heat load in winter

[pramodpatil]

how to calculate heating load in winter? is it same as calculating cooling load?

[dougheret0]

Not exactly. For example, internal loads are not considered, since it is assumed that the coldest period will be in early morning, possibly unoccupied and at least with no lights, cooking, etc. Second, it is OK, even good practice, to oversize a heating system to allow for morning or Monday warm-up after a set-back period. It is definitely not OK to oversize a cooling system, because that will cause it to run less, hence not dehumidify properly. See ASHRAE Handbook - Fundamentals for commercial and residential heating load calculations. Or ACCA Manual J for residential and ACCA Manual N for commercial. Older versions of the Fundamentals Handbook, such as 1985, are often available on e-bay, and are actually easier to follow than the latest.

[back2space]

Not exactly. For example, internal loads are not considered, since it is assumed that the coldest period will be in early morning, possibly unoccupied and at least with no lights, cooking, etc. Second, it is OK, even good practice, to oversize a heating system to allow for morning or Monday warm-up after a set-back period. It is definitely not OK to oversize a cooling system, because that will cause it to run less, hence not dehumidify properly. See ASHRAE Handbook - Fundamentals for commercial and residential heating load calculations. Or ACCA Manual J for residential and ACCA Manual N for commercial. Older versions of the Fundamentals Handbook, such as 1985, are often available on e-bay, and are actually easier to follow than the latest.

Theres an argument that you can oversize cooling systems as long as they are inverter controlled. For instance a 3.5kw system. They quote 3.5kw based on the unit being on high fan speed (which can be noisy) you may run the system in low fan speed most of the time so this may actually be providing the required "sizing" of cooling perhaps 2.1kw. With the unit ramping up and down as required.

On days where it gets extra hot you bump up the fan speed and the extra capacity then comes in handy!

Correct me if im wrong!

[Brian_UK]

Theres an argument that you can oversize cooling systems as long as they are inverter controlled. For instance a 3.5kw system. They quote 3.5kw based on the unit being on high fan speed (which can be noisy) you may run the system in low fan speed most of the time so this may actually be providing the required "sizing" of cooling perhaps 2.1kw. With the unit ramping up and down as required.

On days where it gets extra hot you bump up the fan speed and the extra capacity then comes in handy!

Correct me if im wrong!You are assuming that a heat pump is being used here.

The OP was asking about heating calculations.

[back2space]

You are assuming that a heat pump is being used here.

The OP was asking about heating calculations.

I assumed that because the OP was reffering to cooling in the same post and I assumed that most places use the a/c on reverse if they have cooling in the same system.

Sorry if I was wrong.,

[smpsmp45]

If you are considering India as the place so many factors shall change. & you may still end up with heat load which is @ 40 % lower than summer load calculations

[dougheret0]

Theres an argument that you can oversize cooling systems as long as they are inverter controlled. For instance a 3.5kw system. They quote 3.5kw based on the unit being on high fan speed (which can be noisy) you may run the system in low fan speed most of the time so this may actually be providing the required "sizing" of cooling perhaps 2.1kw. With the unit ramping up and down as required.

On days where it gets extra hot you bump up the fan speed and the extra capacity then comes in handy!

Correct me if im wrong!

You are correct. ;) I was discussing heating calculations and didn't expand my statement about cooling systems. Cooling systems should never be designed with excess sensible capacity. At minimum, a cooling system must be designed with sensible heat ratio lower than the room sensible heat ratio. On the other hand, the total capacity of the unit must be equal to or larger than the total room load. One way to match a cooling unit to the load is to operate it at the lowest allowable air flow, usually 300 or 325 cfm/ton. As you point out, this allows an oversized unit to operate with suppressed sensible heat load, and so to dehumidify effectively.

Air flow is generally not adjustable by owner or occupants after a system is installed. As you point out, there are inverter controlled multi-speed air handlers, but airflow changes are controlled by a programmed thermo-humidistat. These units can be specified with some excess sensible capacity to handle maximum db days, and generally reduce air flow when dehumidification is needed.

This is covered more completely in Chapter 7 of my book HVAC Design Manual, A Mechanical Designer's Guide to Successful Design of Small Commercial and Institutional HVAC systems. The book is available for free download by contacting me at my email address.

[back2space]

You are correct. ;) I was discussing heating calculations and didn't expand my statement about cooling systems. Cooling systems should never be designed with excess sensible capacity. At minimum, a cooling system must be designed with sensible heat ratio lower than the room sensible heat ratio. On the other hand, the total capacity of the unit must be equal to or larger than the total room load. One way to match a cooling unit to the load is to operate it at the lowest allowable air flow, usually 300 or 325 cfm/ton. As you point out, this allows an oversized unit to operate with suppressed sensible heat load, and so to dehumidify effectively.

Air flow is generally not adjustable by owner or occupants after a system is installed. As you point out, there are inverter controlled multi-speed air handlers, but airflow changes are controlled by a programmed thermo-humidistat. These units can be specified with some excess sensible capacity to handle maximum db days, and generally reduce air flow when dehumidification is needed.

This is covered more completely in Chapter 7 of my book HVAC Design Manual, A Mechanical Designer's Guide to Successful Design of Small Commercial and Institutional HVAC systems. The book is available for free download by contacting me at my email address.


I have sent you an email requesitng a download.

You will spark outrage now as so many engineers on the forum say to not oversize units at all.

What manufacturers dont post is specs for what the capacitys are when run in low fan speed.

[stefs_cruiser]

I am confused, this topic is 'Heat load in winter" it then went to talk about a 3.5Kw unit.

so when advocating oversizing, is this still in regards to heating and 3.5Kw units?

[back2space]

The conversation went onto 3.5kw unit as an example, this could mean any capacity unit. We were reffering to 3.5kw unit in cooling. Oversizing was the main point as units tend to be oversized if sized on cooling. There is always more output on heating.

[pramodpatil]

pl tell me how to proceed for calculation of heating load in winter?

[Brian_UK]

You could cheat and use this web based heat loss calculator....

http://www.trademate.co.uk/Services/HeatLoss/default.asp?_brand=CP

[vijay_study]

Hi,

I believe heat load in winter is also application dependant.

For commercial applications in indian context winter heat loads are rarely done except for areas where winters have very low temperatures (Few places in india).

What we do is to add reheat Kw in the heat load sheets till SHF works out to 1.00 OR cooling TR becomes Zero (positive) instead of negative which will be the case in winter since heat flows from building to the atmosphere.

For pharma applications where we maintain relative humidity in addition to temperature reheat is calculated such as to maintain required SHF based on inside design conditions. However, during winter we consider inside design conditions to be around 18°C instead of 24°C in order to have lower heating loads.

Thanks

Vijay Kumar Vishwakarma
HVAC Engineer