How to use existing oil boiler with new Mitsubishi mini-splits

allenm

Earlier this year we had two Mitsubishi Hyper Heat mini-split systems installed, a MSZ-FS06NA-U1 / MUZ-FS06NAH-U1 in the master bedroom and a MSZ-FS18NA-U1 / MUZ-FS18NAH-U1 in the living room at the other end of the house. Both have Kumo Cloud installed and working.

They have been working great for cooling, but now we are thinking about how to use them for heating this coming winter. We still have our Buderus G115 oil boiler, and it works fine.

What we would like to do is use the oil boiler when the outside temperature drops down to maybe 5 degrees F instead of the mini-splits. The specs I find show these Hyper Heat mini-splits are 100% efficient down to 5 degrees and then only 76% efficient from 5 to -13 degrees. Our Buderus is 88% efficient, so it only makes sense to use it from 5 to -13 degrees.

I know I could install a Kumo Cloud Station, an outside temperature sensor, and another WiFi interface just for the station. That's quite a bit of money for the few days we get sub-zero temps here in southern NH.

I'm wondering if there is a way I could use Kumo Cloud to automatically turn off the mini-splits based on the outside temperature reported by for example weather.com? I know Kumo Cloud can connect to IFTTT but have not gotten into that yet. Maybe weather.com or whatever can also connect to IFTTT? I do have a Davis weather station in my backyard, but it doesn't do IFTTT.

If I could use IFTTT to turn the mini-splits off, I could probably also use it to turn my oil boiler on when the temperature drops to 5 degrees.

Any suggestions would be welcome.

GGross

What is the actual COP of the units when running at -13? while you may only get 76% of the rated heating capacity at that temperature I have found that some brands of low temp mini splits are still at a COP of 2-3 which is technically 200-300% efficient this doesn't follow for every brand and model or unit configuration, but modern mini splits can't really drop below 100% efficiency, meaning at their worst it is still a 1:1 of what power you put in you are getting out as heat. One could get into a whole philosophical debate about what true efficiency is, or what the cost per unit is vs gas, and of course if the units won't output enough heat at lower temperatures that would be no good.

If it is strictly raw efficiency the mini splits will always beat out gas, modern gas appliances can never go above 100%. However there are many other factors to consider such as the comfort level from the heat source, and the cost per unit of energy, you obviously don't want to be paying more money even if the equipment is technically more efficient.


As for the setup I believe that @GW has dealt with the Kumo systems a bit in the past

GW

I'm good with Kumo and Station, but the IFTTT is a little over my head. My tech guy is a little better than me at IFTTT and apps in general. IFTTT needs a Kumo and a Nest (or) Ecobee. I saw a video on this a while back; if a gun was pointed at me I could probably figure it out

Jamie Hall

The debate on when -- or if -- to switch from your heat pumps to your boiler is very complicated -- since different folks have different priorities and, perhaps worse, different electrical suppliers have different sources for their electricity.

The simplest approach is to look at the cost, and for that you need to know the cost of a BTU of heat from your oil boiler vs. the cost per BTU of heat from your heat pumps, which depends on your electricity rates. The arithmetic isn't hard -- but you need some sort of figure for the COP of the heat pump at various temperatures, and that may be difficult to get, and you need the cost of your oil and the cost of your electricity.

Just to give you idea, though, where I am located oil is about 3 cents per 1,000 BTU. Electricity with a COP of 2 is about 4 cents per 1000 BTU

Hot_water_fan

You’ve mistaken capacity for efficiency. If the COP of the minisplits is high enough, oil may never pencil. There’s some guesswork involved here because the price of oil changes a lot and so does the COP. If 5 degrees is your goal switchover, then odds are you won’t be able to tell the difference in costs since that weather isn’t all that frequent. You also need not turn one on and one off, try both on at low temps.

Hot_water_fan

@Jamie Hall $/1000 BTU is sometimes unhelpful. Let's use the more common $/1,000,000 BTU, it'll be clearer for people. Otherwise, $3.45/gallon oil looks 50% more expensive than $3.44/gallon oil, which we all know is nonsense.

allenm

It took awhile, but I finally got the COP ratings for my exact mini-splits. The smaller unit has a COP of 2.46 at 5 F and a COP of 2.26 at -5 F. The larger unit has a COP of 2.15 at 5 F and a COP of 2.00 at -5 F.

I found a spreadsheet for comparing various fuel costs that I'll be updating with my ratings and costs. For what it's worth my latest electric bill works out to .3033 cents per KW. I simply divided the total bill by the total KW. My latest fuel oil delivery back in April was $3.70 per gallon.

