Your thoughts on combining my new Mitsubishi mini split heatpump with existing hydronic.

GCR

We installed a 4 zone mitsubishi mini split system in our 1936 colonial in suburban DC -- this is to be our air conditioning since we don't have central air and didn't want to put in ducts. The outdoor unit is the MXZ-4B36NA.



Since it also provides heat, and probably with more efficiency than my current boiler, I'm wondering how to go about combining the two.



The boiler pictured below -- 1 zone, gas, gravity system. No outdoor reset. I have no idea when it was put in, but my home inspector suspects it is at least 50 years old. There's something that looks like a temperature setting set at 160.



So a few questions 



#1, Am I right to try to get as much heat from my mini split as I can and how would I do that? So far I'm thinking I'll just move the thermostat for the boiler to somewhere outside of where it gets a draft from the mini splits, set it for a temperature below what the mini splits are set at, and it will fire when the Mitsubishi system can't provide enough heat for the house. I thought this might cause some short cycling, but figured it's the equivalent of when the outside temperature is warmer.



#2, that boiler is old as is going to go, what's my upgrade path? Mod/Con? A 2 stage boiler with outdoor reset?  Combined with the mini splits, the system is clearly oversized.



Thanks.

nicholas bonham-carter

Heat pump vs boiler

First consider whether the boiler is in good condition, such as clean burners, system well bled etc, and then check out the difference in cost per btu of gas vs electric. Unless your boiler is in bad condition, the cost of heating will be less with gas than electric heat pump, below about 45 degrees.

Depending on the type of radiators you have, the mod con type of boiler will be ideal in the future, especially as it can have outdoor reset, to vary the water temperature with the outside temperature. In this case, you could reserve the mini splits for cooling only, as the gas is used much more efficiently.--NBC

Ironman

The Boiler Plate...

Says "MFG" gas. Are you on Propane?

GCR

radiators

NBC - Attached is the type of radiator that I have. Below 40-something degrees the heatpump can't put out all the heat the house needs. This is a new house for me and I don't know how much attention the boiler has gotten. The folks who put in the heatpump also quoted me a 2 stage Raypak boiler model RH40090-EIWP. I passed at this time.



Ironman. It's on natural gas. There must have been a conversion sometime -- the controls are millivolt but there's a disconnected 24V transformer.



Any idea how old this thing is?

nicholas bonham-carter

Old boiler

This would not be the best time to change a boiler, unless required by emergency, so just make sure it it has no problems. If the pressure gauge is accurate, and the fill valve is closed, you would see the effects of any leak, by a declining pressure. This boiler will give you a bit of warning before it dies. Does it do a good job of keeping the house evenly warm now?

I don't see any draft hood or damper on the flue to moderate the flow of basement air through the sections when not firing, unless it is built into the cabinet.

The advantage of some of the new boilers is their small size, but they do need more regular maintenance.--NBC

GCR

Keep an eye

I'll keep an eye on those. When on boiler only some rooms of the house were warmer than others -- the upstairs about 2 degrees colder.

Jim Godbout

heat pumps

We are in maine with much colder climate than DC

Air source heat pumps have come a long way, they are working extremely well in our climate

the only caution i have is there is a difference in comfort heating with air vs hydronics.

Our company offers a blend of systems when doing air source to provide optimum comfort levels.

Some air source units are 26-27 seer, super efficienct and you will love the ac in hotter months.

good luck,

find a great contractor to guide you.

jim

John Mills_5

We

have a customer in a huge old house with that boiler - just a 400K BTU version! Kept the boiler, put forced air and A/C on the main floor, ducts in the crawl. Put a 3 head Mitsu heat pump system for 3 key rooms (offices now, were bedrooms).  His plan was to use the furnace and Mitsu for heat unless it got too cold for the heat pumps, then he'd fire the old Climatrol. 

Ironman

Near D.C.

Where are you located?



We cover N. Va. If you're in Md. we can recommend a good contractor.

GCR

Location

I'm in Silver Spring, MD and would welcome a recommendation to eventually replace this boiler.

Looking at costs per therm it does seem like below the 40'ish degree line the heatpump is less efficient but for that i'm guessing on the AFUE of the boiler. I think for now I'll just manually change from boiler to heatpump in the shoulder seasons.

RobG

Gravity Heating

One thing that you should do is turn up the aqua-stat (the temperature gauge that is now set at 160 degrees). Those systems were designed to operate at 180 degrees to promote the thermosiphoning effect. Someone previously probably turned it down at some point to conserve fuel, however it actually reduces system performance. That may be why the upstairs is cooler than the downstairs. Turn the water temperature up and turn the thermostat down as once the system shuts down the water will still "carousel" through the piping until it has cooled down enough to stop flow, thus the thermostat will overshoot the set temperature.

Rob 

Ironman

2 Recommendations

Frank Wilsey, "Steamhead" here on the wall.

allsteamedup@verizon.net

Phone 410-321-8116



Dan Foley, Foley Mechanical

dfoley50@verizon.net

Phone 703-339-8030



You won't get any better than either of these guys.