Air to Water Heat Pumps

"Heat Geeks don't normalize or adjust their SCOP calculations."

What I mean by "normalization" is they adjust for climate. The colder out it is, the worse a heat pump is going to perform, that's just their nature. SCOP takes that out of the equation by adjusting for temperature. A heat pump that is running well in a cold climate will score a better SCOP than one running poorly in a warm climate, even if the measured COP is worse.

"In America we don't monitor anything." This I agree with. I think it's crazy that we make installers do Manual J, but there's never any follow up to see how actual energy usage compares to modeled usage.

"I always thought radiant surfaces provided the lowest SWT requirements? Massive amounts of surface area in floors, walls and ceilings. Radiant slabs can work with 95 SWT, what other type of emitters are an option. Jaga forced convectors possibly?"

I'm running forced convectors with my AWHP, along with heated floors and ceilings. But any emitter is going to work with low water temperatures, it just needs to be bigger. Heat output is going to be directly proportional to the difference between SWT and room temperature. Traditional radiators are rated for an aquastat at 180F, which means average water temperature of 170F (20F swing), or 100F above room temperature. If the SWT is at 95F that's 25F above room temperature, you need four times as much radiation. At 120F it's 50F above room temperature, you need twice as much radiation.

A lot of older houses are over-radiated, you might be able to get 120F to work.

"Does a 60C dhw function drive down the SCOP number? It seems that temperature is mandated by legionella codes, across the pond?"

DHW directly from a AWHP is a poor fit. Yeah, you can make it work, but it's not playing to the strengths of the technology. You save on operating costs, installation cost, and complexity and reliability by just using and AWHP. I set mine at 120F but I have chlorinated city water.

One minor comment from an unreconstructed American… with cousins in Scotland.

Quite right. Everyone is buying heat pumps. Some air to water, some air to air. And, at least where my cousins live, they are freezing.

So why do they all buy heat pumps? Because Ed Milliband and his merry crew are forcing them to. They don't have a choice. So far, at least, we haven't gotten to that point on this side of the pond.

"They are forcing you to install HPs here in some parts of America to now."

If you're going to make a claim like that you better have the citations to back it up. Where? Who?

so the UK heat pumps are mainly coal fired?

But they are making progress to get off fossil fuels.Good for them to do the HP pioneering.

The UK is the largest boiler market in the world, also.

Hardly. As was said, the last coal fired plant in the UK was shut down some years ago. There are a few gas fired plants still running. The UK actually has a good deal of renewable power. There are two minor problems, of course — most of it is in Scotland, and the climate isn't exactly favourable at times. The first problem is being attacked by building — or trying to build — a raft of major new power transmission lines, many of which — since they go through things like parks and some very scenic areas — are not wildly popular. The second problem is solved, at great price, by importing power, mostly from France which has a lot of nuclear power (over 70%) and a good eye for business, or from Scandinavia, which has a good deal of hydropower — and an equally good eye for business (I've seen peak rates in the US $10.00 per kWhour range, though $1.00 per kWhour is more normal)..

The problems which the idea are encountering are really two. A good deal of the housing stock is remarkably poorly suited for heat pumps. This is true both in the inner cities and in rural areas — though not in suburban areas in the south of England. The other is that they are simply too expensive, both to install and to run, for all but the reasonably well-to-do.

Mind you, I'm not opposed to heat pumps on any ideological ground — but like any other technology, there is a time and place where they are right, and a time and place where they aren't.

@hot_rod: "I’m all for the A2WHP movement, with caution about over promising, under delivering on performance, cost and ROI."

A big part of being realistic is doing the kind of analysis I did in my post on the first page of this thread from January 16 (I don't know how to make a link to a post).

In your area there is a break-even COP for a heat pump. It has nothing to do with the house or the heat pump, it has to do with the price of electricity and the price of fuel. Once you have the break-even COP, whether a heat pump can achieve it depends upon the climate and the choice of pump.

