Jump to content

Recommended Posts

Posted

I appreciate that this might sound like a rookie question and if, I do apologise.  So I have completed the heat loss calculator and if I use 0c as the outside temp, the hat loss in coming in at 1.7Kw.  If I take this down to -10c the figure rises to 2.6Kw

The reason I need to know the size of the required ASHP is that I need to try and get our electric supply down to 100 Amps.  So on that basis, I assume that if the heat loss is < 3Kw, Then the actual electricity load to generate 3Kw is actually only about 20%, so it would equate to about 600w input?

It's so tricky knowing what supply I actually require.   We need to have an EV charger and I also want to run my woodworking machinery ~ 5 Kw with everything running.  When I plug these number into SSEN, it put me over 100 amps.

Posted

100A should be plenty.

 

100A = 24KWish

 

You are obviously looking at single phase so the biggest EV charger for single phase is about 11KW. 3-phase you can get 22KW ones but your EV has to support it.

 

Assuming you have electric cooking and are not having instant electric water heating/showers it seems like you should be able to have pretty much everything on at the same time without much difficulty. That is not how things are usually planned, it's assumed that not everything demands power at once and if you do get tight you can set the EV charger to charge at times when you are not in the workshop.

 

If you want more than one EV charger either get smaller ones (which are likely all you need anyway unless you do a lot of miles every day) or you can get ones that can share a limited feed, ie you can program them so that they don't draw more than X between them.

 

 

  • Like 1
Posted
8 minutes ago, flanagaj said:

The reason I need to know the size of the required ASHP is that I need to try and get our electric supply down to 100 Amps.  So on that basis, I assume that if the heat loss is < 3Kw, Then the actual electricity load to generate 3Kw is actually only about 20%, so it would equate to about 600w input?

 

Answering your actual question: That is approximately what it would be quite often. Assume a less optimal case and it working harder than planned and you should still be below 1KW, basically inconsequential in terms of large appliances.

 

Your heat gain/cooling load will be different so worth working that out too.

  • Like 1
Posted
49 minutes ago, flanagaj said:

So I have completed the heat loss calculator and if I use 0c as the outside temp, the hat loss in coming in at 1.7Kw.  If I take this down to -10c the figure rises to 2.6Kw

You generally calc at -3 for England and -5 for most of Scotland some bits down to lower.

 

We generally never go above 6kW load for the whole house (our battery inverter size), 6kW ASHP, MVHR, borehole pump, two ovens, induction hobe, microwave , kettle, everything does does demand electricity at the same time.

 

Our largest draw from the grid is battery charging and that rarely even at 6kW.

 

So you can also charge two cars at the same time if you wanted.

  • Like 1
Posted (edited)
1 hour ago, flanagaj said:

I appreciate that this might sound like a rookie question and if, I do apologise.  So I have completed the heat loss calculator and if I use 0c as the outside temp, the hat loss in coming in at 1.7Kw.  If I take this down to -10c the figure rises to 2.6Kw

 

Generally, the design outdoor temperature for places between 51-52 degree north is -2 at 50m above sea level. If you're up higher and in a exposed location this would be slightly different, but -2 - -3 is absolutely fine for most of Wiltshire.

 

1 hour ago, flanagaj said:

We need to have an EV charger

 

Most domestic single phase EV chargers are 7kW I believe and any decent one will have load management to regulate charging according to whole house load, which our does very well. 

 

1 hour ago, flanagaj said:

The reason I need to know the size of the required ASHP is that I need to try and get our electric supply down to 100 Amps. 

 

Calculations for this are a bit of a bogs dinner and highly inaccurate (e.g. using an industry standard method puts ours way over 100amps but is a load of nonsense). I would just ask an electrician to have a look and give you their opinion. If you have both EV charger and Heatpump you will need a DNO application anyway, whereas if it's just one, there are some conditions where notification is all that is required and some where application must be made. 

Edited by SimonD
  • Like 1
Posted
5 hours ago, flanagaj said:

The reason I need to know the size of the required ASHP is that I need to try and get our electric supply down to 100 Amps.  So on that basis, I assume that if the heat loss is < 3Kw, Then the actual electricity load to generate 3Kw is actually only about 20%, so it would equate to about 600w input?

