downstairs (using heat from log boiler) in our new build, and currently considering not fitting any heating upstairs apart from a couple of towel rails and a duct heater linked to the upstairs MVHR outlets.  It's really just to take the chill off the air from the MVHR, but I'd like to avoid the electric duct heaters because logs are relatively free and electricity isn't. We'd like to avoid radiators if possible. We've had a quote for UFH upstairs as well, but it's not cheap and my sister-in-law has her upstairs UFH permanently turned off. The other advantage would be that we could add the duct heater at a later date if it was required... so we wouldn't spend any money unless necessary.

I've found a couple of possible link...
http://www.ves.co.uk/Uploads/Products/66/ProductTechDoc_FILE/Heatline.pdf
http://www.paulheatrecovery.co.uk/wp-content/uploads/2011/08/9_heating_elements_100701.pdf
..but I'd really like to hear anyone's practical experience of fitting/ using one.

Thanks,

Adrian

[dhaslam]

My system heat recovery system has a option  of a heating coil  but  I think it would be a bit ineffective because of the low volume of air transferred.   The air  volume per hour is  something like  300 cubic metres and if that is heated to  thirty degrees above  room temperature it would only use about 1 kW  for the whole house  and  would heat rooms not used as well as ones already warm.  Output would be about four watts per square metre of floor area.      If you could confine the heat to only the rooms which need it then  it should work OK except that an individual room  might only be receiving an air change every two hours so  only receiving about  50 watts  for an average bedroom.     

Although it is difficult to predict  what rooms need  heating  it should be possible  to do a reasonable  guess based on the  direction windows are facing and the  proximity to winter heat sources or well heated rooms.     Since many houses have bathrooms on the north side  they  normally need heat.   I also have to use heat in the main bedroom  because it is at the end of  a long single storey building.    The heat from underfloor heating  is about 20 watts per square metre when on  but the heat is controllable.           

[acresswell]

We're building the house with a higher-than-average thermal mass, so I have to accept that the ability to suddenly warm up individual rooms will be a bit lacking. I'm thinking about only heating the supply air to the bedrooms (by putting a duct heater in the duct that only leads to them, rather than a built-in heating coil which must heat all the supply air).  Bathrooms will be extract only (which is why they'll have a towel rail). Downstairs rooms will have UFH.

Experience of my sister-in-law's house suggests that enough heat from the downstairs will move through the house to keep bedrooms at a sensible temperature... but she doesn't have MVHR where the supply air will always be cooler than the extract air and could therefore cool down an unheated bedroom.  I don't want a lot of watts, just to replace the heat lost during the heat exchanging.

Dhaslam, your point about aspect is a good one. The house is long and thin, so there are 3 bedrooms facing south (which are away from any winter heat sources), plus a small guest room facing north(with the chimney breast running through it).  It would probably make sense to limit the duct heater to the 3 southern rooms (controlled by a thermostat in one of them) and I'll think about an alternative heat source for the guest room (where occupancy and "chimney heat" will be more variable)

[offthegridandy]

We have a stone barn which I converted 9 years ago using large quantities of king span insulation along the way. We have underfloor heating which on solid floors is great but with wooden floors is very inefficient, so much that I removed it. 

Due to the design of the barn we get excellent heat transfer to upstairs from natural circulation. We have 3 "light wells" that allow the warm air to travel up.  Bedrooms have small radiators but the stat is set for 22 degrees and they never come on.

The upstars rads are a seperate zone on the controller and therefore independantly controlled.

Andy

[clivejo]

How big is your duct?  I have a few heat/cold exchangers (ex mushroom growing water to air)  They have two coils one for heat and one for cooling, no reason they cant be both used for heat 

[acresswell]

The duct will probably be 150mm or 200mm, though there's no reason I can't adjust it to be a bit bigger if required

[hiccup]

Hi

Yes, I have this. 

On my MHRV I have a "water heater battery" on the 200mm inlet duct connected as just another radiator on the manifold.

It not only makes up for the losses in the HRV, but usefully heats the house.
When the frost stat kicks in it automatically takes the edge off the frosty incoming air.

Got mine from Lindab along with the rest of the ducting.

HTH

Hic!

[acresswell]

Hiccup,

any idea of the model name?  I've just looked at the Lindab website but can't find any "water heater batteries" except for ones that mount inside a wall and look a bit like a storage heater because of the vent on the front (designed for commercial premises)

Thanks!

[hiccup]

Ooops - I'd forgtton I'd got that from somewhere else as Lindab could not come up with the goods.

http://www.i-sells.co.uk/200mm-water-heating-battery-water-to-air

It is a Systemair VBC

A bit expensive for what it is - probably quite easy to knock something up if you have access to the right tools and parts and maybe use a motorcycle radiator or car heater matrix.

