Why am I putting in an ashp in a passive house?

[CalvinHobbes]

Should it not be warm enough? Like my son lives in a passive house and they use the heating max twice a year. I keep reading of huge electric bills for ashp and that you can't turn them off. So why should I do it? Have we just been conditioned into thinking we need it?

[ProDave]

I guess it depends where you are.  Up here in the Highlands no passive house will be comfortable in winter without any heating.

 

My ASHP bills are not huge, and it is completely controlable including OFF

[joe90]

15 minutes ago, CalvinHobbes said:

keep reading of huge electric bills for ashp

Where they are not installed correctly for a passive house.

15 minutes ago, CalvinHobbes said:

and that you can't turn them off.

Bollocks.

15 minutes ago, CalvinHobbes said:

So why should I do it?

Because people here have proved they work if designed properly 🤷‍♂️(I know mine did).

 

Edited March 23 by joe90

[Iceverge]

Passive houses do need some heating but not central heating.

 

Ours settles at about 17deg in winter when unoccupied if you don't add any extra heat. I reckon it'd be about 19 Deg if occupied with the extra appliances usage and human heat. That would be ok for me but too cold for Mrs and kids. 

 

I couldn't make sense of an A2W ASHP. The capital cost would have never paid back. In the end we didn't put any heating in and have lived 3 years just on a plug in radiator, using about 16kWh/M2/annum. So long as it is used regularly it works just fine. I liken it to a tiny outboard motor on a barge. When up to speed it settles into a hassle free cruise. 

 

I have bought an A2A unit to reduce our bills from about €750 for heating to less than €200. Yet to be installed. I did also put some cheap patio heater style heaters in the bathrooms just to warm your skin when out of the shower. 

 

TLDR. (To Long Didn't Read)

 

Central heating not needed.

Some heating is though (depending on your comfort demands). 

 

A2W ASHP uneconomic unless cheaply bought and DIY installed. 

 

 

 

 

 

[joe90]

But an ASHP will do DHW as well with a COP of 3 - 4.

[MikeSharp01]

In the UK you can, if able to access the grant, get the job done for less than you can DIY it - so that's an incentive. True it is marginal, on ours the lowest quotes are coming in at a couple of grand over the grant - so cost to us being a couple of grand and I cannot get the core components, ASHP and cylinder, for that let alone the G3 certificate and the extended guarantees on the ASHP for that. So despite knowing that my fellow country men and women are paying for the grant, and perhaps therefore some over pricing, although given the MCS overheads these firms carry it is hard to work out how much this might be, we will probably go for this route and ours is passive class and in the SE.

[SteamyTea]

8 hours ago, CalvinHobbes said:

Have we just been conditioned into thinking we need it

Much of it is to do with the language used to describe space heating.

 

Traditional central heating is not really central at all, it is highly distributed i.e. different size radiators, in different positions in different rooms.

Then comes the way it works.  When a gas or oil boiler is delivering heat, it is said to be on.  When it is not delivering heat, it is said to be off.  Sometimes a timer is used to lock out certain times of the day i.e. night time, and sometimes different air temperatures are set for wherever the thermostat/s are located.

 

A heat pump works in the exact same fashion, no different at all, just for some, unexplained, reason, the language used to describe the operation becomes different, and convoluted.

I think this mainly comes from ignorance and prejudice, similar to the debates between petrol and diesel cars in the 1990s.  Back then, the only 'lay measure' of a car's performance was the engine size i.e. why does a 1800cc diesel not go as fast as an 1800cc petrol.  Motoring magazines did some very complicated calculations showing that once running costs were taken into account, you should really be comparing a 1200cc petrol with an 1800cc diesel.  Was a total lot of bollocks.

All they had to do was explain brake specific fuel consumption, but that involves a tiny bit of physics and arithmetic, so scares off 95% of the British, who prefer their own small world view of 'I know for a fact as I have seen it with my own eyes'.  The limes v cement based render debate is a classic.

 

So a quick sum up, heat pumps work exactly the same as a combustion boiler, you put energy in, you get energy out at a different entropy level.

All that means is that as you add energy, you get faster moving molecules, which is what we describe as 'heat'.

Heat is the old term for energy, not temperature, think about energy and it all makes sense.

Energy is not power.

Edited March 24 by SteamyTea

[jack]

8 hours ago, CalvinHobbes said:

Should it not be warm enough? Like my son lives in a passive house and they use the heating max twice a year.

 

The Passivhaus standard sets, among other things, a maximum annual energy budget for heating off 15 kWh/m². On that basis, it's reasonable to expect that heating will be required. The standard says nothing about how the heating is supplied. 