 

I would use a CoP of 3 in your estimate but the load from the HP is the least of your worries. If you can perhaps avoid charging the car while you are using your workshop then I don't think you will have a problem. Choosing the right ToU tariff is key to this.

 

We have a 12kW HP, a 7kW zappi charger, and a Victron inverter which draws over 4kW while charging the batteries at 70A. When it is all going at off-peak rates that makes about 15kW.

 

My DNO has approved all this under a G99 application to run off an 80A/18.4kW supply.

  • Like 1
Posted

Below is my heat loss calculation.  So according to the below I need something that can output ~ 2Kw if I don't want the house to get cold when it is -3c outside.  So a 3Kw unit would suffice, but I assume I also need to factor in the corresponding Kw for hot water heating.  If so, how do you go about doing the calculation?

image.thumb.png.570600bae7072cd481184e9fb3acc6b7.png

Posted (edited)

Looking to heat DHW takes around 1 hour each time. So say doing 2x heats per day. So you have 22 hours to provide 24 hrs of heating.

 

(2142*24)/22, adds a couple of hundred watts only.

 

Small heat loads need some thinking about, because too small an ASHP, DHW can take an age to heat.

 

You really cannot or have zones, even if you wanted. As heat pump would cycle to much.

 

Your heat pump is going to be oversized, but you can live with that with a simple simple system. 

Edited by JohnMo
Posted
1 hour ago, flanagaj said:

Below is my heat loss calculation.  So according to the below I need something that can output ~ 2Kw if I don't want the house to get cold when it is -3c outside.  So a 3Kw unit would suffice, but I assume I also need to factor in the corresponding Kw for hot water heating.  If so, how do you go about doing the calculation?

image.thumb.png.570600bae7072cd481184e9fb3acc6b7.png

What did you use to produce these figures?

Posted
10 minutes ago, flanagaj said:

Below is my heat loss calculation.  So according to the below I need something that can output ~ 2Kw if I don't want the house to get cold when it is -3c outside.  So a 3Kw unit would suffice, but I assume I also need to factor in the corresponding Kw for hot water heating.  If so, how do you go about doing the calculation?

Calculation is a good plan but won't account for absolute capabilities of the ASHP itself so your outcome will inevitably be a compromise between the absolute theoretical demand worst case (which you already have for heating @ 2kW) the rate at which you want to reheat your domestic hot water (DHW) and any additional strategies you may want to put in place e.g. direct electric heating of the DHW to support fast re-heat and your overall control strategy - see below.  You will need to decide what operational strategy you are going for as this also impacts the final system - will you just run the system as a single open loop with weather compensation or have a number of zones all controlled separately by their own thermostats or some other combination. In the end you will arrive at a simple set of high level options all of which will have details that define the physical system and its components:

  1. Small ASHP say 3kW able to modulate down to 1.5kW which will run your heating, with some short cycling (ON/OFF)  because only on the coldest days does the ASHP match the load perfectly on all others it has more output than you need and a slow heating of DHW, if you want faster then the immersion heater is available.
  2. A larger ASHP say 6kW that might modulate down to about 2kW which will run your heating but with more short cycling because it is oversized but your DHW reheat will be much quicker and so direct heating is less likely to be needed.

Do not forget that ASHPs have a standby power drain, keeping the crankcase warm etc, that may be  factor for you and you may want to factor in cooling as a possibility which will ask more questions of ASHP particularly the smaller ones.

  • Like 1
Posted

So you need 3kW of heat.  That would assume your heating is on 24/7.  It won't be, not unless you have something else to heat the hot water.

 

In any event, there are very few 3kW ASHP's made.  the smallest is generally 5kW.  That will do your heating without having to be on 24/7 leaving time for DHW heating as well.

 

A 5kW ASHP will typically use less than 2 kW of electricity so that's about 8 amps.  It won't be the ASHP that tips you over 100A.

 

Re the house going cold.  Up here -10 is not unusual and that is what I used to size mine and the house is still snug and warm at that.

 

So your real concern is your total load.  100A seems way too high. As if you are not applying diversity and assuming everything will be on at the same time?

 

Post your calculations that says you will exceed 100A for us to analyse.

