Running ASHP in 55deg mode for SH & DHW or separately 35deg for SH and 55deg for DHW

[offthepiste]

Am starting to try to narrow down various ASHP options and their associated energy costs for space heating (SH) and direct hot water (DHW).

 

However, I have one (of many) very large area on zero knowledge. I am trying to decide whether to run the ASHP at 35deg, i.e. just for SP, or at 55 deg for DWH.

 

I spotted a thread which seems to address the issue but I want to make sure my understanding is OK.

 

Question: If you run an ASHP at 55deg, achieving say at COP 2.5 - 3.0, how do you handle UFH at 55deg? This seems bonkers. 

 

So there seems to be two schools of thought:

1.     
ref @ProDave
https://forum.buildhub.org.uk/topic/8265-temperature-of-hot-water-leaving-your-ashp/?do=findComment&comment=141288  
who has the funky solution of running the ASHP at 37deg for a period of UFH (I guess during the day) and 55deg for DHW (I guess during early morning to heat up tank) – which means achieving two separate COPs.

2.     
Ref @newhome
https://forum.buildhub.org.uk/topic/8265-temperature-of-hot-water-leaving-your-ashp/?do=findComment&comment=141342  
who I guess runs at 55deg but blends the UFH with cold water and I guess only achieves the lower COP.

3.     
Are there any others?

 

I would have assumed that ProDave solution is the best as you can optimise COP.

 

Cheers

 

 

Edited December 11, 2021 by offthepiste
update

[SteamyTea]

Newhome has resistance heating I think, not an ASHP.

[J1mbo]

Most ASHP will have separate DHW and CH programs and will do only one of them at any given moment in time.

 

When the DHW is programmed to heat, a 3 port valve will be activated that sends the flow from the heat pump through the coil and there are various algorithms used to heat the tank depending on the required recovery time basically.

 

When the valve is not activated, the flow is via the heating circuits only and the ASHP will typically use a weather compensation algorithm to determine the required flow temperature.

[ProDave]

It is usually a function of the ASHP that it sets the temperature of hot water or heating separately.  Unlike a system boiler, they usually only heat one at a time.

 

The detail of exactly how they work varies from one make to another.  But mine for instance you set the on and off times of heating and hot water and the water temperature for each.  The heat pump decides what to do when both are demanding heat at the same time, in my case I have it set for hot water priority, so when the tank needs some heat, it stops heating the house.  In a well insulated house you won't notice that pause in space heating while it heats the hot water tank.

 

Don't over think it.  Buy an ASHP and read the manual or if you have specific requirements we can help you choose, i am pretty sure just about every make of heat pump is in use by someone on the forum.

[SteamyTea]

Depending on the size of heat pump to load, and the thermal heat capacity of the heating system, you can usually turn the heating off for a few hours.

[offthepiste]

Brilliant, thank you all.

 

So pretty much decided on ASHP for both SP and DWH along with Solar PV (potentially Sunpower 400W) and battery (potentially Powerwall) - this simplifies everything. Was starting to get into the long grass .. 

 

Just done some calculations comparing Octopus Tesla (will have insufficient to export) and Go Faster tariffs - this is not going to get implemented until August next year (fingers crossed) and heaven knows what will happen to rates etc. and think on current basis Go Faster, charging Tesla to max and using Solar PV to max during day, is the best option.

 

Now need to decide on ASHP.(am very intrigued by the RED ASHP) and Heat Store .as well as MVHR . house will be 300m2 to an Enerphish standard ... I am guessing perhaps to achieve 45 kWh / m2.

 

Amazingly in our very drafty mid terrace Victorian we currently only consume 30,000 kWh gas for 280m2, i.e. for both SH and DWH.  So am guessing if 80% is SH, then we have a rating of 86kWh/m2  - though we do have a Viessmann 222-f and tend to layer ups in the winter like today - but there are only two of us and we only really use the ground and first floors.

Edited December 11, 2021 by offthepiste

[joth]

16 hours ago, offthepiste said:

to an Enerphish standard ... I am guessing perhaps to achieve 45 kWh / m2.

 

(Assuming that is a typo for EnerPHit, the retrofit standard)

EnerPHit is 25 kWh/m2 per year, so at 45 you'd be a fair way off that standard.

 

The AECB Building Standard is 40 kWh/m2 and 1.5 ACH airtightness,  which would be something reasonable to aim for. 

Edited December 12, 2021 by joth

[offthepiste]

3 hours ago, joth said:

 

(Assuming that is a typo for EnerPHit, the retrofit standard)

EnerPHit is 25 kWh/m2 per year, so at 45 you'd be a fair way off that standard.

 

The AECB Building Standard is 40 kWh/m2 and 1.5 ACH airtightness,  which would be something reasonable to aim for. 

Hi @joth I am erring on the safe side .. have been in discussions with a company that specialises in period property, e.g. georgian & victorian, retrofits. They originally aimed for true Enerphit but have leaned over many years that airtightness measures progressively, sometimes quite quickly, become undone within the fabric of an older building. Hence, whilst we may target 25kWh/m2 I am working on 45kWh/m2 for a potentially realistic view.

 

It would be interesting to hear from experiences of those who have done this sort of thing.

 

One of the challenges I have is a cellar which will be split into plant room and separate fine wine cellar (want to retain current 12 deg and humidity levels) there are lots of potential surface / material interfaces and dimension constraints and want to keep things simple primarily to be able to tolerate any insulation / airtightness deterioration as well as keeping costs down.