My total oil usage from April 2022 to April 2023 was 456 gallons at a cost of $1874. Not real bad for a 53 year old house in NH. But that's not why we went to mini-splits. I'm just feeling too old to be lugging around window AC units. So, being able to use these new mini-splits for most of our heating is a real bonus.

I'm going to use this cost calculator https://us.v-cdn.net/5021738/uploads/editor/li/ikvfch7okbgi.xls and might have more questions.

I am curious about just leaving both the oil boiler and mini-splits on during the heating system. Seems like they would be competing with each other, but I'm not opposed to doing that as long as they won't get into a fight

Hot_water_fan

or what it's worth my latest electric bill works out to .3033 cents per KW. I simply divided the total bill by the total KW.

This doesn't work if you have fixed monthly costs, but it's easy to fix. Simply subtract those out, then divide by kwh.

allenm

Hot_water_fan said:

or what it's worth my latest electric bill works out to .3033 cents per KW. I simply divided the total bill by the total KW.

This doesn't work if you have fixed monthly costs, but it's easy to fix. Simply subtract those out, then divide by kwh.

I'm not sure I understand which charges would be fixed. Here's a snip of my breakdown. Should I take out all delivery charges and just use the supplier charges? In other words, use .162 per kWh in the cost spreadsheet?

GW

Allen that looks like 30 cents per Kw
88/292 = .30

Jamie Hall

What I would probably do -- and, indeed, what I do do with the apartment in one property which has a heat pump in addition to the steam heat in the rest of the place -- is use the steam heat for the base heating load, and the tenant in the apartment can -- if she wants -- turn up the thermostat on the heat pump to top off.

Hot_water_fan

No, just subtract the $13.81. You pay that no matter what, everything else is based on volume. 

pecmsg

You can play with the #’s all you want. Temperature and Wind determine the balance point. When you feel your uncomfortable switch over to your back up. 

allenm

Hot_water_fan said:

No, just subtract the $13.81. You pay that no matter what, everything else is based on volume. 

Ok, so after taking 13.81 out and using my local fuel oil and electricity costs, the final answer is fuel oil would cost me $34.46 per million btu, while my mini-splits would cost me $20.48 per million btu. That's a 41% savings using the mini-splits to heat. My Mitsubishi dealer told me I could potentially expect about a 50% savings, so not too far off.

Of course, I realize that's theoretical and not real world. But I'll be tracking it this winter and making real world comparisons, because shortly after the mini-splits were installed I added an Emporia Energy monitoring system to my AC power panel. I'm monitoring electrical usage on 8 circuits, 2 of which are for the mini-splits.

EBEBRATT-Ed

You can do all the calculations you want. But the calculations have nothing to do with comfort.

What good is energy saving if you're not comfortable. I think from a comfort standpoint you will switch over to the boiler around 25 degrees give or take.

Hot_water_fan

Nothing is set in stone, we don’t have to pick temperatures now. 

The Steam Whisperer

What you are leaving out is the cost of the equipment and installation itself. It seems that heat pumps have about a 14 year life, so you have to replace the complete system every 14 years. So divide what you spent to install the minisplits by 14 to get the yearly cost and add that to your energy costs. You'll also need to figure in cost of maintenance and repairs. If you have a good cast iron boiler, 30 to 45 year life is very likely. At the end of the boiler life usually all you need to replace is the boiler and related pumps and supporting hardware.... the system stays in place. Now divide the cost of the boiler installation by say 37 years to get the yearly cost and add that to the energy cost and maintenance/ repairs.

You may be surprised at which cost the most to operate when all costs are included.

Hot_water_fan

@The Steam Whisperer

Earlier this year we had two Mitsubishi Hyper Heat mini-split systems installed, a MSZ-FS06NA-U1 / MUZ-FS06NAH-U1 in the master bedroom and a MSZ-FS18NA-U1 / MUZ-FS18NAH-U1 in the living room at the other end of the house.

They’re already installed. 

GGross

As for how to handle changeover from heat pump to oil, I generally would tend towards manual changeover. this way you can see if the heat pumps work for you when you hit design day, and don't need to guess, if they can't keep up, or are otherwise not maintaining your desired level of comfort, turn on the oil. I would make a strong recommendation that you keep up on maintenance of your oil equipment as well, and run it during the winter at some point. Since you still have it you have the advantage of a nice dual fuel system and you want to keep your oil equipment in functional condition ready to go if it is needed.