Hope that helps

Regards

Hic!

[acresswell]

Hic,

thanks for that...  I'd agree that £270 isn't cheap, but it's a lot more palatable than the only other one I've found...   (£680)
http://www.vesdirect.co.uk/products/duct-mounted-hot-water-heater-c/w-fitted-controls-hlc200/w/cp-2844.aspx

I'll have a think about the heater matrix idea, too (though it'll be in the loft and any leak could be quite detrimental to my relations with SWMBO so I might have to play it safe rather than "bodging")

[JeffT]

Hi

I am using a water heater battery I made myself using two new car heater matrixs housed in a galvanised filter box costing a total of £70. This is fitted in 150mm ducting just after a VentAxia 350 heat exchange unit. It is fed from a Trianco Activair S500 air to water heat pump 1.2kW input and 3 to 5kW output. Following this is a 1.2kW electric duct heater for use during times of extreme cold e.g. less than -1 Celcius. I have no other heating other than an electric fuel effect fire with built in fan heater in the lounge to provide a top up if necessary and not needed very often.

The problem I have found with the home made battery is getting suitable connectors to couple up to the plastic radiator spiggots. I am using car heater hose to washing machine water hose stubs. This arrangement is prone to leakage at the heater hose connections needing regular tightening of the hose clips. Currently there are no leaks but from experience this will not last.

If there are further leaks I will be getting the £250 160mm unit from i-sells with proper copper pipe in/outs this is rated a 3kW which is pretty much the same as I am achieving with the home made unit. I believe the additional cost is worth while to ensure a water tight system avoiding constant attention.

Jeff

[acresswell]

On my MHRV I have a "water heater battery" on the 200mm inlet duct connected as just another radiator on the manifold.

Hic,

just re-read what you wrote. 

I presume you mean that 1) the water heater is on the duct carrying clean air from the MHRV unit to be distributed to your rooms, and not that 2) the water heater is on the "inlet" duct from outside to the MHRV unit

If it was 2) then presumably the stale air being expelled would be very warm!

[Lincsoldbird]

Hi

I am using a water heater battery I made myself using two new car heater matrixs housed in a galvanised filter box costing a total of £70. This is fitted in 150mm ducting just after a VentAxia 350 heat exchange unit. It is fed from a Trianco Activair S500 air to water heat pump 1.2kW input and 3 to 5kW output. Following this is a 1.2kW electric duct heater for use during times of extreme cold e.g. less than -1 Celcius. I have no other heating other than an electric fuel effect fire with built in fan heater in the lounge to provide a top up if necessary and not needed very often.

The problem I have found with the home made battery is getting suitable connectors to couple up to the plastic radiator spiggots. I am using car heater hose to washing machine water hose stubs. This arrangement is prone to leakage at the heater hose connections needing regular tightening of the hose clips. Currently there are no leaks but from experience this will not last.

If there are further leaks I will be getting the £250 160mm unit from i-sells with proper copper pipe in/outs this is rated a 3kW which is pretty much the same as I am achieving with the home made unit. I believe the additional cost is worth while to ensure a water tight system avoiding constant attention.

Jeff

What you need is boat fittings,I have this problem all the time conecting the hot water to the engine cooling system. Try Aqufax or Midland Chandlies.

[hiccup]

Hi acresswell

Actually it is on the inlet to the MHRV and so yes, some heat is being transferred to the exhaust flow. But this is really only here to prevent frosting in the MHRV unit. Also I can operate the "summer bypass" so that the heat is not transferred to outlet but then the "waste" heat on the exhaust path does not get recovered.

Next stage is to fit two more "heater" batteries after the MHRV unit on the supply to the rooms, one with heat (another radiator loop from the manifold) for the winter, the other with cooling for the summer driven by the PV.

Then the battery on the inlet will only operate in "frost protection mode" - don't want the heat exchanger icing up!

Hey ho!

Hic!

[acresswell]

Just had another (possibly mad) idea...

could I use a double convector radiator (maybe 400mm wide, as tall as possible) and duct the air between the panels?

I'd seal down the 2 edges and bodge an adaptor from the round ducting to the rectangular gap between the panels.  I'd wrap the whole thing in insulation (use celotex to make a sandwich?) to minimise the heat loss from the external surfaces...  but I could get a 400mm wide x 600mm high radiator for less than £40.  If necessary I could even get two radiators and connect them in series...

I know it'd be bigger than a commercially-produced water battery, but space isn't a particular issue.  If I could find a suitable radiator less than 400mm wide I might even be able to hide it under the floorboards between the joists.

If I could get one of those radiators with tappings in all 4 corners then it'd be easy to set it up as a counter-flow exchanger.

Am I missing a big flaw?