 

If your son rarely turns the heating on, what sort of temperature does the house usually sit at during winter? Passivhaus works particularly well in places where there's a lot of winter sun, which doesn't generally describe the UK! 

 

As a point of comparison, our house temperature stabilised at about 14 degrees when we went away for a month over Christmas a few years ago. That was no people, no appliances, no cooking, and no showers for a month. In comparison, some of my friends live in Victorian houses, and during cold weather struggle to get parts of their homes to 16 degrees even with the heating running flat out for hours. I know which I'd rather!

 

9 hours ago, CalvinHobbes said:

I keep reading of huge electric bills for ashp 

 

Have you read that for houses that meet the Passivhaus standard? 

 

One other point: when you say you're building a Passivhaus, do you mean a house that's been designed and modeled to meet all the requirements of a Passivhaus? Or are you building using Passivhaus principles such as using lots of insulation and good airtightness? It does make a difference. I don't think our house would meet the standard due to the size and location of our windows, for example.

 

If it's been modelled, you should know how much energy it will use for heating in a year, and you can then easily work out how much it will cost to run an ASHP to supply that heat. 

 

[Gone West]

9 hours ago, CalvinHobbes said:

Should it not be warm enough?

If it's been designed not to need a wet heating system, then you won't need one. If you build to the PH maximum allowable space heating of 15kWh/(m2/a) then you will probably need a conventional wet heating system. We lived in East Kent near the coast and I designed our PH so we didn't need a conventional wet heating system. As long as the house is designed using the PHPP with the correct weather data, the actual required internal temperature, ours was 23C, then it's feasible. We heated ours with three electric towels, one in each bathroom, 4m2 of electric UFH in the kitchen and warm air heating through the MHRV from an EASHP. The warm air heating only came on when the outside temperature was low for some time. Be careful of people who say, for example, we don't need heating, but don't tell you the bedrooms are at 14C and the living room at 18C. We had the whole house at 23C 24 hours a day.

[joe90]

Why am I putting in an ashp in a passive house?
 

yes, why are you? for me it was a cheap (EBay and self instal) way of getting DHW and providing UFH with a cop of nearly 4 in a well insulated house. If you don’t want to then don't, it’s working out the alternatives I guess. Plenty of of alternatives available.

[JohnMo]

10 hours ago, CalvinHobbes said:

Should it not be warm enough?

Unlikely in the UK generally. Not mentioned above is the other heat requirement of max 10W/m2. This is a design requirement because it's the max heat that can be carried via air heating through MVHR at PH flow rates. So a 200m2 house will most likely require 2kW of heat or 48kWh a day at your design temp. At full price electric that's £13 a day via a panel heater. From an ASHP at a CoP of 3, that's £4.30.

 

10 hours ago, CalvinHobbes said:

keep reading of huge electric bills for ashp and that you can't turn them off

Piss poor install most likely, mine is currently off, a thermostat switches it off.

 

You have 2 aspects to concider, heating DHW and house heating.  You are looking 5,500kWh of energy for a 200m2 house at passivhaus design figures. So direct electric is about £1500 a year. You hold the purse, you choose.

 

 

Edited March 24 by JohnMo

[ProDave]

23 minutes ago, JohnMo said:

You have 2 aspects to concider, heating DHW and house heating.  You are looking 5,500kWh of energy for a 200m2 house at passivhaus design figures. So direct electric is about £1500 a year. You hold the purse, you choose.

 

Our low energy 150m² house is using 1000kWh per year to heat DHW and 1500kWh per hear to heat the house, both measures are electricity consumed by the ASHP.  So £280 for DHW and £420 for heating.  

 

We did once go away in winter and came back to the house at about 12 degrees with no heating and no people in it for 2 weeks.  So even with incidental heating from occupying the house, I doubt it would be comfortable with no heating at all.  

[nod]

I’ve a friend who is a self employed heating engineer Whilst he’s quite capable of installing a HP He’s decided to go down the grant route As he can’t buy the equipment in for anywhere near the 3k that he’s having to pay on top of his grant 

[JohnMo]

37 minutes ago, ProDave said:

low energy 150m² house is using 1000kWh per year to heat DHW and 1500kWh per hear to heat the house, both measures are electricity consumed by the ASHP.  So £280 for DHW and £420 for heating

That's good, but if you didn't have an ASHP, your heating would be 4x that and your DHW 2.5x that, which gets back to the OP question. So comparing direct and ASHP - £700 for ASHP and £2400 for direct heating.  I paid £1300 for an ASHP, my pay back in your house is less than one year.