Condoman

I must have done it wrong. I set my oil boiler for 68 degrees and my heat pumps at 72 degrees. The boiler cycles a few times in the coldest part of winter. This has been done for 4 years now and it is comfortable in all parts of the single floor home.

The boiler gets its exercise from the more often heating water for the megastore.

WMno57

@Hot_water_fan , you missed @The Steam Whisperer point. If the heat pump has a MTBF (Mean Time Between Failure) of 15,000 hours and the Boiler has a MTBF of 100,000 hours you cant just compare cost of energy. You also have to compare cost of depreciation.
Anyone have a Hobbs meter on their boiler?
Same problem with EVs. 2000 charge cycles before the batteries are degraded to a 50 mile range?

Hot_water_fan

@WMno57 no I get the argument, but we’re just making numbers up here. The mini splits are already installed, they can be used for heat. I think claiming that using them for heat and cooling as opposed to just cooling will lead to a shorter lifespan is not supported by any facts. 

Jamie Hall

Oops. Don't compare lifespan in years with lifespan in running hours. It's running hours that is the relevant figure. Using it for heat and cooling both will give you more or less the same number of running hours -- but they will accumulate much more rapidly leading to a shorter calendar life.

Hot_water_fan

@Jamie Hall do we actually know that? Or are we going on a hunch? Seems like an AC in Maine should last a millennium if one in Florida lasts a decade based on cooling hours. 

GGross

The biggest hurdle in that regard, in my opinion, is to have a system that is sized well for both heating and cooling. Starting with a well installed, well sized, and correctly applied unit will always yield better results than the alternative. One point that I think is worth mentioning here is that the warranty period for many top mini split brands, installed by the brand certified professionals, is 12 years for the compressor, whether used for just cooling, or both heating and cooling, contrasted against some cast iron and condensing boiler brands which often top out at 10 years. The manufacturers appear to have pretty close to the same level of confidence in their equipment. I think many mini splits are not meeting the above criteria however, and probably do suffer an early fate partially due to increased run times, but adding to that is the increase in wear from improper sizing/installation etc.

Jamie Hall

Hot_water_fan said:

@Jamie Hall do we actually know that? Or are we going on a hunch? Seems like an AC in Maine should last a millennium if one in Florida lasts a decade based on cooling hours. 

Well, not a millenium -- but a long long time. Several decades at the very least. However, there are, it must be admitted, calendar sensitive influences as well. This is most clearly seen in the aircraft industry, where they really pay attention to such things as breakdowns can be a little worse than nuisances. Consider a typical aircraft piston engine (there are still a lot of them out there). If you read the directions, you will find that it has a "time between overhaul" specification, and there are two times listed: operating hours (typically around 2,000 and 12 years for the iO-360 on my birdy). Why two: a calendar and a operating hours time? Simply because the wear and tear on the engine is related to both operating hours (how many times the pistons go up and down!) and the effects of just sitting out in the atmosphere (rust happens. Rubber oxidizes). The same thing can be seen in cars -- which anyone who works on classics is very familiar with.

To get back to a heat pump or an air conditioner. Some parts will be calendar sensitive (and environmental conditions sensitive) -- a condenser exposed to salt sea air is simply not going to last as many days or years as one sitting in northern Nevada in a desert. Time. On the other hand, anything that moves -- the compressor, any valves, what have you -- is going to have a life limited in running hours or cycles.

pecmsg

The Steam Whisperer said:

What you are leaving out is the cost of the equipment and installation itself. It seems that heat pumps have about a 14 year life, so you have to replace the complete system every 14 years. So divide what you spent to install the minisplits by 14 to get the yearly cost and add that to your energy costs. You'll also need to figure in cost of maintenance and repairs. If you have a good cast iron boiler, 30 to 45 year life is very likely. At the end of the boiler life usually all you need to replace is the boiler and related pumps and supporting hardware.... the system stays in place. Now divide the cost of the boiler installation by say 37 years to get the yearly cost and add that to the energy cost and maintenance/ repairs. You may be surprised at which cost the most to operate when all costs are included.

More like 7 1/2 - 10 years!

ethicalpaul

Just like many cast iron boilers we see here, just saying.

Hot_water_fan

@Jamie Hall I jest, of course operating hours play a role. But it’s probably long. Probably longer than most will live in a house. 

Jamie Hall

Hot_water_fan said:

@Jamie Hall I jest, of course operating hours play a role. But it’s probably long. Probably longer than most will live in a house. 

Quite true. My point of view tends to be skewed... my daughter who now owns Cedric's home is the 8th generation of our family to do so!