Plan for heating in 1910 home renovation
Hi All,
First a note that I’m not a builder or mechanical expert – just a homeowner who spends a lot of time reading all of the wonderful expertise that is shared on this forum, so apologies in advance if I’m not providing enough information or using the wrong words.
This summer we’ll begin renovations on a ~5000 sq. ft. home in zone 5 (Pittsburgh, PA). We’ll be working top down, starting with new roofing and roof insulation to either R-49 or R-60 (roof is directly over a finished 3rd floor) and a gut renovation of the 3rd floor (only), which will include new windows and as much insulation and air sealing as is practical in a 1910 home. Similar work will follow on the other floors, though over the course of many years.
Our dilemma is what to do with HVAC – heating in particular. Currently there is no AC on the 2nd or 3rd floors, and heat is provided by cast iron radiators (hot water) that look to be original. We’ll be installing AC on the 3rd floor, and could probably do a standard heat pump and new duct work to handle both heating and AC and call it a day. But, we’d really prefer to keep a radiant/convective heat source. Preliminary estimates for radiant floor heating seemed cost prohibitive, so this probably means doing something with the existing radiators (refurb or replace). This is complicated by the fact that we would like to go all electric in the future, but we JUST replaced the gas boiler last winter (with a Lochinvar KHB285N) and planning on keeping that in service for it’s useful life (15-20 years?).
I’ve read a good bit on this site about technologies like air to water heat pumps, and that seems like that might be a good option in the future when we need to replace the current boiler. But I also understand they’re currently limited to producing ~120 degree F water compared to the ~180 degree F water of the current boiler.
So my question is basically asking one to predict the future… If we were to proceed with our renovations and refurbish/replace the existing cast iron radiators as we go (which are probably already oversized, even more so when we consider the additional insulation and air sealing we’ll be doing), might it be reasonable to hope that in ~15-20 years air to water heat pumps will have progressed to the point that they can produce higher temp water, OR that running lower temp water through the radiators could produce the necessary heat in the renovated spaces? If not, do we have any other options to maintain radiant/convective heat as we renovate?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I'd first install ductwork for the other floor(s). There's no real downside to installing a ducted heat pump - you get AC, which you already planned on, and a second heat source for a marginal cost. You could stop right there - the ducted heat pump would likely be able to cover all or most of your heating. After a winter with a well installed and spec'ed heat pump, you could be happy with the heat pump and stop there. I've lived with radiators and a cold climate heat pump, and the hydronic system was not more comfortable in my experience.
If not, your options are keeping the cast iron and hoping it's oversized enough to use lower temps, or proactively install larger radiators - this could be cast iron or could be panel radiators, which are more compact. The ductwork could also function as a supplemental heat source. The issue isn't that air-to-water heat pumps can't reach high temps, they can, but the efficiency is poor. There's no expectation that they'll ever progress on that front.
Thanks, Paul. I would have thought the same about hydronic vs. forced air, but having lived in places with forced air heat all my life, moving here and experiencing radiators for the first time was a bit of a comfort revelation, even with the house itself having original windows and being fairly leaky. That aside, I really like the idea of having both available, and it seems like we have a good path to get there since we'll be installing AC anyway.
Have you done a Manual J? In zone 5, it wouldn't surprise me if your cooling load is half your heating load. So you size the heat pump for your cooling load, it will be way undersized for heating but you use the radiators for heat. Initially you just use your boiler, which presumably is adequately sized. (It may be oversized after you improve the insulation and air sealing as part of your renovation, but that's OK).
That kicks the can down the road until the new AC and new boiler are at the ends of their useful lives. At that point, we may have CO2 air-to-water heat pumps that deliver 170F water, and you just drop one in as a replacement for your boiler. In the worst case the available air-to-water heat pumps are no better than what you can get today, but with your radiators running at 120F and the heat pump from the HVAC you can get through the coldest days. On milder days the radiators are fine.
The real question I see is whether today you go with conventional, mini-split or ducted mini-split.
And if down the road you decide you want to use more electric and less gas, you could switch it up and use the heat pump when it's mild and only turn on the gas when the heat pump can't keep up.
Thanks for the thoughts. We're going to engage a mechanical engineer soon, which I assume will include Manual J calculations. Your intuition about heating vs. cooling load sounds right based on a year and a half or so of living here.
Re: conventional vs. ducted mini-split, I haven't done my homework there. I associate ductless mini-splits with wall-mounted units, which we'd prefer to avoid for aesthetic reasons. Ducted is an interesting option, though. I'll have to read up on that option.