Heat pumps cast iron radiators.
I have a 100 year old old bungalow with cast iron radiators and cast iron boiler. Always loved cast iron radiators and the gentle radiant heat the produce. I just think there’s no better heat in a cold climate in a older not so tight or heavily insulated home.
My question here is with the push to go all electric and the push for heat pumps, what will happen for those of us who want to keep our radiator heating systems? Are we going to be forced to scrap them because there is no future for this type of heat? Certainly hope not I don’t want to part with them.
I’ve heard mention of hot water heat pumps but don’t know if that’s a real thing or not. I can’t see heat pumps generating enough hot water to heat cast iron radiators but I could be wrong. Also, I see them as being ridiclously expensive.
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Air to water heat pumps will cover them. But - radiators are extremely unpopular in the US, so housing churn might eliminate them anyway.
At 130 degrees it will just take longer to heat them. I’ve heard people have great experience with cast iron baseboard using low temperatures with a mod con boiler so it would be very similar.
Caveat is you probably would need to add a lot more radiators to equal the heating when it was at 180 or even worse if it was steam
I am working through this same issue. I want to keep my radiators. I am getting the typical non listening mechanical contractors when I explain I will have electric "boiler" hot water system. They just shut down their listening assuming that all radiators must be run at 180 degrees. But when a old home is upgraded with insulation and air sealing it is possible to just run the radiators at 105 to 110 degrees because the load is greatly reduced. I have done this on a home before and it worked. The only difference was the boiler was run on gas versus electric. The need on that home for hot water quantity was much higher.
All old radiator houses were designed with no insulation and single pane glass. The modernizations from the 70s significantly changed the output required.
Do some heat load calcs with R1 windows and R4 walls[1920], then R2 windows and R11 walls[1980], then R20 walls and R4 windows, and you will see how the BTU needed drops, and thus the max water temp drops.
My own house has baseboards and was designed with a lot of single pane glass. Since we have effectively tripled the insulation of the house my boiler is set at a max temp of 165 and can be circulating 110 degree water in the shoulder seasons.
Do you think that if someone has existing single pipe steam heat and really tightens up the house and pumps up the insulation, there would be some type of problem with maintaining a steady temperature.
The output of an emitter like a radiator is determined entirely by its surface temperature, specifically the difference between the surface temperature and the environment. For a radiator, the surface temperature is going to be close to the temperature of the water.
Boilers usually had an aquastat set to 180F, with a 20F swing. So the burner turned on when the water got to 160F or below, and turned off at 180F or below. The average temperature f the water supplied is assumed to be 170F. Usually old systems were designed for a 20F drop in water temperature, so the water would leave the boiler at 170F and return at 150F, so average system water temperature was assumed to be 160F.
Heat pumps are a little different, the one I'm most familiar with has a thermostat setting of 113F (45C), but it modulates the compressor to try and maintain that temperature as a return water temperature and keep the delivery temperature about 10F higher. So average system water temperature is about 118F.
So if your house is at 70F, with an old-style boiler you have 90F of temperature difference, and with a heat pump 48F. So basically you get around double the heat output with the old boiler. But those radiators might work anyway. As others have noted, you might be able to tighten up your house. More important, it's likely those old radiators were bigger than they needed to be, systems weren't really designed back then and it was safer to go bigger with radiators.
There were many different ways of plumbing radiators, and the way the radiators is plumbed may not be compatible with a heat pump. Also, there's just a question of magnitude. A small boiler might have been rated for 100,000 BTU/hour. A medium sized heat pump might put out 20000 BTU/hr at design temperature. If you have a big old leaky house with a big old boiler, it might be infeasible to install heat pumps, you'd need so many of them, even though it might be theoretically possible.
Thanks for the info. The house has storm windows but no insulation. The walls are lath and plaster over furring strips mounted to brick walls about 10" thick. The plaster is in good shape and the house is full of beautiful oak trim and built ins that I don't want to touch.
The radiators are huge, all placed neatly under windows. It's a two pipe system, all original gravity piping converted to forced circulation with a newer cast iron natural draft gas boiler with 140,000 BTU input . It heats the house well but it does seem to cycle on and off a lot. The aquastat is set to 180 deg. I'm wondering with the short run times if that would mean temps don't need to be set that high.
First step is to figure out what your heat loss is. Since you have fuel use data for the past heating season, it is pretty straight forward. Follow the steps here:
https://www.greenbuildingadvisor.com/article/replacing-a-furnace-or-boiler
Once you have that you have to measure your radiators and see how much output they have. Since you won't find this printed on the unit, you'll have to do an estimate. You can use manufacturer data like this and find the closest match to your units:
https://www.expressradiant.ca/pdfs/product_classic_sizing_how_to.pdf
If your rads can deliver the heat required from the first calculation with a mean water temperature of 120F, you can swap out your boiler for a heat pump. Most air to water heat pumps loose capacity in colder temperatures, so make sure you check output at your outside design temperature.
As for turning your existing boiler down, you can do that if you have condensing boiler. If it is a modcon, most have outdoor rest function, setting that up properly will probably save 5%-8% fuel. If it is a non-condensing unit, you need the return water temperature of at least 140F. In that case you can turn it down a bit, but not too much.