New guy here: looking for a boiler to dual fuel my Bosch IDS 2.0 heat pump

EvilDragon

Hi everyone! I've a 2600 sq. ft. home with a dying cast iron oil boiler (3 zones) and a Bosch IDS heat pump in the Boston area. The oil is going to be replaced with propane. I would like to be smart about replacing the boiler, because I would like to take advantage of the heat pump during the warmer days - what's the proper way for me to set up a dual fuel system? What are the recommended boilers and controls? I would assume that the Bosch Greenstar boilers and their BCC thermostats are the way to go, but would like to get an opinion of someone who has dealt with this type of setup.

Thank you in advance for any help.

D.

Jamie Hall

You would size and choose the boiler in the same way as you would without the heat pump: it needs to match the heat loss of the house, if this is hot water, or the installed radiation, if it is steam. There's no magic to it.

The fun comes in integrating the boiler with the heat pump, so that you are using the heat pump when it can keep up with the load at less cost than the boiler, and the boiler when it is colder out. It does help if all the components are from the same manufacturer, but it's not mandatory.

EvilDragon

Are there some standard controls that folks are having luck with?

Jamie Hall

Unfortunately, while there are standard controls for heating systems and for heating combined with A/C, so far there aren't for the combination. Some thermostats have some provision for setting the changeover point, but each make (and often model!) is different.

Ironman

We do this with a Honeywell TH6320 stat programmed for dual fuel sensing outdoor temp.

The “W” connection on the stat is used to energize a relay coil whose “NO” contacts control the boiler. The “NC” contacts control the fan on the AHU so it doesn’t run when the boiler does.

You’ll need a DPDT 24v relay and a thermostat wire between the the AHU, where the relay will be added, and the boiler.

pecmsg

Why LP? that's a lot more dollars into the replacement.

What do you have for hot water?

EvilDragon

Separate heat pump for hot water. LP because no natural gas.

The wrinkle is that they are not on the same thermostat - I've three zones on separate thermostats for the baseboards, 4th thermostat for the heat pump that's one zone across the house.

Hot_water_fan

What's your cost/kwh for electricity?

Jamie Hall

EvilDragon said:

...
The wrinkle is that they are not on the same thermostat - I've three zones on separate thermostats for the baseboards, 4th thermostat for the heat pump that's one zone across the house.

Well that makes it much more complex. I suspect you will do bests to do as @Ironman said for one of the zones, to handle the changeover. Then set the other two zones to a somewhat lower space temperature, but still comfortable. Then, when the one zone switches over, the other two won't get heat pump heat and will shortly operate perfectly normally on the boiler.

Ironman

Jamie Hall said:

... The wrinkle is that they are not on the same thermostat - I've three zones on separate thermostats for the baseboards, 4th thermostat for the heat pump that's one zone across the house.

Well that makes it much more complex. I suspect you will do bests to do as @Ironman said for one of the zones, to handle the changeover. Then set the other two zones to a somewhat lower space temperature, but still comfortable. Then, when the one zone switches over, the other two won't get heat pump heat and will shortly operate perfectly normally on the boiler.

Or, combine all three hydronic zones into one.

If one zone is satisfactory for the heat pump, why not for the hydronic?

Or, if there’s a hydronic zone panel, simply run the end switch connection for the boiler through the “NO” contacts of the relay that I previously described.

EvilDragon

Are there any connected thermostat solutions that do something similar? I'm kinda surprised that's not the case with something like EcoBee.

Ironman

Probably not, but I don’t try to keep up with every thermostat that comes along.

You have an unusual arrangement and if there’s not a large enough of a market for it, thermostat manufacturers aren’t gonna address it.

We run into this situation a couple of times a year and what I recommended is the simple solution. All that you need is a $20 relay and some stat cable.

EvilDragon

Cost per kwh is around .12, but I have solar panels and generate about 7.5 MWh per year, feed it back into the grid for credits (Massachusetts).

