Can Heat Pumps heat Hot Water Cast Iron Radiators for Radiant Heat?

woofenme

i’m new to this form but was referred here. Yesterday got notice from our oil company when they were servicing our oil furnace that after we’ve been in our house for 24 years and the furnace was here when we bought it that it’s time to get a new furnace. So I’m looking at all sorts of options. We have hot water radiators cast-iron radiators in this old house was built in 1900 in New England. We could just get a new oil furnace or even propane gas furnace, but my mind was going, “Could you even a heat pump work to heat enough water to a needed temperature to heat water for these radiators. Currently we have two zones using radiators in our house. We have a heat pump hot water heater that works great for domestic hot water. But looking at efficiency cost Savings and effectiveness at new sources of radiant, hot water heat in these radiators that we have. Any thoughts? Thanks.

hot_rod

Maybe, especially if the home has had energy upgrades.

I've run cast radiators on 120 supply and they still give some heat.

So you need a room by room heat load. Then determine the existing radiator output at lower temperatures. See if they can meet the heatload at 120 supply water temperature.

Some ideas and options here.

Alan (California Radiant) Forbes

I’d say probably not, even in my neck of the woods unless you’re willing to layer up with sweaters when it gets cold. You will probably still need a backup boiler.

And what do you pay for a heat pump install these days? Not cheap.

bburd

Unfortunately, unless your older home has had very significant envelope upgrades like full insulation and draft sealing, the answer is: probably not.

Your existing radiators were probably sized for an average water temperature of 170°F on a design (very cold) day. An air to water heat pump can produce 120°F supply water, which means the average will be closer to 110 to 115°F. Conversion charts show the heat output of cast-iron radiators at various water temperatures; in this case you would get about 40% of the original design heat output.

You can survey your radiators using charts that show the square footage of heating surface per section for the various types and sizes. At 170° average water temperature the heat output is 150 Btuh per square foot. Then apply the correction factor for the lower water temperature, which will tell you how much heat you will get with an air to water heat pump.

Compare that to a Manual J or other heat loss calculation of the heating load for the house as it now exists. This can also be estimated from your fuel consumption over several years.