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Jim_UK · 07-27-14, 07:34 PM

This has turned into a long thread, but totally worthwhile.

Thanks to all contributors for the last 4 years or so of fantastic information and inspiration.

Apologies from me for making a long post, but on topics like this enough detail needs to be given to solicit quality responses without lots of follow up posts!

----

I'm researching the design/construction of a HRV for an old wooden boat we're completely restoring/renovating. I'd like to run some design aspects/choices past the audience and ask for feedback/advice please before they get locked in.

To empathise, as it's a boat, power (aka never enough of it, aka electricity) and keeping costs under tight control are the two major factors.

Also due to the nature, I need to plan all of the major works (electrical, plumbing, HVAC etc in whilst I'm working on the Hull/Superstructure). So important to get it 'mostly' right from the start!

So, to our own HRV design...

Proposed HRV design

Three Counter Cross Flow cores in series. Each ~243mm x 243mm x 346mm (~ 9.5" x 9.5" x 13.5"), constructed from 2mm Correx (extruded polypropylene, aka the estate agents sign material as discussed at length here). The size is partially determined by the size of the fans.

I appreciate that 3 cores is perhaps overkill and suffers from diminishing returns. Still, the Correx is very cheap and a small proportion of the costs of building a HRV. Lots of surface area and the 2mm has thin walls.

If air resistance of the 3rd core proves to be too high a cost to pay (compared to the extra ~10% efficiency) then I can leave it out but I'll start off with it in.

Airflow

4x 48V DC 13W 0.27A high efficency axial fans, each yields 226CFM. 172mm (6.75") in Dia. 47 db(A) at full tilt means they are fairly quiet and when they are enclosed with the HRV cores and tucked away in the 'mechanical' room means we'll never know they are running (cross fingers).

Two of these on the input and two on the output. A total of 452CFM for each of the flow and return BUT I only need 400CFM to acheive 6 Air Changes per Hour. I'll PWM these down to the 85% of flow for 6ACH, 42.5% for 3ACH (or drop down to 1 fan per leg) etc (whatever saves the most power).

Depending on time of day and temp/humidity conditions measured throughout will influence the tradeoff between ACH and maintaining the target temp/humidity selected on the 'thermostat'.

Filtration

A cheap, fairly coarse, air con/HRV disposable filter on the Fresh Air In, and I'm hoping to source a HEPA H14 filter for the Fresh Air Out.

A HEPA H14 grade filters down to 0.3µm so is good enough to stop Legionella spores. I don't know if the resistance of this filter will be a show stopper or not yet. I could add more fans (I can get them dirt cheap, and they are energy efficient).

Heating/Cooling

I've also pencilled in two water/air heat exchangers (Honda motorcycle 'radiators') that I'll be passing water through.

Hot through one, cold through the other (not at the same time, motorised ball valves under microprocessor control which, if either, is on). I'm expecting the hot radiator to add 2-6°C depending on air flow. The cold radiator won't be nearly as effective, possibly 1-2°C due to a much lower deltaT, but every single degree of cooling helps.

(Two radiators make the plumbing design simple)

Condensate drainage

I've allowed space for the condensate to collect and the microcontroller will monitor the water level and activate a small DC pump to remove that from time to time.

Humidifier/De-humidifier

Pulling apart a cheap ultrasonic humidifier could be a nice touch. Removing the integral large water reservoir and feeding the water from an external source make them quite compact.

A cheap one can inject 480 ml/hr. In rough numbers that is enough to drop the temp by 2.5°C (I think). However 3 ACH's drop this to @1°C of course, 6ACH's = ~0.5°C. Perhaps something that can inject 1000 ml/hr may be a better choice.

Of course maintaining RH is important but remember this is a boat so higher than average RH on the input compared to a house.

De-humidifier. I'm not so keen on integrating this internally. I can always add this 'feature' at some point if humidity is ever a problem by using one of the portable units as my gut tells me it may only be an issue in rare circumstances (given the number of ACH's the HRV can do, after all a HRV acts somewhat like a dehumidifier as a side effect).

Some background info/factors:

I've roughly calculated that due to the volume size, the amount of insulation etc. that a 5KW water heater (for heating & domestic hot water supply) will ensure a ~20°C internal temp IF we fit a HRV that manages 50% or better under external conditions down to ~-5°C.

Volume: It's not a massive boat, in round numbers 4,000 Cu. Ft.
Insulation: Good, not far off a typical house (double glazing etc).
Location: UK (so Winters can be cold - say -10°C. Summers can be hot - 32°C yesterday for example)
Power: 48VDC (from batteries, generator & solar - shoreline 230VAC at times too)
Air Changes per Hour: between 1 & 6, depending on time of day and environmental circumstances (IE it will be dynamic)
Control: Microprocessor controlled, Temp/Humidity constantly measured everywhere of course.
Heating: As mentioned, a 5KW diesel water heater provides hot water. This would be used for domestic hot water and underfloor radiant heating.
Cooling: As it's a boat, we're sitting on an unlimited supply of 'cold' water. 5-12°C is typical. This would be utilised via a water/water heat exchanger to the 'dirty' water isn't used directly inside the boat of course. This cooling source will be used for other things too.

