MVHR - duct heater using water

[dhaslam]

A 6" pipe won't carry all that much heat.   A cubic metre of air  only carries .5 watts per degree differential  and and it needs a very good airflow to get 1000 cubic metres through a  150mm pipe in an hour.  Consequently you only need a relatively small coil to carry the heat  via water to keep it supplied.   

[knighty]

the extractor fan is inverter driven, so changes speed depending on temp/humidity.... but on full belt it's a 2kw motor


the heaters inside the machine are normally 65kw

but I set them into start (instead of delta) because I was worried about tripping fuses when we have big motors etc. starting up


not sure what power rating they will be now (at 240v instead of 415v) but after watching it for 15 min they seam to be on more than off
(and that was near the end of a 6 hour run when everything was up to temp!)

[acresswell]

If it's really something like 65kW, then I'm guessing that the exhaust air must be pretty warm.   Can you rig up a heat exchanger to recover some of that heat, rather than pre-heating the airflow with hot water?

[knighty]

the output air is the same temp as the chamber temp, so about 75'C

looking to get an MVHR to pre-heat the incoming air with outgoing air
(it's crazy that it doesn't have one as standard... I bought it second hand, but it's a £80k machine!)

I'm not sure which MVHR to go for, or if I'd be better making something myself, it's a drying machine, so the outgoing air is very damp = lots of condensation inside the HVHR.... I've looked for more 'industrial' air to air heat exchangers, but there's not much around

and thought I should add some heat exchangers to boost the incoming temp up a bit ?

[Countrypaul]

Sorry for resurecting an old thread, but it is now highly relevant to our situation.

We have UFH on the gound floor but no heating on the first floor. We are finding that the bedrooms on the first floor are cooling down too much, and part of this appears to be due to the MVHR system, as the air it is pumping in is cooler than the actual room temperature. This is probably true of the groud floor too, but the large thermal mass in the UFH screed hides the effect downstairs.

I am therefore looking at various options for warming, or at least not cooling down the bedrooms on the frst floor. Each bedroom (carpeted) has an air inlet and the bathrooms (tiled) have air extracts, so the bathrooms have a small amount of UFH and heated towel rails otherwise the floors would be too cold (as other members of the house would be sure to highlight repeatedly).

I am therefore looking at putting some heating into the supply air to the rooms, basically to take the chill off the air rather than to have any large heating effect. I have come across both electric and water based "duct batteries" which appear to do what I hope for. Given that the MVHR system is located close to the UFH heating manifold it would be relatively easy to run another circuit from the UFH supply to a water based duct battery in order to heat the supply air to the rooms. Given that the UFH is driven off a thermal store which is heated from E7 electricity and in the future an ASHP it appears to me to make far more sense to go for the wet solution.  In our case all the ducting is well with in the thermal envelope and most is between the gound floor ceiling and first floor deck surrounded by rockwool insulation.

Has anyone got one of these working at present, and if so what experiences have you had with it?  How hot, for example, does the water circulating in the duct battery have to be to ensure the supply air is at a minimum of room temperature? I know it will also depend on flow rates and outside temperature but some examples would help.

Thanks

[TT]

I use wet systems at work, and have retro fitted electric inline heaters to supplement heating on inefficient 20 year old duct run.

Try a small fan heater to see how much extra heat is required to boost the room temp.
That will give you a size/ watts/btu required.