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Comment by twic | Hacker News Reader

Heat pumps are obviously great in many ways. But fitting one is a big deal - they are quite expensive, they take a lot of work to fit, really require extensive additional insulation, and there are all sorts of caveats to them that mean they aren't a straightforward replacement for a boiler.

So i'm slightly mystified that we basically don't hear anything about fitting drainwater heat recovery [1], in which the lukewarm drain water from your shower is used to pre-warm the incoming cold water. It's extremely simple, pretty cheap, simple to fit, and can recover ~50% of the waste heat, of something which is tens of percent of the energy consumption of a household.

By all means, get a heat pump. But get a heat exchanger on your shower first!

[1] https://www.energy.gov/energysaver/drain-water-heat-recovery

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> But fitting one is a big deal - they are quite expensive, they take a lot of work to fit, really require extensive additional insulation

I think this somewhat overblows the complexity of a heat pump.

They are simply Air conditioners that can be ran in reverse. One valve is the difference between a heat pump and AC. Why they are so expensive is really just price gouging because they aren't as common.

If you are in a home with forced air AC then installing a heatpump system couldn't be simpler and should be something you consider when updating your AC system. It's really just a matter of updating the compressor and maybe adding some smarts to avoid condensation. In fact, it's shocking to me that AC -> heatpump conversion kits haven't hit the market.

> So i'm slightly mystified that we basically don't hear anything about fitting drainwater heat recovery [1], in which the lukewarm drain water from your shower is used to pre-warm the incoming cold water. It's extremely simple, pretty cheap, simple to fit, and can recover ~50% of the waste heat, of something which is tens of percent of the energy consumption of a household.

Really interesting idea, but I assume you have to rip up the shower to accomplish this right? Also, I'm guessing clogged wastewater piping might be an issue right? That said, that'd be easily overcome by just running the lines side by side with maybe a simple copper connector for heat exchange.

What is mystifying to me is why governments allow air conditioners without that reversing valve to be installed. A simple change, and the more people installing a heat pump the cheaper those valves get.

I regret getting just an AC a while ago. I could have gotten a heat pump and used it for most of the heating. Even if I didn't get heat pump installed, one that could have been converted to heat pump without complete replacement would been nice.

Agree with your take. Recently helped install a heat pump at a single-family home (rental property) in the SF Bay Area. Actually, they aren't that much more expensive than AC's (mine cost about $9k for everything, but again, I did 35% of the work myself for a ducted 3 ton heat pump). And yeah, the installation is almost exactly the same as what you'd do for an AC.

And, you don't need to add insulation, but we did (to the crawlspace), costing about $2.5k

And, it's been running for about 3 months, and saved about ~$175/month in heating. wow. It'll pay for itself in about 7-8 years.

My views are coloured by the fact that i live in the UK, where almost no houses have air conditioning.

That makes sense then.

How are heat pumps typically installed? Are they integrated into the radiator/hot water system? Or is some sort of forced air system setup installed?

I'd assume a mini split setup wouldn't be too uncommon. (Probably the cheapest route for most homes in the UK).

In the US the vast majority of homes and apartments are setup with forced air heating and cooling which really simplifies transition. I'd assume the UKs setup where everyone uses heated water to heat homes would make transition more expensive and costly.

Heat pumps here heat water. In a new build with a heat pump from the start, or a big refurb, the hot water is used for underfloor heating. In a smaller refurb, or where that's not possible, it's used in radiators, although I think you typically fit special radiators and bigger pipes which can work at a lower temperature.

Some new builds have mechanical ventilation heat recovery, which is a ventilation system with a heat exchanger. But I don't hear much about air to air heat pumps.

It's very possible I'm wrong in my perceptions here!

Is the water ever chilled for cooling?

I'd assume that would cause a lot of issues if it is as chilled water would generate condensation (and the UK is nice and humid 24/7)

some air-water heat pumps _can_ run in cooling mode, but speaking to a heating engineer the other week he recommended not doing this for exactly this reason.

You're right; 80% of US homes are single-family with forced air and ducts. Most heat pumps are swap-in replacements for these systems. One central air handler is connected to ductwork and vents to distribute the heat and cooling—one outdoor unit, with refrigerant lines connecting the indoor and outdoor units.

Europe has more hydronic/radiator mono-block heat pumps, with no refrigerant work needed in the field, which makes them easier to retrofit, assuming all the other plumbing is in place. Refrigerant work requires more training due to the high pressure and the potential for the high-GWP refrigerant.

