Michaelm

Ashp without a tank

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Hi advice forum. 

 

I'm going to replace our combi boiler with an ashp next year. However, there is no room for a tank (nor do I want one)

 

Is it not possible to install the ashp for heating only, and for the hot water we use an under sink pump with an electric heater or digital shower?

 

And 

 

Is the reason why this isn't common due to the power required for all electrics at once, and potentially blowing the fuse?

 

Thanks for any help, learning from scratch here-

 

Mike

 

 

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1 hour ago, Michaelm said:

Hi advice forum. 

 

1. I'm going to replace our combi boiler with an ashp next year. However, there is no room for a tank (nor do I want one)

 

2. Is it not possible to install the ashp for heating only, and for the hot water we use an under sink pump with an electric heater or digital shower?

 

And 

 

3. Is the reason why this isn't common due to the power required for all electrics at once, and potentially blowing the fuse?

 

Thanks for any help, learning from scratch here-

 

Mike

 

 

Hi Mike.

 

1. This raises lots of questions as to the suitability of your property to have an ASHP and or separate hot water.  Whilst none of our business it might be good to share some details. There are so many alternatives for secondary hot water that without knowing things like what you want it to do - showers? Baths? 8 people? and so on that you could end up putting the wrong thing in.

 

 

2. Sure, you can use ASHP for heating only. 

 

3. No. 

 

Good luck

 

Marvin

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Well, the reason the combi can provide DHW without a storage tank is that it’s likely 30kW or more. Running a good shower or filling a bath will use all of that. An air source Heat pump connected to a single phase supply will be at best 15kW and the outdoor box for such a unit is physically large. It also takes some minutes to “get going” whereas the gas combi can fire in seconds.

 

A buffer tank will probably also be needed for the heating side unless there is a lot of water in the heating circuit.

Edited by J1mbo

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As above no reason you can't use an ASHP for heating only.

 

But an electric water heater will cost more to run and even that will need a small unit containing a tank, often fitted under a kitchen sink unit.  That will be fine for filling a sink of washing up water or a basin of water in the bathroom, but don't expect to fill a bath from it.  If you have no bath and are happy with an electric shower that will probably work.

 

But also as hinted above there are aother things you need to check before swapping from a combi to an ASHP, like what is the actual heating load, how well insulated is the property, how is the heat delivered i.e. radiators or UFH etc.  Failure to design it properly will likely result in disappointing results.

 

What is the reason for making the change?

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I think there is a Sunamp that can run off a heat pump.

Expensive though.

Can you fit a cylinder in the loft, or outside in an insulated box?

Or just under the stairs.

 

I have a very small house, and have a 200lt cylinder, with enough room above it for another 100 lt.

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We have Sunamp heat batteries running from a heat pump. They work well and are quite small (small enough to fit in a kitchen cupboard) but, as @SteamyTea said, they're expensive.

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Thanks all for the advice, I did wonder if there might be more to it than I initially thought.

 

So it's a 2 bed, insulated with DBL glazing. Double panel rads. 80s build. It's small though, tiny. 

 

I suppose I don't like the idea of a tank because-

 

1) it takes up space, we want to convert the loft in a few years.

2) the idea you can run out of water. All 4 of us shower in the space of 2hrs. In the evening.

3) I don't like the idea of spending energy keeping water hot for 22hrs in a day, completely unused. I only need it for 2 hrs. Out of principal this seems very inefficient.

 

There may be a very good reason no one does this, but id rather have ashp for heating and on-demand hot water for when we need it. (Ashp+electric shower+ 2x under sink heaters)

 

Cheers

 

Mike

 

 

 

 

 

 

 

 

 

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Electric shower will be 11kW best, combi shower is probably 30kW. The rate of flow with electric will be basically 1/3rd.

 

Standing losses with heat pump cylinders are about 2kWh per day for a 200l cylinder (approx 18p per day at current electricity prices with a COP of about 2.5) when stored at 60°C. If the water is lower temperature so the losses will also be lower (maybe 12p).

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2 hours ago, Michaelm said:

3) I don't like the idea of spending energy keeping water hot for 22hrs in a day, completely unused. I only need it for 2 hrs. Out of principal this seems very inefficient.

Not if you insulate it well and store the water at a relatively low temperature.

My cylinder looses about 0.5 kWh in the time it is idle (and washing up/hand washing), that is on a useage of 2 to 3 kWh.

Seems high, but as it is heated by E7, it is 35p/day, standard rate electricity would now cost around 50p/day.

