Short cycling

[pyropaul]

Originally posted by thorton

Originally posted by jeffshvac
i wonder if a load calc was ever done 45,000 btu furnace with a nearly 2000 sq ft home?sounds a little undersized.

Certainly does sound undersized. A furnace that small in a 2000 sq ft home should be running almost all the time, right?

Thorton

Then how can it be undersized? If it is undersized, it will not be able to meet the demand at the design conditions, will run continously and not get the temperature to what the theromstat is asking for. If it is oversized, then it will short cycle.

For what it's worth, I have a 1950 square foot house in a much colder climate (average 7560 degree days per year). The house is old (108 years) and not too well insulated by today's standards. At the design temperature of 14F outside, 69F inside, the heatpump runs continuously and supplies about 40kbtu/h. I'm always astounded at the seemingly massive furnaces people talk about installing here - it's almost as though there's some macho pride in saying "yeah, I have a 185kbtu/h furnace". All that says to me is either their house has crappy insulation and/or they've wasted their money on equipment which is completely mis-matched to the task at hand.

Paul.

[Advanced Response]

Jack,

Check to see if the location of the thermostat is too close to a supply registar or if the hole for the t-stat wire is not sealed behind the t-stat...

Good luck
J

[thorton]

Originally posted by pyropaul

Originally posted by thorton

Originally posted by jeffshvac
i wonder if a load calc was ever done 45,000 btu furnace with a nearly 2000 sq ft home?sounds a little undersized.

Certainly does sound undersized. A furnace that small in a 2000 sq ft home should be running almost all the time, right?

Thorton

Then how can it be undersized? If it is undersized, it will not be able to meet the demand at the design conditions, will run continously and not get the temperature to what the theromstat is asking for. If it is oversized, then it will short cycle.

For what it's worth, I have a 1950 square foot house in a much colder climate (average 7560 degree days per year). The house is old (108 years) and not too well insulated by today's standards. At the design temperature of 14F outside, 69F inside, the heatpump runs continuously and supplies about 40kbtu/h. I'm always astounded at the seemingly massive furnaces people talk about installing here - it's almost as though there's some macho pride in saying "yeah, I have a 185kbtu/h furnace". All that says to me is either their house has crappy insulation and/or they've wasted their money on equipment which is completely mis-matched to the task at hand.

Paul.

Obviously, something else is wrong if a furnace this size short cycles in a house this size!!!

Thorton

[mayguy]

On pretty much on all Honywell I set it at 3 (High Eff.) CPH.

Sounds like it's on 5 cph right now.

Mine some how at one time got changed to that, and it did the same thing as you were saying. it was at 5.

If it was undersized, it would be running non stop and not keeping up to temp.

[jack david]

I appreciate all your posts, and found the advice to be useful as this isn't the end. The homeowner is going to follow it through. I really need to know how the CPH is set up in this particular t/stat: T8112D 1005. Honeywell 5+2 programmable.

[Wild Leg]

Originally posted by jack david
I appreciate all your posts, and found the advice to be useful as this isn't the end. The homeowner is going to follow it through. I really need to know how the CPH is set up in this particular t/stat: T8112D 1005. Honeywell 5+2 programmable.

I hope this helps:

http://customer.honeywell.com/Techli...0s/69-0917.pdf

[jack david]

I would like to thank all of you again for your time, and bawl2, that was the iceing on the cake. Thanks.

[Wild Leg]

Originally posted by jack david
I would like to thank all of you again for your time, and bawl2, that was the iceing on the cake. Thanks.

Sometimes, even a blind hog finds an acorn.

You're welcome!

[odcman]

For what it's worth, I have a 1950 square foot house in a much colder climate (average 7560 degree days per year). The house is old (108 years) and not too well insulated by today's standards. At the design temperature of 14F outside, 69F inside, the heatpump runs continuously and supplies about 40kbtu/h. I'm always astounded at the seemingly massive furnaces people talk about installing here - it's almost as though there's some macho pride in saying "yeah, I have a 185kbtu/h furnace". All that says to me is either their house has crappy insulation and/or they've wasted their money on equipment which is completely mis-matched to the task at hand.

Paul. [/B][/QUOTE]

Totally agree ! How can anyone make that determination without seeing the house ? Especially when short cycling is the problem, not running flat out ? I have a 4500 sq. ft. house with a 59000 Gross output Buderus gas boiler. We have 3750 degree days where I live and on a 14F design day the heat loss (measured by boiler run time) is 21000 BTU. My boiler is way oversized for space heating, but when you add a 40 gallon buffer tank, it runs 20 mins every 60 mins on design day. That's 1 cph. Sounds like some people here would like to put a 185kbtu furnace into our house just based on square footage, but ignoring the fact that it's about 60% more effcient than an R2000 house.
Heat load calcs are really easy to make in an existing house. Just clock the boiler over 1 hour and if you know it's gross output, you know what your heat loss is.