High Velocity ducts with Heat Pump?

cbandes

Hi!

I have a ~20 year old High Velocity AC system in my home (3000ish sqft, built 1960) - the current system is a 4 ton WeatherKing condenser and a Unico MB4860 air handler, both are from 2004.

The system works, but it has some issues - it cycles very frequently, it is very noisy (particularly the outdoor unit sometimes sounds like a jet engine, and sometimes makes a weird clanking noise), and the lights dim everywhere in the house when the system cycles. Last year I had a new hard start capacitor installed in the condenser to see if that would mitigate the dimming lights but it didn't really make much difference. We've confirmed that the circuits these are on are sufficient amperage so we don't really think it is an issue with the wiring. I would like to replace this system before it breaks once and for all.

Anyway, we're thinking that it is probably time to replace the system, and ideally we would like to replace it with a heat pump. (We live in a colder climate (Massachusetts) and we don't want to disconnect our gas hydronic boiler, but would love to use electric heat from the heat pump at least part of the year.) I've started getting bids and I am getting a lot of contradictory information which is hard for me to figure out.

So far we've gotten one proposal for a Bosch condenser with a Unico air handler, and I am waiting for other bids to come in. We thought we had a bid for a Gree heat pump system, but the contractor backed out because his equipment was apparently incompatible with high velocity ducts. I have heard that high velocity systems are much less efficient than traditional duct work and that a heat pump is unlikely to give the maximum benefits in our layout. I have also been told that Unico's newer air handlers might need different ductwork than what we currently have, which would be a pretty big problem for us.

Massachusetts has very good incentives (MassSave) for installing heat pumps, but I'm told they may not apply to my situation because there are no approved 4-5 ton air handlers for high velocity systems.

So... I wonder if you have any advice for me? Such as:

- Brands to look for? (Is Bosch a good option?)
- Is a heat pump a good idea in my situation?
- If not, should we just go for an AC-only system?
- Do you know if there are 4 or 5 ton high velocity heat pump systems that quality for MassSave? (Doesn't look like it from the spreadsheet I saw)

mattmia2

How big is the current AC system? If the heat pump needs to be bigger because the heating load is bigger than the cooling load then it will most likely need bigger trunks and more outlets to move that larger volume of air.

That doesn't mean you couldn't have a smaller heat pump that is only used in milder weather with the current duct system.

Jamie Hall

Couple of thoughts -- and I'm not an expert by any stretch. First, keep in mind that high velocity systems have different requirements for the air handler. There is a much higher pressure drop involved to move the air, and as @mattmia2 said you will need to move a lot more air for heating than you do for cooling. This could be a problem with any installation.

Second, the lights dimming. It is perfectly possible that your internal wiring is fine, or at least adequate -- but the dimming all over the house means that the lines from the transformer to your main panelboard are not, and it is quite possible that the main panelboard isn't up to the loads, either. If you have less than a 200 ampere main service, it needs to be uograded.

mattmia2

The service size depends on the other loads. if it is a modest sized house with all gas or oil appliances a 2 or 3 ton condenser on a 100a service should be fine. If the load is reasonable i would look for loose connections in the panelboard between buses, between the busses and the breakers or fuses, connections to the wiring in the panelboard and meter socket, bad main breakers and bad connections in the drop from the utility.

Big Ed_4

Could be a weak ground too ...

pecmsg

High velocity systems when designed properly are almost silent. 

If you’re hearing noise there’s issues. 

Get those corrected first. 

Jon_blaney

Unico design and installation manuals are available online. These may answer some of your questions. You may need to do a little digging to find them.

mattmia2

Big Ed_4 said:

Could be a weak ground too ...

An ac condenser is a 240 v only appliance, it only has line connections, it does not use the neutral and will not change the balance between the lines if there is a bad neutral.

mattmia2

Jon_blaney said:

Unico design and installation manuals are available online. These may answer some of your questions. You may need to do a little digging to find them.

It sounds like their noises are from the condenser, probably one with a poorly chosen fan that is noisy and probably some of the sheet metal parts that need some adjustment or cushioning to stop vibration. Maybe the bushings under the compressor have rotted away or hardened too. Unless it is slugging, probably all but the fan is easy and inexpensive to fix.

cbandes

Yes, the noise is from the condenser outside. It sometimes makes a jet-engine noise, and sometimes makes a weird clanking noise. The indoor unit in the attic doesn't make much noise at all, we just hear the air coming out of the vents.

We have 200amp service in the house, the condenser is on a 40amp breaker and I believe the air handler is on a 20amp breaker.

I did get a proposal today that appears to qualify for the rebates with a 4/5 ton air handler, so that's exciting.

mikeg2015

Gree makes a Universal Heat Pump, which is a minisplit type outdoor unit, but operates with any indoor TXV and just matches the load. So you can use a 2 stage indoor blower with it. Will be quiet and low startup torque.

Similar in operation to the Bosch but less expensive, a little lower efficiency since it's a VS rotary not a VS scroll.

The bosch is a good unit. I did replace a control board in one, but I think it took a near lightning strike.

With either inverter unit, you might want a surge suppressor.

mikeg2015

High velocity can be quiet, but you need a LOT of supplies. The one in my house was really noisy on high stage, ok on low stage. But so is the system in the tract home we are renting now.

cbandes

Jamie Hall said:

Couple of thoughts -- and I'm not an expert by any stretch. Second, the lights dimming. It is perfectly possible that your internal wiring is fine, or at least adequate -- but the dimming all over the house means that the lines from the transformer to your main panelboard are not, and it is quite possible that the main panelboard isn't up to the loads, either. If you have less than a 200 ampere main service, it needs to be uograded.

I do have 200 amp service, but maybe something is up with the wiring, that would be really unfortunate, but I guess no time like the present to check it out...

cbandes

The new quote is for AHRI 208518975 which is a ECOer condenser and a Unico air handler, looks like it qualifies for rebates which sounds good to me. They have add-on options like a surge protector, air purifier/filter, uv purifier - idk if any of those are important, or if it is just an upsell?

mattmia2

Th dimming lights could be problems with the utility or things that are undersized from the utility like the dropwire or the transformer.

As I said above, it also could also be bad connections in the service equipment.

acaldwell44

@cbandes I have a very similar situation. I am moving into my new house next week in North Andover MA, and have the High Velocity ducts. We know the whole AC system needs to be replaced and we were told by our inspector to use mass saves and get a Heat pump for heating and air. Reading about your issues with finding a system that qualifies for the rebates, I see you went with the "AHRI 208518975 which is a ECOer condenser and a Unico air handler". Can you share who you ended up using for install? were you happy with the outcome?