[joth]

43 minutes ago, offthepiste said:

 

It would be interesting to hear from experiences of those who have done this sort of thing

 

We recently obtained our EnerPhit plus certification, with airtightness of 0.6 ACH which is good enough for a new build passivhaus.

We could easily of exceeded even that (our initial test was under 0.5), except for a couple very annoying leaky components that weren't really anything to do with doing a retrofit:

- The ground floor shower trap is "low profile" and got sucked dry on the depressurisation test. (Should have left the shower running during the test!!)

- the stupidly expensive PH certified Moralt garage door was specified as outward opening and the threshold leaks like a sieve

- one of the internorm windows was poorly adjusted and leaky (but we fixed that med test).

 

 

Really interesting comments about the longevity of the airtightness work on a retrofit. May thought was by overachieving we'd eventually settle to a "reasonable" level, but there is a lot depending on sticky tape so time will tell

 

 

 

 

 

Edited December 12, 2021 by joth

[offthepiste]

2 hours ago, joth said:

 

We recently obtained our EnerPhit plus certification, with airtightness of 0.6 ACH which is good enough for a new build passivhaus.

We could easily of exceeded even that (our initial test was under 0.5), except for a couple very annoying leaky components that weren't really anything to do with doing a retrofit:

- The ground floor shower trap is "low profile" and got sucked dry on the depressurisation test. (Should have left the shower running during the test!!)

- the stupidly expensive PH certified Moralt garage door was specified as outward opening and the threshold leaks like a sieve

- one of the internorm windows was poorly adjusted and leaky (but we fixed that med test).

 

 

Really interesting comments about the longevity of the airtightness work on a retrofit. May thought was by overachieving we'd eventually settle to a "reasonable" level, but there is a lot depending on sticky tape so time will tell

 

 

 

 

 

Agree re stick tape. Current thinking is to try to minimise use by leveraging plaster, either existing if appropriate or new to close gaps, etc. but obviously will need a different solution re joists and their holes (not sure if there is a technical term) etc.

 

Am having a "little bit of a debate" at home regarding the need to avoid a letterbox!!!! I have found a few quite smart looking, i.e. not tweeeeee, standalone cabinets that take letter and possibly parcels. Do not want to fix to wall and thinking of semi hiding behind hedge ... we will have the educate the postman ... once the debate has subsided.. One attached to rear of front wall would have been ideal but our house is located just at the distance that local yuffs start to get rid of the remains of their trans (a word much in fashion these days )fat foodand packaging... a public facing letterbox would be toooo tempting.

[joe90]

My ASHP (kingspan) only heats at one temp, I set that to 48’ to give me a reasonable COP, this  heats the DHW tank and a buffer tank fir UFH, giving priority to DHW, THE Tank stat on the buffer is set to 35’ and blended down again on the manifold to 25’. The control unit is soo complicated I don’t use it, just the stats on the tanks and a room stat to call for heat (which runs the pump on the manifold). During the non heating months I simply disconnect(  buy a switch) the buffer tank stat so this tank is not heated.

Edited December 12, 2021 by joe90

[ProDave]

Re "sticky tape"  Where I could, e,g, where the tape was just joining adjacent strips or air tight membrane, I screwed ripped strips of OSB over the taped joint, to mechanically hold it together in the vain hope of preventing the taped joints opening over time.

 

Also I use the Barriar air tight membrane which comes with a tape built in which is supposed to seal overlaps, but because I did not board straight away, I fortunately found the built in tape was useless and started to open up so I gave it a layer of Tescon tape as well.  Had I fitted the Barriar using it's own tape and boarded straight away I would not have know the tape was failing until the air tight test and then it would be a bit late.

 

I am surprised the air tight test blew the water out of a shower trap. It is only supposed to pump up to 50Pa which is only a few mm, way lower than even the shallowest shower trap.  But that is a well known problem when doing a drain pressure test, I could not get above 75mm before it just blew bubbles though the traps.  I did however have to make some temporary wedges to keep the lift out buckets in the 2 shower trays down in place for the drain pressure test.

[saveasteading]

4 hours ago, joth said:

Should have left the shower running during the test!!)

 

Or used sticky tape, as you can over fans etc too.

 

For long-term quality though I suspect most sticky tape fails.

I've just brought my decorations boxes down from the attic and most of the closing tape has failed, regardless of being packing tape, gaffer tape or supposedly gorilla (or whatever).

It will fail in cavities and over leaky details too, where it can never be seen to have failed, and can't be replaced.

 

I suspect a lot of commercially built properties are taped up for the test.

[J1mbo]

Air tight is all very well but air needs to get in to allow extractors (cooker hood and bathroom) to do their job and in equal measures.

[saveasteading]

Just now, J1mbo said:

Air tight is all very well but air needs to get in

Airtightness first, ventilation second. It is for the avoidance of uncontrolled draughts that we do the airtightness, and then the extracts and intakes can be opened up again.

[ProDave]

22 minutes ago, J1mbo said:

Air tight is all very well but air needs to get in to allow extractors (cooker hood and bathroom) to do their job and in equal measures.

Which is why most people that bother about making a house air tight, also fit MVHR to give controlled air in and out with heat recovery.

 

In fact if the air test is under a certain number, building control here insist on mvhr.