If you're not saving money, or even increasing the fuel bill, the customer isn't going to be happy. Full stop.

Similarly, if the device isn't sized to meet the entire heating load, the customer isn't going to be happy. Full stop.

I'm going to go out on a limb and suggest two other absolutes:

1. Getting DHW directly from the heat pump is never going to be a good deal for the customer. An indoor HPWH is always going to be better.
2. Any sort of dual fuel setup is never going to be a good deal for the customer. If you can't make a heat pump work as the sole heat source, you can't make a heat pump work.

If you look at the analysis I did, there's just no savings in trying to go dual fuel. But it basically doubles the installed cost of the system and makes it enormously more complicated. Boston is about as close as you can get to break-even between the cost of electricity and oil. That means that if oil were any cheaper, the heat pump wouldn't make any sense at all. Similarly, if electricity were any cheaper there's zero advantage to dual fuel.

@John Ruhnke : "The data you have provided me is assuming you have an American designer installing a HP rated at 2.7. Assuming he is making mistakes, which I am sure he is. The real SCOP is lower. It isn't actually 2.7 when installed in the field."

To be clear, that's not at all what I was saying. I used the data sheets for the 5-ton Arctic that you provided. I said nothing about SCOP. Rather, I estimated the average COP for the year, in Boston. That's not SCOP.

SCOP is kind of misnamed at "seasonal" COP, the S really should be for "standardized." This document gives an overview of how SCOP is calculated: https://www.chiltrix.com/documents/App-B-Teknical-SCOP-NEF%20-EN14825.pdf

But basically you take your observed COP measurements for different temperatures over whatever time interval you're using, and you weight them to reflect a standardized distribution of temperatures in an "Average" climate. The SCOP uses the climate in Strasbourg, France, as it's benchmark, the average winter temperature there is several degrees above freezing.

Now, my estimates are the best I could do. I don't have access to the part-load efficiencies for the Arctic heat pump, as I note above, part-load efficiencies tend to be substantially higher than full-load. If I had them, I would use them.

In economics, there's a saying, "all models are wrong, some are useful." I can't claim my model is 100% correct, it's based upon assumptions. But I believe it is useful.

I guess it's in another thread. Too many of them… I get tangled.

But I just did the analysis on dear old Cedric. He's running just under 86%, measured by comparing actual fuel firing rate to actual pounds of steam output. The only really meaningful measurement. (and a big thank you to @Charlie from wmass for getting him set so well!)

I'm dreaming based on twenty years of records… which are, I assure you, meticulous. I don't play games with my numbers, friend. Sorry. You measure power output — no big deal — and power input —also no big deal — and divide. What is your definition of system efficiency?

Pounds of steam — or condensate — per hour.

@John Ruhnke I'd love to check out your Euro Boiler Study sources, but I can't find a link to the full study.

However, I have checked out and can provide the Brookhaven National Lab study that measured useful heat output of cast iron boilers and forms the basis for the AFUE rating. And contrary to myth, the Brookhaven AFUE methodology does factor in standby losses. But, the AFUE number does ultimately depend on operating conditions. And unfortunately, what happens is that the boiler mfr cherry-picks unrealistic operating conditions so that his AFUE ratings come out as high as possible. That's the real problem. It's like letting car mfrs advertise only highway mpg ratings when the customer is only going to be doing city driving.

Brookhaven's analysis used cast iron boilers supplying a range of heating loads while maintaining a set internal water temperature. (This mimics the conditions for DHW production.) Over a range of conditions, they measured boiler overall efficiency (useful heat output divided by total BTU input) ranging anywhere from about 5% to 70+% (steady state efficiency) depending on operating conditions. If you have almost no demand while the boiler maintains water temperature, most of the oil you burn gets wasted as jacket and flue loss, and you get efficiency of 10% or less. OTOH, if you have endless high demand, the burner runs constantly at steady state, and you get a steady state efficiency which is close to combustion efficiency (which is your limiting factor) at 70+%.

