Discuss ? gas/air powered heatpump

[scottishjohn]

interesting alternative to ASHP

 they claim its cheaper to run than electric only + it can provide real hot water at 65-70c with no penalty .

maybe good choice for old  big house where you need rads as well as UFH and lots of bath rooms 

 

http://www.roburheatpumps.co.uk/products/heating-only/robur-k18-mini-air-source/

 

http://www.roburheatpumps.co.uk/wp-content/uploads/2019/01/Robur-K18-brochure.pdf

 whats your thoughts guys ?

  

[ProDave]

That link is short on details, so I did some googling.

 

I assume it works on the absorption principle like a gas powered fridge?  A bit about that here https://www.energy.gov/energysaver/heat-pump-systems/absorption-heat-pumps

 

If so I am surprised it is only 38% more efficient than an electric heat pump.  Given that gas is about 1/4 the cost of elctricity, I would have expected an astoundingly low cost per KWh of heat from a gas heat pump?

 

Of course it has all the gas disadvantages of another standing charge to pay and the need for a gas safe installer.

 

[scottishjohn]

you need to change the costs to uk prices and it could end up better 

 but 38% better than ASHp --your getting greedy now .LOl

and you can have real hot water .Idid see inoen of thier downloads you can run the ufh water down to 20c -- that would give some more savings proably in a well insulated house maybe

[PeterW]

1 hour ago, ProDave said:

That link is short on details, so I did some googling.

 

I assume it works on the absorption principle like a gas powered fridge?  A bit about that here https://www.energy.gov/energysaver/heat-pump-systems/absorption-heat-pumps

 

If so I am surprised it is only 38% more efficient than an electric heat pump.  Given that gas is about 1/4 the cost of elctricity, I would have expected an astoundingly low cost per KWh of heat from a gas heat pump?

 

Of course it has all the gas disadvantages of another standing charge to pay and the need for a gas safe installer.

 

Depends which one they are talking about as they have two models, one is a gas booster on the end of a standard air source heat pump. Uses the heat pump at max CoP (so A7/W35) then uses a small modulating gas boiler to pull the temperature up from W35 to W65. It can use the low return temp to manage the condensing on the gas so it’s basically maximizing both energy sources. 
 

The gas absorption ones are ok but they need to be big - think 18kW is the smallest they do. 
 

 

[ProDave]

4 minutes ago, PeterW said:

Depends which one they are talking about as they have two models, one is a gas booster on the end of a standard air source heat pump. Uses the heat pump at max CoP (so A7/W35) then uses a small modulating gas boiler to pull the temperature up from W35 to W65. It can use the low return temp to manage the condensing on the gas so it’s basically maximizing both energy sources. 
 

I can't figure how that would work.  Typically when heating DHW I observe a flow temperature of about 55 degrees and a return temperature of about 47 degrees.  Under that scenario the HP would be doing nothing and it would just be a gas boiler heating my DHW.   

 

On that basis the HP would only operate when heating the UFH, and if that is the case surely you might argue to have 2 separate systems for UFH and DHW?

 

What I am saying is these systems that claim to heat first by the HP then boost it with another heat source require a massive delta T that never happens in practice.

[Ed Davies]

1 hour ago, ProDave said:

If so I am surprised it is only 38% more efficient than an electric heat pump.

 

Do they say that? I can see where they say it's 38% more efficient than a condensing boiler.

 

(BTW, it''s “kWh”, not “KWh”).

Edited October 27, 2019 by Ed Davies
Add link for kWh.

[Ed Davies]

2 minutes ago, ProDave said:

On that basis the HP would only operate when heating the UFH, and if that is the case surely you might argue to have 2 separate systems for UFH and DHW?

 

Agreed, I've been puzzled by such things in the past. The only way I can see it working is if you run two systems in series so the flow from the HP goes first to the DHW tank then to another cooler heat user before returning to the HP. The cooler heat user could be a DHW pre-heat tank or UFH.

[joe90]

1 hour ago, scottishjohn said:

and you can have real hot water

 

Not sure what you mean, @ProDave and I run our DHW at 48’ and we don’t need it hotter and tank losses are minimal. (Accepted that DHW tank may need to be slightly bigger).

[ProDave]

6 minutes ago, joe90 said:

 

Not sure what you mean, @ProDave and I run our DHW at 48’ and we don’t need it hotter and tank losses are minimal. (Accepted that DHW tank may need to be slightly bigger).

Indeed.  I arrived at 48 degrees as that is the hottest I can (just about) hold my hands under for any length of time.  I see no point in having it hotter than that.  If I need really hot water for some specific and unusual task I will boil the kettle.

