Storage justification ...

[gorsuchmill]

I've been reading some great things about storage, but wonder how much difference it would make in my case. I live in a 3,900 sf house in the Baltimore area and heat with a combination of radiators (1/3 of the house) and hydronic coils in a forced air system (2/3). During work days (M-F) I "warm" the house to 64* from 6 a.m. to 8 a.m., then drop back to 58* until 3 p.m., then back to 64* until 10 p.m. and back again to 58*. Essentially, when we're awake in the house the temp is 64* and it is reduced to 58* when we're away or asleep. On the weekends the temp is 64* from 6 a.m until 10 p.m., then back to 58* overnight. The house isn't particularly comfortable, but my heating oil bill is killing me. I currently use about 1200 gallons of oil from October thru April. Anyway, based on Brownian Joe's formula, 1000 gallons of storage at a Delta T of 30 (assumes a storage tank temp of 190* and usable water down to 160*) I'd be able to get approx. 4.5 hours of btu storage at my average rate of 55,000 btu/hr (50% of my calculated peak load). Installing a larger storage tank would be difficult.

It seems the most benefit is gained using a radiant system, given the large Delta T and ability to store a lot of usable btu's; however, the benefit is greatly reduced when radiators are involved.

A few additional questions:

1) Is it reasonable to achieve 190* storage in an unpressurized tank?

2) Is it reasonable to be able to charge a 1000 gallon tank from 160* to 190* on a single firing of a 140k btu boiler?

3) In order to improve the delta T, what is the lowest usable temp for hydronic coils and cast iron radiators that is still suitable to return to a wood or oil boiler?

Thanks in advance for all your help. This site is great.

Lee

 

[BrownianHeatingTech]

I've been reading some great things about storage, but wonder how much difference it would make in my case. I live in a 3,900 sf house in the Baltimore area and heat with a combination of radiators (1/3 of the house) and hydronic coils in a forced air system (2/3). During work days (M-F) I "warm" the house to 64* from 6 a.m. to 8 a.m., then drop back to 58* until 3 p.m., then back to 64* until 10 p.m. and back again to 58*. Essentially, when we're awake in the house the temp is 64* and it is reduced to 58* when we're away or asleep. On the weekends the temp is 64* from 6 a.m until 10 p.m., then back to 58* overnight. The house isn't particularly comfortable, but my heating oil bill is killing me. I currently use about 1200 gallons of oil from October thru April. Anyway, based on Brownian Joe's formula, 1000 gallons of storage at a Delta T of 30 (assumes a storage tank temp of 190* and usable water down to 160*) I'd be able to get approx. 4.5 hours of btu storage at my average rate of 55,000 btu/hr (50% of my calculated peak load). Installing a larger storage tank would be difficult.

It seems the most benefit is gained using a radiant system, given the large Delta T and ability to store a lot of usable btu's; however, the benefit is greatly reduced when radiators are involved.

A few additional questions:

1) Is it reasonable to achieve 190* storage in an unpressurized tank?

You can go up to 200, without trouble, as long as the tank structure can withstand it. Certain EPDM liners won't, but if you can use a metal tank, you're all set.

2) Is it reasonable to be able to charge a 1000 gallon tank from 160* to 190* on a single firing of a 140k btu boiler?

It would take about 250k to make that temperature change in one hour. So 140k would do it in a bit under two hours. If that's the "input" value for the boiler, then you should take the efficiency into account, as the output will be lower.

3) In order to improve the delta T, what is the lowest usable temp for hydronic coils and cast iron radiators that is still suitable to return to a wood or oil boiler?

A proper installation will need to include return-water tempering for the boiler, anyway, so that's really not a concern. You can use a thermostatic valve (eg, Termovar) or a motorized three-way valve, or a bypass pump to blend a little bit of supply water with the return water, thereby protecting the boiler when the return water is too cold.