Edited by SteamyTea

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2 hours ago, Michaelm said:

Thanks all for the advice, I did wonder if there might be more to it than I initially thought.

 

So it's a 2 bed, insulated with DBL glazing. Double panel rads. 80s build. It's small though, tiny. 

 

Ok, insulated how?  Loft insulation - how thick?  Cavity wall insulation? Floor insulation - how thick?  Do you have trickle vents in the windows. What is the approximate floor area (upstairs and downstairs) in square meters. What emitters (radiators or underfloor heating?) are you thinking of using?

 

A lot of people who contribute on build hub have masses of experience so its good to listen to it all.  @TW9 has one of the answers which would save you space, but its as my brother says, its not if, but how much.

 

Good luck

 

M

 

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11 hours ago, SteamyTea said:

Not if you insulate it well and store the water at a relatively low temperature.

My cylinder looses about 0.5 kWh in the time it is idle (and washing up/hand washing), that is on a useage of 2 to 3 kWh.

Seems high, but as it is heated by E7, it is 35p/day, standard rate electricity would now cost around 50p/day.

 

It's also worth noting that the savings made by using a heatpump at ~2.5COP for DHW will still be cheaper than running direct electric taps & showers at a COP of 1. 

 

Also in the winter any lost heat from the tank will also be heat gain for the house. And in summer whilst the same is still true, the COP should be much higher than 2.5 due to the warmer weather. 

Edited by Luke1

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Why do you want to change from a combi boiler to an ASHP with such stringent conditions?

 

Does the combi need replacing?

 

An ASHP is going to be a worse fit for your lifestyle and cost more to run as well as having a big ugly box outside. It's likely cost a lot to install too.

 

You are proposing a system with greater complexity for no benefit.

 

I don't see the point, perhaps you can enlighten us.

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If the combi is at the end of it's life you could replace it with another and spend the rest of the money on improving insulation and airtightness.  You could look at UFH on the ground floor.  You will need at least 150mm insulation under there to make it work.

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Mr Punter, seems to make the best suggestion.  Combi boiler £1500 is less installed.  An ASHP, will need an ASHP, £2-3000, plus larger radiators (to cope with lower circulation temperature), maybe new piping depending pipe sizes you currently have, you easily eat into £5-7000.

 

This leaves several grand to improve insulation and drafts.  Giving you low ongoing bills for heating.

 

Get a low energy house first.  ASHP good for well insulated and airtight buildings, they need a cylinder to work for DHW.  Really wouldn't want to suggest something not fit for purpose, especially as you don't want a cylinder.

 

A good combi, good showers, a good electric shower, worse than a rubbish combi.  Both cost the same to run.

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1 hour ago, JohnMo said:

ASHP good for well insulated and airtight buildings

Or lower capacity ASHPs work well in a lower needs house.

Large ones will work well in a leaky house, it is just a case of sizing for the power needs.

Don't make financial sense, but that is a separate issue and must not be confused with very basic thermodynamics.

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If you could get 3-ph electric then the Steibel Eltron 27kW ( 3x9 ) instant water heater is a very good bit of kit. Rated to fill a bath, and I’ve witnessed it running a shower and it’s very very good. 
Compare that to the same unit in single phase, and I can piss faster than it. Totally useless. 
Also, if you go for a huge electric shower, and decent instantaneous water heater, the cold mains will not be sufficient to run both at full capacity. You’ll end up using the shower on full

power only if nobody else in the dwelling so much as opens a tap to fill the kettle. They’re HUGELY cold mains dependant so if you do go for an all instantaneous solution, expect to be fitting a ~200L cold mains accumulator to support the cold mains flow rates needed for reliable operation. 
Furthermore, factor in for much poorer performance in the winter, where the cold mains water is much colder……

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20 hours ago, Michaelm said:

Thanks all for the advice, I did wonder if there might be more to it than I initially thought.

 

So it's a 2 bed, insulated with DBL glazing. Double panel rads. 80s build. It's small though, tiny. 

 

I suppose I don't like the idea of a tank because-

 

1) it takes up space, we want to convert the loft in a few years.

2) the idea you can run out of water. All 4 of us shower in the space of 2hrs. In the evening.

3) I don't like the idea of spending energy keeping water hot for 22hrs in a day, completely unused. I only need it for 2 hrs. Out of principal this seems very inefficient.

 

There may be a very good reason no one does this, but id rather have ashp for heating and on-demand hot water for when we need it. (Ashp+electric shower+ 2x under sink heaters)

 

Cheers

 

Mike

 

 

 

 

 

 

 

 

 

 

 

do some more research. your rads will need to double in size, if they are fed by microbore pipe this needs to be replaced. 