So you can make a cast iron boiler run anywhere from 5% to 70+% overall efficiency , and it all depends on whether you have a DHW coil maintaining constant water temp, and the % demand on the boiler while it maintains that water temp. That's illustrated in Figure 2, pg 4 of the study linked below.

We do not have a DHW coil, and therefore we do not have a boiler sitting around trying to maintain water temperature while there is no demand. As a result, we have no standby losses while there is no demand, which is much of the time because our oversized boilers run 25% or lower duty cycle even in cold weather.

If you look at page 13 of the report, Brookhaven measured a seasonal efficiency of around 64%-70% for dry base boilers burning about 1200 gallons per year in New York, which happens to be almost exactly our seasonal consumption, in the same climate.

https://www.osti.gov/servlets/purl/5263192

So I'm quite willing to agree that our boilers are probably somewhere in the 64%-70% overall efficiency range. As I've said, our combustion efficiency alone knocks us down to an upper limit of 75%, and our thermal losses could be 5%-10% even though we have cold start and cold finish after post-purge.

But again, look at Figure 2, pg 4. You can make a cast iron non-condensing oil boiler run anywhere from 5% to 70+% overall efficiency, depending on the operating conditions. This is why making sweeping statements tarring all CI boilers as 40% efficient is not helpful.

@John Ruhnke : "I also think the way of measuring COP with a electric meter before and BTU meter after is producing different SCOP numbers that are more accurate than what is coming from that "App-B-Teknical-SCOP-NEF" doc you are referencing from. The U.K. climate is most likely warmer than Boston so SCOP measured in the same way in a colder climate would have a SCOP that is less."

SCOP is a climate-independent measurement of performance. Observed COP in Boston is going to be lower than in the UK, the SCOP adjusts for those differences, the same equipment in both places should give the same SCOP. That's the whole point of it. So when you read those HeatGeeks bragging about their 4.5 SCOP's you can't compare it to a 2.8 average annual COP measured in Boston unless you do the same adjustment. SCOP is not the same thing as annual average COP.

The article doesn't present a different way of calculating SCOP, it presents the way of calculating SCOP.

This does raise a very important point, which is that modeling heat pump performance is more complicated than modeling boiler performance. With a boiler, a BTU is a BTU pretty much, just take the number of heating degree-days and you can estimate gas or oil usage pretty accurately. Or take fuel usage and heating degree-days and you can estimate boiler output.

With a heat pump, COP varies with the outside temperature. It also varies with the load, with higher efficiencies at lower loads. How much it varies depends upon the particular piece of equipment. So to estimate the instantaneous efficiency at any given moment you need to know the outside temperature, the performance curve for the equipment you have, and the heating load of the building. And to estimate the annual average COP you need to know the average temperature distribution for where you are. Or put briefly, heat pump performance depends upon the location, the building and the equipment. And it is something that is spectacularly poorly suited to estimating using rules of thumb.

That's why I started making those spreadsheets.

Chapter 3 in Modern Hydronic Heating and Cooling 4th edition talks about boiler efficiencies. The formulas Siegenthaler put together:

Efficiency as desired output quantity ÷ necessary input quantity

Steady State` Efficiency as heat output rate ÷ energy input

Cycle Efficiency as total heat output over a period of time ÷ energy content of the fuel consumed over that period of time

Run Fraction Burner on time ÷ total elapsed time

When all these numbers are crunched using the Brookhaven graph, you get the actual operating efficiency, installed on the job. Which is really what you want to know at the end of the day, not just the number from the manufacturers slick.

Here is my minimum wage, non-degreed, non-certified, attempt of running those numbers:)

Graph used with permission from the author

I would think the heat pump should be looked at with the run cycle and cycle efficiency applied. Buffer tanks probably help those numbers?