07-27-14, 07:34 PM	#446
Jim_UK Lurking Renovator Join Date: Jul 2014 Location: UK Posts: 4 Thanks: 4 Thanked 0 Times in 0 Posts	This has turned into a long thread, but totally worthwhile. Thanks to all contributors for the last 4 years or so of fantastic information and inspiration. Apologies from me for making a long post, but on topics like this enough detail needs to be given to solicit quality responses without lots of follow up posts! ---- I'm researching the design/construction of a HRV for an old wooden boat we're completely restoring/renovating. I'd like to run some design aspects/choices past the audience and ask for feedback/advice please before they get locked in. To empathise, as it's a boat, power (aka never enough of it, aka electricity) and keeping costs under tight control are the two major factors. Also due to the nature, I need to plan all of the major works (electrical, plumbing, HVAC etc in whilst I'm working on the Hull/Superstructure). So important to get it 'mostly' right from the start! So, to our own HRV design... Proposed HRV design Three Counter Cross Flow cores in series. Each ~243mm x 243mm x 346mm (~ 9.5" x 9.5" x 13.5"), constructed from 2mm Correx (extruded polypropylene, aka the estate agents sign material as discussed at length here). The size is partially determined by the size of the fans. I appreciate that 3 cores is perhaps overkill and suffers from diminishing returns. Still, the Correx is very cheap and a small proportion of the costs of building a HRV. Lots of surface area and the 2mm has thin walls. If air resistance of the 3rd core proves to be too high a cost to pay (compared to the extra ~10% efficiency) then I can leave it out but I'll start off with it in. Airflow 4x 48V DC 13W 0.27A high efficency axial fans, each yields 226CFM. 172mm (6.75") in Dia. 47 db(A) at full tilt means they are fairly quiet and when they are enclosed with the HRV cores and tucked away in the 'mechanical' room means we'll never know they are running (cross fingers). Two of these on the input and two on the output. A total of 452CFM for each of the flow and return BUT I only need 400CFM to acheive 6 Air Changes per Hour. I'll PWM these down to the 85% of flow for 6ACH, 42.5% for 3ACH (or drop down to 1 fan per leg) etc (whatever saves the most power). Depending on time of day and temp/humidity conditions measured throughout will influence the tradeoff between ACH and maintaining the target temp/humidity selected on the 'thermostat'. Filtration A cheap, fairly coarse, air con/HRV disposable filter on the Fresh Air In, and I'm hoping to source a HEPA H14 filter for the Fresh Air Out. A HEPA H14 grade filters down to 0.3µm so is good enough to stop Legionella spores. I don't know if the resistance of this filter will be a show stopper or not yet. I could add more fans (I can get them dirt cheap, and they are energy efficient). Heating/Cooling I've also pencilled in two water/air heat exchangers (Honda motorcycle 'radiators') that I'll be passing water through. Hot through one, cold through the other (not at the same time, motorised ball valves under microprocessor control which, if either, is on). I'm expecting the hot radiator to add 2-6°C depending on air flow. The cold radiator won't be nearly as effective, possibly 1-2°C due to a much lower deltaT, but every single degree of cooling helps. (Two radiators make the plumbing design simple) Condensate drainage I've allowed space for the condensate to collect and the microcontroller will monitor the water level and activate a small DC pump to remove that from time to time. Humidifier/De-humidifier Pulling apart a cheap ultrasonic humidifier could be a nice touch. Removing the integral large water reservoir and feeding the water from an external source make them quite compact. A cheap one can inject 480 ml/hr. In rough numbers that is enough to drop the temp by 2.5°C (I think). However 3 ACH's drop this to @1°C of course, 6ACH's = ~0.5°C. Perhaps something that can inject 1000 ml/hr may be a better choice. Of course maintaining RH is important but remember this is a boat so higher than average RH on the input compared to a house. De-humidifier. I'm not so keen on integrating this internally. I can always add this 'feature' at some point if humidity is ever a problem by using one of the portable units as my gut tells me it may only be an issue in rare circumstances (given the number of ACH's the HRV can do, after all a HRV acts somewhat like a dehumidifier as a side effect). Some background info/factors: I've roughly calculated that due to the volume size, the amount of insulation etc. that a 5KW water heater (for heating & domestic hot water supply) will ensure a ~20°C internal temp IF we fit a HRV that manages 50% or better under external conditions down to ~-5°C. Volume: It's not a massive boat, in round numbers 4,000 Cu. Ft. Insulation: Good, not far off a typical house (double glazing etc). Location: UK (so Winters can be cold - say -10°C. Summers can be hot - 32°C yesterday for example) Power: 48VDC (from batteries, generator & solar - shoreline 230VAC at times too) Air Changes per Hour: between 1 & 6, depending on time of day and environmental circumstances (IE it will be dynamic) Control: Microprocessor controlled, Temp/Humidity constantly measured everywhere of course. Heating: As mentioned, a 5KW diesel water heater provides hot water. This would be used for domestic hot water and underfloor radiant heating. Cooling: As it's a boat, we're sitting on an unlimited supply of 'cold' water. 5-12°C is typical. This would be utilised via a water/water heat exchanger to the 'dirty' water isn't used directly inside the boat of course. This cooling source will be used for other things too.