Fresh air with heat pump or minisplit
TfMWMX1nC2ZsbO
| Posted in General Questions on
Good morning. I’ve been reading on this site (and others) about getting fresh air into the house. I’m in Climate Zone 4 in a 1970’s ranch house (rectangle, basically; 1,600 s.f. on main level) with a basement (partially finished). Heat pump is ~20 years old, and the air handler and supply / returns are in unfinished attic space. I’m planning to replace the heat pump and air handler soon, but am looking for the best way to get the fresh air. We have high radon and CO2 levels (based on AirThings Wave Plus device), and was reading on here about using an ERV to combat both. I had a blower door test a couple of years ago and had 1983cfm at 50pa, but that was before I insulated and airsealed the rim joists (which had 0 insulation).
I’ve talked to a couple of HVAC contractors about replacing the HVAC, but get the impression they haven’t done many ERV’s. I did a simple Manual J and got 35,000 BTU.
1. I’ve seen recommendations for the ERV, but also for a whole house ventilator / dehumidifer. Is one preferred over the other in my situation? If so, why?
2. Any particular brand you’d recommend? Or avoid?
3. We’re potentially adding a 2nd story. Does that affect the recommendation? Or would we just add a second ERV / ventilator?
Since the existing ductwork is in the attic, I’m going to abandon it and either add new ductwork to the basement with a heat pump / air handler, or use a minisplit (or two). Both contractors recommended two minisplits due to needing 5-7 heads (3 bedrooms, kitchen / dining are open, living room; potentially basement and mud room).
4. Both the ERV and whole house ventilator would require new ducting, so could I use either with my current setup (and wait until the current heat pump / air handler dies)?
5. Any issues with ceiling cassettes since they and the lines running to them would be in the unfinished attic space? It has R-49 blown-cellulose.
6. One contractor said mini-splits don’t last as long as heat pumps, and that since it’s an advancing technology still, sometimes parts aren’t available for older units (e.g. he had to replace a whole unit because a part for the head wasn’t available). So I was leaning towards adding ductwork with the heat pump. Is this a legitimate consideration?
7. Would I need two minisplits? Or use one with (5) heads and a transfer box?
8. Another contractor said minisplits are susceptible to power surges, so he adds surge protection on the units (~$2k add he said).
9. The basement has a fireplace / chimney that we don’t plan on using. Could it be repurposed with a fan of some kind to pull air out? Would still need a supply of fresh air, though.
Sorry for the long post; was trying to paint as clear a picture as possible, so hope I didn’t overdo the questions. I appreciate the help.
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Replies
I’m not able to answer all of your questions.
First, mini splits are heat pumps.
Second, never install a ductless head in a bedroom if you can avoid it. That load is too small for 5-7 ductless units and you’ll experience terrible efficiency in a set up like that. The manufacturers of these products have explicitly told installers NOT to do this, yet it keeps happening.
IMO, the two best options by far are keep the existing ductwork as is or add new ductwork to the basement.
Thanks for the quick reply and thoughts, Paul.
You're correct, I was just trying to distinguish between a "traditional" heat pump and the mini-splits.
So you'd pick a "traditional" heat pump with basement duct over a mini-split with a couple of heads?
If no heads in the bedrooms, would you do it in the hallway to try to serve all of the bedrooms? Does it work with doors closed? We have large undercuts on the doors now, if that matters.
This is why “mini splits” are so confusing! A ducted heat pump can be either traditional or a minisplit. The indoor unit isn’t what makes something a minisplit, the outdoor unit does.
I’d recommend a ducted heat pump and I’d run ducts to the bedrooms (and all rooms if possible). Everything else is a downgrade that doesn’t come with a discount.
Insulate ducts in the unconditioned space. You can spray them or you can wrap them with the foil bubble wrap. That is what I did and it worked well. Make sure the ducts are sealed very well (especially the return path) most so there is no unconditioned air infiltration- that is far more important than duct insulation actually.
Plumb fresh air into the return duct. Erv or whatever strategy you choose.
Multi splits are a disaster in smaller rooms- they are grossly oversized and will short cycle the outdoor badly. Even larger bedrooms require load that is x3 or more times smaller than the smallest head.
Good luck
Thank you for your thoughts. I did seal the attic ductwork and then buried it with blown-in cellulose. There's only a central return in the hall ceiling, so I'd have to give some thought to how to get fresh air ducted to that. But for various reasons, I'd prefer to bring the duct into the building envelope.
Good to know about the mini-split heads. I had originally decided to go with a ducted "traditional" heat pump, but then didn't know if the ERV / HRV would be better with a minisplit. Doesn't sound like it.
2k for surge protection sounds crazy high. You can get surge protective breakers for under $200. This is what our Daikin guy recommends
A Lunos type HRV might be a potential solution for fresh air, especially if going with ductless split.
I would recommend taking additional steps to mitigate radon in addition to fresh air.
Thank you for the additional input. I was going to check what the radon levels did when adding the ventilation; if it didn't drop enough, then I was going to use the sub-slab suction.
Good Morning TfM… et al,
Some thoughtful and thoughtless observations by an NRG hack still seeking answers to ventilations perplexing questions.
A. If piggybacking ventilation on the H&C ductwork, discourage ERV supply air from exiting at the return air grill;
1. Make the connection downstream of the Fan Coil. The filter should provide high enough s.p. so fresh air flows to diffusers (path of least resistance) OR
2. Add an interlocked (or “gravity” ) damper at the return air grille - to be opened when “Fan” in Fan Coil operates, and closes when it doesn’t. OR
3. Get an ERV without any integral fans and make sure the air handler can operate at a low enough speed to do ventilation with modest recirculation. (Check out the economy grade ERV from Reversomatic). It likely needs cooperative interaction of a volume damper , grille damper and fan speed controller so include a healthy dose of voo-doo.
4. If current H&C distribution includes diffusers in bathrooms, laundry and or kitchen, consider reducing discharge or closing them in conjunction with ERV exhaust grilles.
B. Regarding Manual J and other approaches to assessing your envelope effectiveness:
1. Correlate current system effectiveness with weather, indoor temperature, system efficiency to characterize your house’ characteristic Btu/ hr F* .
a. My direct envelope calculations were consistent with an adjusted J-calc , but may be inconsistent (higher) based on our experience using a 12kBtu portable AC /heat pump for at least one winter…
b. Thinking? out loud, you could install a 9kBtu mini split with wall unit (simplest, cheapest, most efficient approach) while the existing ing air handler is retained as a circulation ventilation fan to move the heated air from the living room to other places. (Leave the older heat pump off , just run in fan mode) .
2. We have just had two 9kBtu mini splits installed, on in LR and one in basement.
a. The tabulated capacity at -5F is 13.3kBtu. It’s the extra cold climate model -
b. As I see it, each one is the backup for the other, and
c. it’s the only way to get adequate dehumidification in summer (moist zone 5) , as well as shoulder season modulated heating down to 3400 Btu/hr.
- Sterile and cuckoo west of Boston
p.s. it’s not easy being green, and getting right sized equipment. Proposals ranged from 24kBtu ducted to 60kBtu ductless. I believe this is driven in part by the rebate rules.
I am now needing to finally scheme the ERV
Thanks for your reply / thoughts!