 

You WILL be paying a heck of a lot more money on your heating bill compared to gas, maybe close to treble depending on how insulated the house is.

 

Also take into account the racket the outside unit makes if your neighbours are close to you.

 

Not many pro's and a lot of cons unfortunately. You will be pushed to get unbiased facts here as you will no doubt read ....

 

 

 

 

Edited by Dave Jones

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4 minutes ago, Dave Jones said:

do some more research. your rads will need to double in size, if they are fed by microbore pipe this needs to be replaced. 


Fact Check #1 - no you won’t, it is around 30% increase in output and many rads are already oversize so do the heat loss calculation first. 
 

5 minutes ago, Dave Jones said:

You WILL be paying a heck of a lot more money on your heating bill compared to gas, maybe close to treble depending on how insulated the house is.


Fact Check #2 - for Gas at 88% efficiency and Electricity at CoP 2.5 average (which is low) then it will be around 20% more. How insulated the house is is irrelevant when comparing heat input sources. If you have a big gas bill now, an ASHP wont reduce your heat demands. 
 

7 minutes ago, Dave Jones said:

Also take into account the racket the outside unit makes if your neighbours are close to you.


Fact Check #3 - modern units are around 52dB or less. Some it is near impossible to hear them from 4-5m away. 
 

 

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1 hour ago, PeterW said:

Fact Check #3 - modern units are around 52dB or less. Some it is near impossible to hear them from 4-5m away. 

 

I can barely hear our 6 year old unit standing right next to it when it's running flat chat. I can't hear it at all when it's modulated down to do space heating.

 

1 hour ago, Dave Jones said:

You will be pushed to get unbiased facts here as you will no doubt read ....

 

Nearly every post above yours says that an ASHP at best isn't a drop-in replacement in these particular circumstances.

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3 hours ago, PeterW said:

Fact Check #2 - for Gas at 88% efficiency and Electricity at CoP 2.5 average (which is low) then it will be around 20% more.

 

That really depends on your rates. Here, using those values, the heat pump would cost 37% more to run at night rates and 92% more to run at day rates. And, of course, if you have over sized radiators the boiler should be running at a lower return temperature with efficiencies of well over 90%.

 

(I've been looking at the feasibility of a heat pump, and with a few radiator enlargements (doubling some of them) and a bit of pipe enlargement it looks possible to get a system with a cop of over 3, which is approaching acceptable running costs (a mere 25% higher than gas), which could be hobby project to waste a few thousand on to get rid of the gas connection.)

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1 hour ago, billt said:

(I've been looking at the feasibility of a heat pump, and with a few radiator enlargements (doubling some of them) and a bit of pipe enlargement it looks possible to get a system with a cop of over 3, which is approaching acceptable running costs (a mere 25% higher than gas), which could be hobby project to waste a few thousand on to get rid of the gas connection.)

 

R290 and R32 systems will deliver SCOPs in the region of 3.9 with 50°C design flow temp.

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On 02/12/2021 at 19:41, Michaelm said:

I suppose I don't like the idea of a tank because-

 

1) it takes up space, we want to convert the loft in a few years.

2) the idea you can run out of water. All 4 of us shower in the space of 2hrs. In the evening.

3) I don't like the idea of spending energy keeping water hot for 22hrs in a day, completely unused. I only need it for 2 hrs. Out of principal this seems very inefficient.

 

 

1)  A tank only takes up the space of a floor-to-ceiling cupboard and if you put it in the loft then any heat lost from the tank will be not be wasted when you convert the loft.

2)  You will not run out of water if you size the tank correctly.

3)  You really use no hot water apart from for showering?  What is also very inefficient is to heat your water using direct electrical heating when you can heat it for 1/2.5 (ish) of the electricity and cost using a heat pump.  

4)  You are likely to need a buffer tank for your central heating.  Often this is placed below the hot water tank as a single unit.  

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17 hours ago, Dave Jones said:

You will be pushed to get unbiased facts here

I think you will find most opinion here is based on real life usage (like my own).

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14 hours ago, J1mbo said:

 

R290 and R32 systems will deliver SCOPs in the region of 3.9 with 50°C design flow temp.

 

ITYM "some" systems will do that.

 

The Vaillant arotherm+ 5kw has a COP of 3.41 at 50C, the 7kW has a COP of 3.65 at 50C, those aren't in the region of 3.9, but they are over 3.

 

Yes the 12kW just about gets to 3.9, but that's one of a range of 5.

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