Hot_water_fan

Cost per kwh is around .12, but I have solar panels and generate about 7.5 MWh per year, feed it back into the grid for credits (Massachusetts).

That’s what I pay. If that includes all volumetric charges (my utility has probably a half dozen $/kwh charges), then propane isn’t competitive on a cost basis so you could skip the boiler entirely. Now that you’ve saved a bunch of money there, you have options. They include, but aren’t limited to (nor are these mutually exclusive): using backup electric strips integrated into the air handler (cheap), using electric baseboard (zones easily and provides redundancy cheaply), and/or using an electric boiler with the existing baseboards. Down the line, an air to water heat pump does this more efficiently, which would favor keeping the existing baseboards. 

If you want propane for non-economic reasons, a propane furnace probably fits best, gas/heat pump hybrid systems are super common. You’d lose the different zones though. I’m sure Bosch has controls for dual fuel. 

EvilDragon

The cost of propane to met with a 1000 gallon tank is 2.1 per gallon. That is a lot cheaper than electric.

Sounds like I don't have as straightforward of an option for controls. Thank you, everyone!

Hot_water_fan

The cost of propane to met with a 1000 gallon tank is 2.1 per gallon. That is a lot cheaper than electric. 

Not quite, run the numbers: 

Propane: $2.1x (1,000,000/91500) / .95 = $24/MMBtu with condensing boiler. 

Electric: $.12 x (1,000,000/3412) / 3 COP= $11.72/MMBtu with heat pump. 

50% cheaper on an annual basis. On the coldest day, maybe you have a COP of 1.5. That’s about equivalent to propane and you don’t have to buy a boiler or drop $2100 on fuel for the 1% of the year it might be cheaper. Save your money and use the tech you already bought. 

EvilDragon

So a side note here - my existing oil equipment is at the end of life, boiler about to die and the tank is also due for replacement soon. So either way I am going to do a switchout. The propane equipment is useful also because the tank will power my backup generator.

Your calculations are precisely why I like a dual fuel system, I want to run the heat pump as much as is feasible. I wish there was a good way to monitor the heat pump energy consumption data to get a proper COP curve vs. temperature to figure out the switchover point to propane. During the real cold weather, I would much prefer the baseboard hot water that's propane powered.

Hot_water_fan

@EvilDragon exactly, propane has plenty of non-heating benefits (backup, cooking, grilling, etc.) I think the heat pump itself will always be cheaper than propane @ these energy prices. The nuance comes in with the electric resistance - the Bosch should always maintain a COP > 2, so until the heat load is around 50%/50% heat pump and resistance I think electricity wins. Of course, air-to-water heat pumps would give you both the baseboard comfort and savings, but these are less common at present.

hot_rod

Here is a formula that goes along with a 3 way valve that we sell. The valve selects from HP or boiler. Really between any two heat sources. The control has and outdoor sensor.


I'll see if I can get the formula it tweaked for LP, and english units.

Problem with NG is knowing the fuel value. I think it changes from the utility when gas supplies run low and they add??
O2 maybe? methane?

rsilvers

I live in MA also and power is more like 0.22, no 0.12. I think you are just seeing the generation charge and not the delivery charge.

Heat Pump in MA is the same cost per BTU as Oil or Propane.

That being said, heat pump with solar is interesting. Problem is, my solar production of about 28 MwH per year is already used up from other stuff.

What kind of heat pump is it?

hot_rod

as @rsilvers mentioned, you need to plug in the actual energy cost with all the add on fees. The bottom dollar amount divided the actual meter reading.

Here is a formula that Siggy reworked. Just knowing the actual COP which can change second by second is one challenge to get exact numbers.

A control build to do the switch over may want to have a time delay function to prevent bouncing back and forth between the units a weather, or loads change constantly.

JUGHNE

Are above ground propane tanks allowed in your area?

Some places have restrictions. Are there other tanks you see in the area?