[JohnMo]

4 minutes ago, nod said:

he can’t buy the equipment in for anywhere near the 3k

Really depends how you source your equipment. Plenty on here have paid less than £1k for a heat pump, currently there are ones on eBay for £1k new, not the most sophisticated, but can work well.

[nod]

12 minutes ago, JohnMo said:

Really depends how you source your equipment. Plenty on here have paid less than £1k for a heat pump, currently there are ones on eBay for £1k new, not the most sophisticated, but can work well.

The eco that we’ve just had installed

Theres the same model on eBay for 4K 2018 Then you have the tanks 

[Iceverge]

DHW with an ASHP gets quite nuanced.

 

The COP like @JohnMo says is closer to 2.5 Vs the 4+ you would expect for a well set up space heating. 

 

Then you are limited really to about 50⁰ storage temperature for efficiency so you can only store about half then energy that you can with an immersion driven UVC at 70⁰-80⁰. If you operate on a TOU tariff like we do the sums get interesting. 

 

We can heat all our 10kWh of DHW overnight in a 300L direct UVC for 18c/KWh or a cost of €1.80/day. 

 

I estimate that if we had an ASHP we would heat half per day and half at night. 

 

5kWh* COP@2.5 * 18c/kWh = €0.225

5kWh * COP@2.5 * 36c/kWh = €0.45

 

Total €1.13/day savings. Assuming that an ASHP UVC would be €1000 more that a direct one and a heat pump would be €3000 and DHW was half the use load you could say €2000 was the extra spend on the DHW side. Payback 5 years. Pretty respectable. 

 

Unless you add solar PV. 

 

Take a 4kWp system and dump say 3MWh into the DHW/year. Then the averaged daily savings drop to €0.2. 

Payback for an ASHP becomes 27 years. 

 

TLDR

 

A2W ASHP is probably worth it for DHW unless you plan for solar PV anyway. 

 

 

 

 

[MikeSharp01]

7 minutes ago, Iceverge said:

We can heat all our 10kWh of DHW overnight in a 300L direct UVC for 18c/KWh or a cost of €1.80/day. 

 

I estimate that if we had an ASHP we would heat half per day and half at night. 

 

5kWh* COP@2.5 * 18c/kWh = €0.225

5kWh * COP@2.5 * 36c/kWh = €0.45

 

Total €1.13/day savings. Assuming that an ASHP UVC would be €1000 more that a direct one and a heat pump would be €3000 and DHW was half the use load you could say €2000 was the extra spend on the DHW side. Payback 5 years. Pretty respectable. 

 

Unless you add solar PV. 

 

Take a 4kWp system and dump say 3MWh into the DHW/year. Then the averaged daily savings drop to €0.2. 

Payback for an ASHP becomes 27 years. 

 

We need a spreadsheet for this lot, anyone up for experimenting?

[JohnMo]

9 minutes ago, Iceverge said:

A2W ASHP is probably worth it for DHW unless you plan for solar PV anyway

Just to add a couple more lines to the spreadsheet - I would say there are advantages with PV with an ASHP also, a marginal PV day may not give enough PV to fire up the immersion via a diverter to any degree that will provide usable heat.  ASHP, would amplify that PV out by 2.5. Then the maths gets way to difficult.

[Iceverge]

I did one.........

 

Must modify it for the new electric rates. 

 

In short for a passive house long term there was almost no 25 year difference between. 

 

A2A + ESHP

A2A + PV diverted into DHW

ASHP 

 

ASHP+PV is dearer long term. 

 

Shorter term the lower capital cost of the A2A and PV wins. I think up to about year 10 or 15. 

 

 

[Iceverge]

12 minutes ago, JohnMo said:

Just to add a couple more lines to the spreadsheet - I would say there are advantages with PV with an ASHP also, a marginal PV day may not give enough PV to fire up the immersion via a diverter to any degree that will provide usable heat.  ASHP, would amplify that PV out by 2.5. Then the maths gets way to difficult.

 

Not having either an ASHP or PV I thought the conventional wisdom had become not to use the PV for ASHP due to he high start up loads and the extra wear on components when otherwise it could be rested for long periods of the year.

 

Also take a situation on a winters day. Say an ASHP needs 800w minimum to run but you only have 400w of PV. You can run the ASHP for 1hr to top up the DHW but need to buy in 0.4kWH. 

 

If you had 8hrs of weak sunshine at 400w it would do the same thing but with no bought in electricity. 

 

 

 

 

Edited March 24 by Iceverge

[Stones]

If memory serves, the original Passivhaus heating standard was designed so that all of the additional heat requirement could be delivered via a ventilation system (resistive heater).