The lowest usable temperature will depend upon the size of your radiation. For example, if you have a zone heated by a hydronic coil, and that zone needs 20,000 btu, you then look at the performance chart for your coil. It will list outputs at various combinations of supply temperature, water flow rate, and airflow rate. You may find that your coil was oversized, and will produce 20,000 btu with 140-degree water, or it might have been sized barely big enough, and can only produce that output with 200-degree water.

Each situation will be different, depending on who did the original install and how the equipment was sized.

Joe

 

[gorsuchmill]

I'd have to go EPDM with the liner given the limitations of the opening to my mechanical room. That is, unless I can have a steel tank welded in place. I have an uncle who worked in the shipyard for many years - maybe we could partially assemble in the yard to minimize the amount of welding indoors. Seem reasonable? Any other metal tank ideas?

 

[BrownianHeatingTech]

I'd have to go EPDM with the liner given the limitations of the opening to my mechanical room. That is, unless I can have a steel tank welded in place. I have an uncle who worked in the shipyard for many years - maybe we could partially assemble in the yard to minimize the amount of welding indoors. Seem reasonable? Any other metal tank ideas?

Nofossil has a metal tank under his deck, outside. Heavily insulated, of course.

Joe

 

[SE Iowa]

I have a question about cycling the temp up and down throughout the day. Does this really save much money? To me it would seem that the rate of heat loss is the same regardless of what the house is set at. In otherwords, allowing the house to cool when not using does save energy, but then it takes more energy to heat back up, rather than just maintaining the same temp all day. Especially considering that hydronic systems do not heat up or cool down the house temp very fast? I do know that it is dependent on the diference in temp between the inside and the outside, but could someone show me thermodynamically how this works?

 

[BrownianHeatingTech]

I have a question about cycling the temp up and down throughout the day. Does this really save much money? To me it would seem that the rate of heat loss is the same regardless of what the house is set at. In otherwords, allowing the house to cool when not using does save energy, but then it takes more energy to heat back up, rather than just maintaining the same temp all day. Especially considering that hydronic systems do not heat up or cool down the house temp very fast? I do know that it is dependent on the diference in temp between the inside and the outside, but could someone show me thermodynamically how this works?

Think of it this way...

Let's imagine that your heat loss is 140ktbu/hr at 70 degree indoor temperature. At 60 degrees indoors, the heat loss is only 122500 btu/hr.

We're going to give you a 100% efficiency oil boiler, just for convenience. So at 70 degrees inside, you burn 1 gallon of oil per hour. At 60 degrees, you only burn 0.875 gallons of oil per hour.

Let's also say that it takes half a gallon of oil to raise the indoor temperature from 60 to 70.

So, you go away for eight hours and turn the thermostat back. In that time, you would have burned 8 gallons, but instead you only burn 7 gallons. Then you come home and waste half a gallon heating the place back up, so you burned 7.5 gallons instead of 8.

Joe

 

[SE Iowa]

Thanks. So you do actually use less energy to maintain a lower temp? I just figured that going from 59F to 60F (for example when your thermostats call for heat) would take the same energy as going grom 69F to 70F. Could you show me the equation of how you came up with those numbers? All I remember from thermodynamics was deltaG = DeltaH - T*delta S.

By the way, on your example you'd also save some energy as the house cooled from 70F to 60F. There would not be any boiler providing heat during that cooling period and would therefore increase your savings, right?

 

[BrownianHeatingTech]

Thanks. So you do actually use less energy to maintain a lower temp? I just figured that going from 59F to 60F (for example when your thermostats call for heat) would take the same energy as going grom 69F to 70F. Could you show me the equation of how you came up with those numbers? All I remember from thermodynamics was deltaG = DeltaH - T*delta S.

The heat flow is related to the delta T. In that example, I used a -10 outdoor temperature, so the delta went from 80 to 70 degrees. So the 70-degree delta yielded a heat loss of 7/8 the 80-degree value.

By the way, on your example you'd also save some energy as the house cooled from 70F to 60F. There would not be any boiler providing heat during that cooling period and would therefore increase your savings, right?

Yes. Just kept things simple.

Joe