This sounds like an American viewpoint. In Australia, New Zealand and much of Europe they are pretty cheap to install, and rarely require additional insulation.

It may be that they're relatively new to your area, so the demand is outstripping supply of installers who have been doing this for 30+ years and as such are charging to that effect.

For most single family US homes (which are single story) this would likely be a nightmare to install and maintain.

Definitely a cool idea but a leak or any maintenance might quickly undercut the cost savings

Just as an FYI:

> Information obtained from the US Census Bureau’s Survey of Construction (SOC) and tabulated by NAHB, shows that the rising trend of single-story homes reversed in 2021. The share of single-story homes decreased in 2021 and the share of two or more stories homes started was greater than one story homes. This is in line with recent NAHB analysis of new single-family home size trends.

https://eyeonhousing.org/2022/07/share-of-two-or-more-storie...

Shouldn’t be a problem in any home (1 story or 2) with a basement or taller crawlspace, which is a significant fraction of U.S. homes.

Speaking of heat pumps and water heat, I installed both a heat pump and hybrid water heater to save on electricity. The water heater doesn't seem to have made much of a difference in my electrical bill, my guess is the problem is peak demand so I should have gotten a much bigger tank than before. Ambient garage temperature may also be an issue but it exhausts outside and the garage is insulated so it generally stays above 55 degrees.

Make sure you are in energy saving mode. The default for these systems is to try and do quick recovery which likely isn't what you want.

Ah. I installed one in a uninsulated garage and it was really a big savings. It’s basically a free ac for whatever space it’s installed in

While those save money, most people don't really use a lot of hot water and so the amount they can save is limited.

they also aren't very hot and wont work in northern cities, but people will install them there anyway. "boiler" - is that jargon for multiple kinds of heating units or actually just boilers?

Are you saying heat pumps won’t work in cold climates? That hasn’t been true in some time. https://www.greenbuildingadvisor.com/article/heat-pumps-for-...

Heat pumps work down to -25C. Cold climates get colder than that. Sure most cold climates are typically above that, but it only takes one night every 10 years to hit those temperatures and a heat pump alone does not work. You need a backup. Of cousre once we have a backup we can ask if it is cost effect - heat pumps get less efficient as temperatures go down - when you need the most heat, so it may be most cost effective to install a heat pump that doesn't even work to that cold (perhaps it does but is under sized?) and just use backup heat when it is cold.

One night where the outside temperature gets below -25C is not a serious problem. If you had the heat pump keeping your house at a comfortable temperature the day before, you aren't going to die of hypothermia or have pipes freeze overnight. You'll just wake up to a house that's chilly inside and takes most of the day to get back up to where you want it.

Sustained cold spells are where you need an alternative heat source.

Also electric heaters are dirt cheap - get a bunch 20$ ones

Or conveniently mount one inside your air handler!

I presume you’re talking about air source heat pumps here. Some will go down that far, but it’s not common. If installed in a location where winters will routinely see temperatures below -10/15 degrees C, it’s going to be far more efficient to do a ground source heat pump, ideally with a deep bore hole where you can get a heat source that is continually 10-20 degrees C

What you’re talking about is factored in to system design - it’s called the 1% design temperature.

https://www.greenbuildingadvisor.com/article/design-temperat...

As CP says, you design your system for the conditions 99% of the time, because designing for that last 1% leads to a terrible cost benefit calculation. Same reason you don’t daily drive an armored personnel carrier with a 300-gallon gas tank.

I live in one of these areas and I just use a couple electric space heaters a few days a year - my mini splits are still great otherwise.

By boiler i mean the big box on the wall in the kitchen which burns natural gas to heat water. This may be a UK specific usage.

I think this is known as a tankless water heater in the US. (Tankless because you said it's on the wall.)

Boilers in the UK are usually mounted on the inside of an exterior wall to provide for venting exhaust gases outside. They may provide hot water on-demand without a tank (combi boiler) or fill a hot water cylinder on a schedule (system boiler) depending on type. Heating is typically provided by the boiler pumping hot water through a circuit of wall-mounted radiators regulated by TRVs (Thermostatic Radiator Valves).

Boilers are definitely much more common in the UK than US. I believe New England is one of the few areas of the US that has a relatively high percentage of homes with hydronic+boiler heating setups. In most other parts of the U.S., many people probably have no idea what a boiler even is.

Pretty common in the Northwest part of the country as well.

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