  • Like 1
Posted (edited)
23 minutes ago, MikeSharp01 said:

with some short cycling (ON/OFF)

Some cycling is possibly more correct. Short cycling really refers to very short run times and short off times. If you are getting short cycling your system design is poor, no matter what size heat source company to heat needed. You size system volume to ensure you get 10 to 12 minutes of run time no matter what. Then you have controlled cycles, which yield a great cop as well.

 

Our ASHP is huge compared to heat demand, 6kW output from plus 30 to minus 15 degs with a minimum output closer to 4kW. Heat demand closer to 3kW at -9. Will happily supply house with sub 1kW heat demand, with very controlled cycles, just using floor as a buffer.

Edited by JohnMo
Posted
41 minutes ago, MikeSharp01 said:

Calculation is a good plan but won't account for absolute capabilities of the ASHP itself so your outcome will inevitably be a compromise between the absolute theoretical demand worst case (which you already have for heating @ 2kW) the rate at which you want to reheat your domestic hot water (DHW) and any additional strategies you may want to put in place e.g. direct electric heating of the DHW to support fast re-heat and your overall control strategy - see below.  You will need to decide what operational strategy you are going for as this also impacts the final system - will you just run the system as a single open loop with weather compensation or have a number of zones all controlled separately by their own thermostats or some other combination. In the end you will arrive at a simple set of high level options all of which will have details that define the physical system and its components:

  1. Small ASHP say 3kW able to modulate down to 1.5kW which will run your heating, with some short cycling (ON/OFF)  because only on the coldest days does the ASHP match the load perfectly on all others it has more output than you need and a slow heating of DHW, if you want faster then the immersion heater is available.
  2. A larger ASHP say 6kW that might modulate down to about 2kW which will run your heating but with more short cycling because it is oversized but your DHW reheat will be much quicker and so direct heating is less likely to be needed.

Do not forget that ASHPs have a standby power drain, keeping the crankcase warm etc, that may be  factor for you and you may want to factor in cooling as a possibility which will ask more questions of ASHP particularly the smaller ones.

Thanks.  It's all so complicated.  Makes my head spin.

Posted
3 minutes ago, flanagaj said:

Thanks.  It's all so complicated.  Makes my head spin.

Not really complicated, just new.

 

Simple rules

1 zone

1x 3 port diverter valve

With a decent screed thickness the floor will act as a buffer, so no need for volumiser or buffer cylinder.

Cycling isn't bad as long as it's controlled.

The more the heat pump is oversized the flatter the WC curve becomes, mine is almost flat for heat and is flat for cooling.

 

If you are going for a grant, all you need to do is a reality check on system design and make sure heat pump isn't stupidly over sized.

 

Low flow temperature design, each 1 Deg lower represents approx 3% in system efficiency.

  • Like 1
Posted
35 minutes ago, ProDave said:

 

In any event, there are very few 3kW ASHP's made.  the smallest is generally 5kW.  

@flanagaj  Prodave is right.  Note however that there are quite a few ashps which are labelled ~5 kW that actually are ~8kW models down rated in the firmware.  Avoid these.  The most prominent/common one is Daikin, but there are others.

  • Like 1
Posted
16 minutes ago, flanagaj said:

Thanks.  It's all so complicated.  Makes my head spin.

And there was me trying to simplify it down to 2 choices.

 

One thing Buildhub is very good at is offering a range of tried and tested schemes none of which ae quite perfect even for the people who advocate them. The challenge is to make your choice from that lot or synthesise your own solution as a combination of several.    

 

Perhaps what we need to do on BH is develop a definitive guide to this fundamental decision that has longevity at the start of any build. Otherwise you start off in one direction and end up swithering all over the place. We are past first fix now and have just decided - yesterday to install a fan coil coil unit in each of the bedrooms - the location of which will be sub optimal because we didn't factor it in at the start. We did think of cooling and designed in a passive stack system which has proved very useful over the past few weeks but still has the other half worried the bedrooms will be too warm. Same has gone for our change from gas boiler to ASHP but that, at least, was a choice we made way back but not until after we had moved the gas meter and installed the gas pipe in the slab!

  • Like 2
Posted
19 minutes ago, flanagaj said:

Thanks.  It's all so complicated.  Makes my head spin.

Deal with one decision at a time.  Don't overload yourself with all details and choices.

 

At the moment the task in hand is determine the size of your ASHP and sort out your maximum electrical load.