 

I know someone here who designed specifically to Passivhaus accreditation level, but still requires additional heat on certain days. Average solar gain for example is all very well, but rarely do you get the gain evenly spread.

 

When I was looking at it for our house (2016), I looked at a range of options and heat requirement levels.  The conclusion was that for both heat and DHW requirement, if the requirement was below 2500kWh annually for each, you would over a 10 year time frame be better off using straight forward (and inexpensive) resistive heaters.  Above that level of requirement, the figures stacked ever more favourably in the direction of a heat pump.  Energy prices have of course significantly changed since, so the cut of kWh requirement point will have changed.

 

 

[ProDave]

22 minutes ago, Iceverge said:

 

Not having either an ASHP or PV I thought the conventional wisdom had become not to use the PV for ASHP due to he high start up loads and the extra wear on components when otherwise it could be rested for long periods of the year.

 

Also take a situation on a winters day. Say an ASHP needs 800w minimum to run but you only have 400w of PV. You can run the ASHP for 1hr to top up the DHW but need to buy in 0.4kWH. 

 

If you had 8hrs of weak sunshine at 400w it would do the same thing but with no bought in electricity. 

 

 

 

 

That is just over thinking what will the PV power?  On that dull day, it will power whatever happens to be on in the house at the time, and if there is any surplus after that it will divert excess to the immersion heater.  It needs no user input to achieve that.

 

So the key thing with solar PV (without battery storage) is move as many of your electrical loads as possible to the daytime.  For instance I have my ASHP timed to only start DHW heating at 11AM.  By then the sun should be up and PV generating.  So at least some of what the ASHP is using is coming from the PV.  And all big appliances get used near the middle of the day.

 

And there is nothing wrong with the ASHP heating the DHW AND surplus going to the immersion heater as well at the same time.

 

PV conflicts with a TOU tariff such as E7.  The TOU tariff suggests you want to do as much as you can at night, PV says do as much as you can in the day.  I don't personally like TOU tariffs, they come with a much higher day rate.

[JohnMo]

Just now, Iceverge said:

had become not to use the PV for ASHP due to he high start up loads and the extra wear on components. 

PV is connected to consumer unit after inverter (normal system), consumer unit provides electric to where ever or whatever wants it.  Not directly coupled to HP.  Just that the HP is another user from CU.

 

Our HP (like many others) slowly ramps up when doing DHW, so with start flowing in the late 20s and over the next hour get to 55 degs.  So the early part of DHW is actually pretty good CoP, the later part is a dreadful CoP. So not straight forward.

 

5 minutes ago, Iceverge said:

ASHP needs 800w minimum to run but you only have 400w of PV

It could be putting 2kW or more out for 400W off the grid. The 400W provided by PV the rest of the time would have powered the background loads to the house. Not easy to make the electric go to just the immersion though, unless directly wired.

 

Add battery it makes things smooth out, currently paying 13p per kWh for 7 hours, utilisation over winter has averaged at 83% of night rate, so 17% at expensive day rate. Very patchy day for solar today, lots of clouds etc.  Charged battery 70% last night, heat pump been ticking away most the morning, will be off at 12pm.  Battery currently at 68%, and generated 5kWh from PV, with peaks (5kW) and troughs (0kWh), would have been useless for PV directly driving the heat pump and wasteful driving an immersion.

 

 

 

[Iceverge]

Theres many variables here. 

 

I'll try to list them, in no particular order

 

1. kWh price

2. TOU unit price

3. Annual Space Heating demand

4. Annual DHW demand

5. ASHP Capital cost. 

6. ASHP lifespan

7. Future energy costs

8. Cost of credit

9. House occupation patterns

10. FIT rates

11. Battery costs

 

The only one you can make a big difference to is #3. Get your demand as low as possible. Then even the worst case doesn't look too bad. We are heating the house and providing DHW for 5 people for about €1500 annually doing it the most expensive way possible. Resistive heating and DHW.

 

 

A €1400 A2A would save €700/year (2 year payback) . €800 total bill

A €1900  ESHP for DHW would save € 265/ year (7 year payback)  €1235 total bill

A €5500 A2W ASHP would save €950/year (5 year payback) . €550 total bill

A €3750 of 4kW Solar PV would save €750/year. (5 year payback) €750 bill

A €100 second hand storage heater would save €288/year €1212 total bill, 

 

 

For us I decided that an A2A HP and 4kW of PV would virtually eliminate bills. Time will tell.

 

A2W ASHP and PV will be cheaper to run but cost much much more to install inc UFH etc.

 

 

 

 

 

 

 

 

 

 

 

 

 

Edited March 24 by Iceverge