Cast iron hot water radiators

hotquestions

Hi, Has anyone run a hot water cast iron radiator system off an air to water heat pump? 

Ironman

An A2W heat pump will only produce about 120* water. Most standing cast iron radiators need around 160* water during the coldest weather.

Of course, you could use the A2W during milder weather and have the boiler staged to kick in during colder weather.

kcopp

It would work IF you size them to run at lower water temps. May need some good sized rads...

hotquestions

@kcopp by good sized rads you mean large or small? ☺️☺️

@Ironman by staged, you mean have it run off gas/oil during colder weather?

Our idea is to run these with a A2W heater that goes up to 160 F, supplementary to mini split heating (which we don’t like very much). So not looking to have the house be toasty but comfortable. 

psb75

Let us know what region of the country you live in. That makes a big difference for the answer to your question. Also what size, vintage and type of dwellling do you have? In general the more cast iron radiation you have--the better. A2W heat-pump will likely do fine most of the winter. Its efficiency will drop off radically during the coldest part of the winter. But it will do fine certainly in the shoulder seasons of spring and fall, and early and late winter. A2W also matches well with high-mass radiant floors.

hotquestions

@psb75 we’re in New York City. I’m thinking of the SANCO2 which apparently goes up to 150 F(though I’ve read it can also do 170 F)
https://www.smallplanetsupply.com/sanc02

dko

Sanco does not permit the use of the unit as space heating only.
150F is the max temp.
Maximum space heating permitted is 8,000 BTU.
Total 15,000 BTU with DHW.
They mandate the Taco X-block system if used for both.

hotquestions

@dko it would be DHW + space heating…

Jamie Hall

What @kcopp meant by "good sized" radiators is... BIG radiators. You need almost twice the radiation on 140 F water to heat the way 180 F water does.

Ironman

The first things that you need to do are a Manual J heat loss calculation and then an accurate radiation survey.

The Manual J will tell how much heat the structure requires on the coldest night of the year (design temperature). And the radiation survey will determine how much heat the radiators can produce at a given AVERAGE water temperature. This data is then compared to the performance data of the A2W heat pump.

The performance of a heat pump drops off as it gets colder out side. At the same time, the heat loss of the structure increases as it gets colder. The temperature at which these intersect is called the “balance point”. At any outdoor temperature below that point, the heat pump requires supplemental heat to carry the load.

A number of 160* is a best case number under ideal conditions. If you check the performance data, I’m sure you’ll find it’s no where near that at 10* ambient which is what you can expect in NYC.

And I’ve never seen a house with standing cast iron rads that had a heat loss of 8k btus. It usually at least 10 times that.

Designing a system is based on worse case scenario, not best case.

hotquestions

Thanks for all the info!