[PeterW]

So “real hot water” has a maximum temperature of 47c as set by building regulations (in Scotland) via a TMV2 mixing valve so any hotter is pointless ... 

 

I store at 62c and then accept the losses but also use a lot of hot water. 

[Onoff]

To have one of these I'd first need gas.

[scottishjohn]

3 hours ago, PeterW said:

So “real hot water” has a maximum temperature of 47c as set by building regulations (in Scotland) via a TMV2 mixing valve so any hotter is pointless ... 

 

I store at 62c and then accept the losses but also use a lot of hot water. 

as in if you need to run raditoars as well on big old house  and or large volumes of dhw 

your ashp is not good at getting hig temps for DHW 

[PeterW]

So an ASHP will get to 48c on DHW without a problem. It will also get to 56c if you let them a keep going but the CoP will drop toward 1 the higher the temperature. 
 

And if you have a big old house, an ASHP isn’t the ideal solution anyway ... 

[SteamyTea]

13 minutes ago, PeterW said:

And if you have a big old house, an ASHP isn’t the ideal solution anyway

Can you explain why not?

1 hour ago, scottishjohn said:

your ashp is not good at getting hig temps for DHW 

It may loose the CoP as temperature rises, but if you have enough storage at the lower temp an ASHP can easily deliver at, where is the problem.

 

Edited October 27, 2019 by SteamyTea

[ProDave]

1 minute ago, SteamyTea said:

Can you explain why not?

 

Heating my modern well insulated house with low temperature UFH works very well.

 

Thinking back to a previous 1930's house I owned, and how much heat it needed, and how quickly it cooled down, I very much doubt you could get enough heat out from UFH to keep it warm in the middle of winter, so it needs radiators to pump enough heat into it, which a heat pump is not really suitable for.

 

I am glad not to be pumping that much heat into a leaky box any more.

[SteamyTea]

9 minutes ago, ProDave said:

Thinking back to a previous 1930's house I owned, and how much heat it needed, and how quickly it cooled down, I very much doubt you could get enough heat out from UFH to keep it warm in the middle of winter, so it needs radiators to pump enough heat into it, which a heat pump is not really suitable for.

If he heat pump and the emitters (radiators, UFH, forced air or whatever) are size correctly, there will not be a problem.

I think the problem is, that as a nation, we are used to the idea of a gas boiler.

There is nothing inherently wrong with a gas boiler, they are cheap to buy and install, are small, can throw out a lot of heat, but also throw out a lot of pollution.

There is nothing to stop a house having UFH and radiators, even in the same room, if that is what is needed (what is wanted is another thing).

[PeterW]

The issue is how quick an ASHP can get heat into the water - even an 18kW will struggle as it’s about a small volume of water getting the heat so the response time is slow. With a gas boiler, you’re getting a rapid heat up which is what is expected with rads. 

[Ed Davies]

1 hour ago, PeterW said:

With a gas boiler, you’re getting a rapid heat up which is what is expected with rads. 

 

Maybe it's that expectation that's broken. The house I'm in now is not large (haven't measured but a bit less than 80m² I guess) but it's not particularly well insulated and not airtight but still the 18 kW oil boiler cycles and short-cycles madly however cold the weather is. Last December (which included a week when I ran the heating continuously) the boiler only ran 25% of the time so the equivalent of a 4.5 kW full-time boiler. The average flow temperature to my study radiator over the whole month was 30.37 °C.

 

Running, say, a 6 or 8 kW continuously ASHP with some modulation would give an equivalent effect. On the other hand, if the house did cool down for any reason (e.g., if you were away over Christmas (I wasn't)) then heating up again afterwards would be painfully slow. So an ASHP might be suitable for such a pile if it's more-or-less continuously occupied - kids, people working at home, retired, sick, whatever.

Edited October 27, 2019 by Ed Davies

[Nick Thomas]

Bit of thread necromancy, but I was researching these a bit today. Strictly hypothetically, something like https://www.remeha.co.uk/products/heat-pumps/gas-absorption-heat-pump#product3 could be interesting as a district heating solution for a small group of houses, or a block of flats. With a SCoP of 4.28, you're looking at ~2.3p/kWh of heat output if you fire it from natural gas at current prices. Worth noting that in principle you could fire one of these from anything, not just natgas - fuel oil, wood, solar thermal, they're all options.

 

It's very loud and emits NOx in operation, but still, interesting.

 

They could also be useful for heating municipal swimming pools, etc? Anyone aware of existing deployments?