Lennox heat pump in cold climate?

EdBoston

Hi all, we are trying to improve the heat to our attic and were looking at adding mini-splits. The attic has its own recently installed (~2010ish) Lennox central AC system so our HVAC company advised that we use that setup and convert the system to a Lennox heat pump. They are also a Mitsubishi dealer but felt it wouldn’t be worth adding another outside unit and multiple interior heads when we already have a good quality air handler and ducts. Also, there are 4 rooms up there in 1000 sq ft and it’s not necessarily clear where best to place the heads.

My concern is, with the cold climate here in Massachusetts we spend a lot of time below 25 degrees. Are we just going to wind up relying on backup heat most of the time anyway? Is it worth the expense and trouble to get mini-splits?

HVACNUT

The Lennox system serves only those 4 rooms?

The contractor is looking to remove the existing Lennox A/H and condenser and replace it with a heat pump system with electric heat for 2nd stage/emergency?

I dont believe adding a ductless system will help unless the existing equipment and ductwork is not properly sized.

What's the heat source for the rest of the home?
Is there a hydronic boiler?

EdBoston

Yes, Lennox system is only for the attic. Rest of the house has forced hot air (gas) and central AC but the ductwork going up to the attic is poor so previous owners put in the Lennox system with separate attic ductwork for cooling only. Contractor says the current system is well-sized for the space (they installed it and still have records) and they plan to revise the current Lennox AC system into a heat pump. They say they can modify the A/H, add backup electrical heat, and change the outdoor unit to heat pump. They’ve done a real Manual J, checked the equipment, etc.

Jack

How many sq ft per room? How are the rooms laid out? I would suggest you add a gas fired Rinnai Energysaver Direct Vent wall furnace and if necessary a couple Tjernlund Airshares for the adjoining rooms. I would not do the Airshares until I see how the air transfers into the adjoining spaces.

EdBoston

Pic is attached, hopefully it comes through.

How much space would the furnace take up? May be a challenge to find a spot for it, and to run a gas line up there from the basement.

JUGHNE

Does the existing Lennox duct work satisfy the cooling needs of that floor? If it does it will most likely do the heating needs also.

All heat pumps have claims of providing heat in what we consider sub-zero conditions.....some heat but not necessarily enough. You will most likely need supplemental heat with mini splits also.

You have some heat from the forced air furnace in the basement....yes?

I would go for the Lennox HP with some elements installed in the air handler. IMO it is the best way to go at this time.

You have survived with only forced air from the basement this long so you don't perhaps need a lot of heat up there.

EdBoston

We have lived here a couple of years and mostly abandon the attic during Jan/Feb but the heat coming from below can keep it in the 50s at least. So hopefully the heat pump does enough to make it comfortable without blowing up our electric bill. Yes, the current Lennox AC easily cools that space so that is encouraging too.

The wall furnace idea is interesting too, I’d like to explore how feasible it might be. Although the Lennox HP seems like a more elegant solution, and I’m impressed with the quality of this HVAC company’s work so far.

HVACNUT

Your limited on the SEER and EER ratings due to the existing evaporator so if , for example the existing is 13 SEER, your going to see higher electric bills as opposed to say a 16 SEER.
As far a the electric heat kicking in, other than when the condenser is in defrost mode, the thermostat might not even hit the 2nd stage differential, especially if kept at or below 60°.

Make sure the contractor sets the condenser above the snow line.

DZoro

I would go with a good quality HP in the ductwork. Look at a 2 stage system, that will give you some extra power in the heat mode, cooling mode you may only be in 1st stage. Electric coil in the plenum could be also added now or in the future if needed.

EdBoston

This is all very helpful information, thank you. I should get the quote this week and can see what Lennox unit they specify. He did mention he would size out different options by price.

We do get some big snowstorms a couple of times each winter. He offered me a choice of 6” or 12” snow legs, I would rather keep them shorter for aesthetics but if it makes a big difference I could go higher.

Jack

Does the Lennox have a base pan heater to prevent freeze-up of the condensate in defrost mode. If you get substantial snow the taller legs will probably be best. An iced up unit isn't very aesthetically pleasing either;) One of the advantages of the Rinnai DV is that it is another heat source entirely. I have run my EX22 off a deep cycle marine battery and a good sine wave inverter. In some conditions you could find yourself huddled in the attic.

JUGHNE

IMO the taller legs will raise the unit so that during defrost of the outside unit the melting icewater will fall to the ground and refreeze there. You do not want icicles solid from the unit to the ground. You want the water from the next cycle to be able to leave the bottom of the coil.
It is an advantage to have the HP unit see some sun in the winter to avoid the ice puddle under the unit.
I have seen lower tubing crushed by the accumulation of re-freeze of defrost water/ice.

EdBoston

Thanks very much for the input. It sounds like taller legs are the way to go.

Jack I did really like the idea of sticking with gas heat. But my wife hates the idea of running a gas line up there. I was also worried about consistent heat across all the rooms and placing the Tjernlund vents would have been a big challenge.

Jack

I would not use the TJ units until it is proven that the air would not circulate into those rooms. Given the 7 stage modulating burner and blower you will generally see a 2-3* temperature difference in adjoining rooms. That depends upon lay-out of course, but you can also just short cut the doors a bit and put a though the wall register up high on an adjoining wall. Heat wants to go to cold. This is kind of the old straw in a glass of water trick. If you put you finger over the straw you can lift the water out of the glass. Remove your finger and the water flows nicely. Air is a fluid and be haves the same way. Rinnai units are designed to do two thing. Number one, satisfy your comfort demands. Number Two is to do it at the lowest possible input so the warm air will travel to those adjoining areas. I can't help you with the gas line and more importantly, your wife;)...but I'll be happy to talk heating with you!

EdBoston

Thanks for your help Jack! I’m going to hold on to the Rinnai idea too... especially if the heat pump quote comes in higher than our budget.

EdBoston

Quick update: I received the proposal from my HVAC contractor. The quote looks fine and we're going to move forward with changing the condenser to a Lennox heat pump and adding electrical backup heat to the existing handler.

He quoted a Lennox XP14, which is a smaller single-stage unit. It does look fine however and has good reviews. Our existing air handler is single stage so we are limited by that. How much advantage would there be in going for one of the more expensive "Signature" line heat pumps? Someone suggested to me that I check AHRI to see the difference - what data should I focus on there?

https://www.lennox.com/products/heating-cooling/heat-pumps/xp14

Thanks again!