Dedicated ERV with Ducted Heat Pump System
The HVAC is recommending for a hybrid ERV system since I’m doing a full duct heating heat pump system (Mitsubishi Zuba Central). They suggest only doing an exhaust system in the bathrooms and kitchen but supplying the fresh air by using the heating/cooling ducts. They said by doing this it would save cost because of less duct having to be installed and a small ERV could be used.
Is this recommended or should I stick with my original design by having a fully dedicated duct system for both the ERV and the heating/cooling system?
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The fully ducted and independent ventilation system is still considered the best. Whether it's better enough for the cost difference is up to you. Note that the system they are proposing may cost less to install, it costs more to run.
Thank you for your input. My understanding was the same that a shared duct ERV system would cost more to run due to the duct size being large than what is required for the ventilation system.
The HVAC contractor also mentioned that he would need to install 2kWh preheater if I went with a dedicated ventilation system but wouldn't need one if I incorporated it with the heating system because the incoming air would be diluted with the recirculation trunk.
The all makes since except for the shoulder season or during mild summer period during when neither the heat or AC is running.
Am I missing something or is this just an HVAC contractor trying to do the typical installation and not knowledge this house will be highly efficient?
You don't need a preheater for most ERVs. What is the model?
In the shoulder seasons, the air handler will still need to run. Even in the heating/cooling seasons, it needs to run a lot more than it would for heating/cooling purposes. The heating/cooling requirements will rarely align perfectly with ventilation needs, which is why keeping those ducts separate is preferable.
Are you going to run the furnace fan all the time? Check the numbers - say the furnace fan is off, your house needs 100 CFM and your closed door master bedroom is ducted to receive 10% of the air (based on heating/cooling load). It's only going to get 10 CFM* of fresh air - where you need about 30 CFM of fresh air for healthy CO2 levels for two people. Heating/cooling distribution ratios and ventilation ratios are different - something a shared system can't do.
* - maybe. At low volumes, the over-sized duct flow ratios change in hard to predict ways.
I was not planning on running the furnace fan 24/7 and for the ventilation system, I was thinking it would run for sometime like 20 minutes every hour.
I went through some numbers and a closed door bedroom needs a lot more mostly recirculated air than it does 100% fresh air. For example, 30 CFM of fresh air vs 100 CFM of a 90/10 mix of recirc and fresh air. Anything less than fan on full all the time is unlikely to provide sufficient airflow for reasonable closed-door bedroom CO2.
Note that excess airflow over an evaporator coil has a large negative effect on latent performance. Ideally airflow would be bypassed around the evaporator when the compressor is low/off - but I've never heard of a system that does this.
It sounds that ideally it is best to have dedicated ventilation system and keep the heating/cooling on it's own.
I just need to figure out if it's worth the additional cost.
I'll throw out another option that could be cost effective. Use shared ducts for the majority of ventilation air distribution and a supplemental spot ERV in problematic closed door bedrooms and offices.
Most modulating heat pumps run the blower 24/7, provided your supply is sized correctly, generally you don't have to worry about enough air changes. There is usually no need for any timers.
With the hybrid ducted option you can always include a stale air pickup in bedrooms. This way even if the air handler is off (ie shoulder season), you'll get air changes in the closed rooms. Drawing stale air from a room is not as effective as supplying fresh air directly but works well enough. It also saves you having to interlock the air handler with the ERV. When the air handler is running your supply ducts distribute the fresh air through the house, when the air handler is off, the fresh air is supplied to the house through the return registers.
They also modulate the fan, which then doesn't provide nearly enough mixing airflow for examples I looked at. On the other hand, some people can leave the bedroom door open.