Temp. efficiency question between Mr. Slim and in-floor electric radiant heat

[tjwalker]

I have a newly built 1008 sq. ft patio home (on slab) in central Minnesota.

I have a Mitsubishi Mr. Slim split heating/cooling system, outside unit is model MUZ-FE18NA and inside unit is model MSZ-FE18NA. I also have a single zone in-floor radiant heat system powered by an Electro Industries 9kw mini-boiler.

My Mr. Slim operator manual shows the lower limit for heating using this model is -13F dry bulb and -14F, and that my unit puts out 100% of rated heating capacity at +5F and 80% of rated heating capacity at -13F.

I have lived in the house for a year now and absolutely love the Mr. Slim! It's super quiet and efficient and has done what I've asked of it so far.

My question is in regards to "heating" using the Mr. Slim with the cold temps coming. We can have some brutally cold winter temps here in MN, and I am looking for recommendations as to what temperature would be a reasonable point to stop using the Mr. Slim for heat and start using the in-floor radiant heat?

I should also mention that I am on a "dual fuel" program with my electric cooperative that sells me electricity for both Mr. Slim and radiant boiler at the exact same half price cost of 4.2 cents per kwh.

I am receiving some mixed signals so far, with one saying to discontinue use of the Mr. Slim at +20F and others saying that because of the efficiency of the Mr. Slim, that I would be best served to continue to use the Mr. Slim down to the -13F temperature. That's why I'm posting here to get the views of pros in the field.

Any recommendations would be much appreciated as well as pros and cons considerations (wear and tear on the Mr. Slim at sub-zero temps, mold potential, etc.) other than just the cost of electricity.

Thanks in advance very much for your replies!

[BaldLoonie]

So you have 1 indoor unit for the whole house? How does it condition all the rooms evenly?

That place must be EXTREMELY well built to heat on 18K even at 5° out. My 1000 sq ft home sure couldn't do that.

Obviously the heat pump even in bitter weather is cheaper heat. But how comfy is it? At some point, probably closer to 0° I might give it a break and go for the comfort of the radiant floor even it it cost more. But that's personal opinion.

[tjwalker]

Thanks for your reply. Yes, one Mr. Slim indoor unit for the whole house. The house is a VERY open design with no halls; as long as the doors are open, the single Mr. Slim indoor unit which is centrally located does a decent job.

I agree that comfort is the one non-tangible that will be in play here....

[hvacvegas]

If the Mr. Slim can keep you comfortable, I'd go with it. For ever.
Unless there's a minimum run temperature for it that I'm not aware of.

I just glanced at some data for an 18seer convential goodman heatpump, and it produces 50% more heat than electric heat, at the same energy consumption, at -10*.
The heatpump at -10* has a COP (coefficient of performance) of 1.53, your electric heat has a COP of 1.

Thats an 18 seer, convential. You have a more efficient unit than that (most likely).

Since comfort is probably an option, run the mr. slim till your not comfortable, check the outdoor temp, and thats when you know you need to run electric heat.

Even better, you could get your HVAC professional to wire in some controls that has your electric heat come on automatically at that temperature.

[tjwalker]

My Mr. Slim is one of their high efficienty "hyper heating units" 21,600 btu capacity heating capacity, 20.2 SEER, 14.2 EER and 10.3 HSPF if that helps any.

I just looked at another site that shows my Mr. Slim unit has 82% heating capacity at -4F and 62% at -13F. Not sure why different sites show slightly different specs than what I outlined earlier or which is actually accurate, but you get the basic picture I think on their efficiency.

So it does look like my Mr. Slim efficiency does drop pretty quickly at around zero degrees F?

How difficult/expensive to install controls that switch between the two heat sources at a predetermined temperature?

Keep the ideas coming guys! Appreciate it.

[hvacvegas]

My Mr. Slim is one of their high efficienty "hyper heating units" 21,600 btu capacity heating capacity, 20.2 SEER, 14.2 EER and 10.3 HSPF if that helps any.

I just looked at another site that shows my Mr. Slim unit has 82% heating capacity at -4F and 62% at -13F. Not sure why different sites show slightly different specs than what I outlined earlier or which is actually accurate, but you get the basic picture I think on their efficiency.

So it does look like my Mr. Slim efficiency does drop pretty quickly at around zero degrees F?

How difficult/expensive to install controls that switch between the two heat sources at a predetermined temperature?

Keep the ideas coming guys! Appreciate it.

Not expensive, not difficult if you have some experience. It's honestly nothing special.
What I'm thinking, wouldn't turn off your mr. slim. It would just prevent the infloor from coming on above your "comfort" outdoor limit that the mr slim can handle.

We can't discuss pricing, but I would suggest asking a local professional for a quote. If I was in your situation, I know I would do it.