 

a 5KW ASHP will do your heating and hot water.  Which make and which heating scheme is a decision for later.

 

Now lets look at your total load that you fear is going to exceed 100A and put that issue to rest so you can move on.

Posted
3 minutes ago, JamesPa said:

@flanagaj  Prodave is right.  Note however that there are quite a few ashps which are labelled ~5 kW that actually are ~8kW models down rated in the firmware.  Avoid these.  The most prominent/common one is Daikin, but there are others.

 

Putting this a different way. Manufacturers who produce a range of heatpumps will likely only produce a small number of sizes and artificially down rate the others. Avoid any that are down rated even if it means buying bigger than you need.

 

So get the smallest size heatpump for your needs that also is not limited by software. Given whatever you get will probably be oversized, look for ones with the highest range of modulation.

Posted
10 minutes ago, -rick- said:

highest range of modulation

Which almost impossible to find out from datasheets.

Posted
20 hours ago, flanagaj said:

I also want to run my woodworking machinery ~ 5 Kw with everything running

You cannot operate that much unless you have very large plant - in which case you should be on 3ph, on your own, assuming 1kW extraction plant and perhaps 1.2kW motor on the machine you are running your need is actually only 2.2kW so that's an example of the diversity that @ProDave was talking about. Only the background load of the place will run 24/7 so needs slicing off the top everything else will only run occasionally and rarely together. There must be a standard way of working out the diversity, somebody on here must have discussed it, or at least estimating it, e.g  assume 75% of the 'equipment' running worst case doe that get you under the 100A?

Posted (edited)
10 minutes ago, MikeSharp01 said:

You cannot operate that much unless you have very large plant - in which case you should be on 3ph, on your own, assuming 1kW extraction plant and perhaps 1.2kW motor on the machine you are running your need is actually only 2.2kW so that's an example of the diversity that @ProDave was talking about. Only the background load of the place will run 24/7 so needs slicing off the top everything else will only run occasionally and rarely together. There must be a standard way of working out the diversity, somebody on here must have discussed it, or at least estimating it, e.g  assume 75% of the 'equipment' running worst case doe that get you under the 100A?

My PT is 2.2Kw, Extractor 1.2Kw and bench saw 2.2Kw.  These will all be running together as sometimes I am moving between the two machines frequently.

The electricity company has said that there is 2 phase and mentioned 46KVa, so I assume that would mean two 23 Kva lines into the consumer unit.  So on that basis maybe it will be ok.

I think my issue, is that I have no idea how I go about calculating the size of the ASHP and the Air conditioning unit.  I know people have said to go with fan coils upstairs, but given the recent warm weather and the large amount of glazing, I want a simple solution that I know is going to work.  Turn on the AC and within no time the rooms can be chilled to 18C.  No faffing around.

We won't be using electric showers as we intend on having a HW cylinder.  Dual ovens and an induction hob.  Apart from that, there isn't anything else that is significant.
 

Edited by flanagaj
Posted (edited)
26 minutes ago, flanagaj said:

Turn on the AC and within no time the rooms can be chilled to 18C.  No faffing around

So why bother with 2x systems just do A2A for heating and cooling or A2W for heat, cool and DHW. I touched my cooling switch in April not touched it since, will most likely touch it again in October to flick over to heat. Nothing else in the house.

 

Why have 2x systems and 2x the cost? Two systems to size, two outdoor units, two different companies. Double your spend. Twice the thinking time. Your just making your own life complex.

 

Stick in either a direct cylinder for immersion heating or a heat pump integrated cylinder if you go A2A.

 

A 100A supply is all you really need.

Edited by JohnMo
  • Like 1
Posted
8 minutes ago, flanagaj said:

My PT is 2.2Kw, Extractor 1.2Kw and bench saw 2.2Kw.  These will all be running together as sometimes I am moving between the two machines frequently.

Ah yes but will you also be showering and cooking at the same time? Anyway as you say just dump the electric showers and all will be well.

 

Back to sizing the ASHP - you have all the data you need so looks to me like you need a 5kW unit. The one we are looking at is 6kW the modulates down to 1.8kW - according to the blurb, we have a heating load of of 1.2kW at -1oC (We are in Kent so not the -10oC of @JohnMo and @ProDave )

  • Like 1

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...