Looks too good to be true... Looking for recommendations on ASHP size.

[ChrisDL]

Evening,

 

I have been looking, sourcing and trying to design a heating system for our barn conversion. I initially contacted several reputable GSHP/ASHP suppliers and they have come back with system designs with pumps in the region of 25kW to 30kW. Granted our property is quite large but I have been suspicious over their assumptions and calculations as none of them asked me any detailed questions about our build materials etc.

 

Before just slinging something in I have consequently made the effort to  use @Jeremy Harris Heat Loss Calculator, whilst at it I also used @Lilly_Pines Solar Gain Calculator.

 

At the worst in January I have calculated a hourly requirement of 4.41 kWh if we exclude any solar gain it is 5.82 kWh. Now I can't understand why I would need a 30kWh capable pump?

 

Either my calculations are too good to be true and the building will never perform so well in real life/a input mistake has been made... or the suppliers are massively overcompensating. Or am I missing something else?

 

The next issue is (whether the above is right or wrong) is installing a system that can run on ASHP and another heat source. We are off-grid - so no choice but to use another source of heat. Probably oil for ease of use. However I am seriously considering using a boimass boiler or woodburner with back boiler, this would have to be connected up to a thermal store which from most forum posts is unpopular compared to UVC. (Why is that?)

 

I have provided monthly solar generation from a 33kW Solar PV array as per https://re.jrc.ec.europa.eu/pvg_tools/en/tools.html I assume it is accurate enough - it is suggesting a PV generation in winter which is 19% of summer output which I have seen bounded around.

 

If my calculations are correct I have expanded Jeremy's spread sheet to estimate oil usage and it is relatively negligible so burning wood seems unnecessary unless my calculations are all wrong... I have attached my spreadsheet for those interested/willing to have a look - I have noticed @SteamyTea is pretty good at crunching the numbers.

 

Appreciate anyone's thoughts and opinions on the matter.

 

Edit: this is a continuation of thoughts from a post a few months ago - appreciate the responses from everyone there but realised I lacked useful detail and my own calculations.

 

 

 

 

 

 

 

 

 

 

Heat loss calculator - My Barn.xlsx

Edited September 4, 2022 by ChrisDL
To link past post.

[JohnMo]

Sounds like the designers are very lazy and not doing the job they should.  Maybe avoid.

 

Quick question are you sure your coldest day never goes below zero?

 

Jeremy spreadsheet shows slightly lower than SAP (for me), so may be worth looking at your SAP report also.

 

You need to basically size for your coldest day, with no solar gain.  Plus you need an allowance for DHW heating.

 

But if your having an oil/biomass boiler it may make sense to run a smaller heat pump for heating only and leave the oil/bio and solar to sort out the DHW.  That way you get away with something like 5-6kW heat pump.  The benifit of a smaller heat pump is no buffer likely and it energy use will be low and you make use of running on solar in the summer for cooling 

[ProDave]

Yes I found Jeremy's spreadsheet very accurate, more accurate than the SAP calculations (which had all the same input figures)

 

If your max heating demand is just over 4kW then I would be fitting a 10kW ASHP to allow it some time off, and some time to heat DHW.

[ChrisDL]

12 minutes ago, JohnMo said:

Sounds like the designers are very lazy and not doing the job they should.  Maybe avoid.

 

Quick question are you sure your coldest day never goes below zero?

 

Jeremy spreadsheet shows slightly lower than SAP (for me), so may be worth looking at your SAP report also.

 

You need to basically size for your coldest day, with no solar gain.  Plus you need an allowance for DHW heating.

 

Yes they do - I have struggled across the board to find reassurance from a lot of companies that they know what they are looking at -  but to be this far apart on the calculations has me worried I have it wrong.

 

Well it is Mean minimum OAT - Devon doesn't get many sub-zero days. I have already included DHW (over compensated this one) If I drop the outside temperature to -3 and remove all solar gain we calculate a hourly heat requirement of 7.88 kWh. Without any solar the Oil will be running anyway as we will be conserving battery power for electrical amenities.

[ChrisDL]

18 minutes ago, ProDave said:

Yes I found Jeremy's spreadsheet very accurate, more accurate than the SAP calculations (which had all the same input figures)

 

If your max heating demand is just over 4kW then I would be fitting a 10kW ASHP to allow it some time off, and some time to heat DHW.

 

Good to know it is accurate - and I am confident my inputs are right. But as stated it worries me how the 'professionals' can be so wrong.

 

I was looking at about 10-12kW. Samsung 12kW comes to mind... not sure if they are any good.

 

Would you have a recommendation for a cylinder size? We are a family of four (might grow to five one day) but we are likely to have guests over, hence I have based my DHW requirements for 6 at 75l each. Which will mean I need to be able to store at minimum of 450l. Then there is the requirement for underfloor heating. I keep leaning towards a thermal store, out of a odd desire plumb in a back boiler wood burner and not waste any heat. But UVC's seem to be championed on the forums so recommendations on sizing / buffer required would be great.

[SteamyTea]

23 minutes ago, ChrisDL said:

Well it is Mean minimum OAT - Devon doesn't get many sub-zero days.

I have had none this year.

 

[SteamyTea]

5 minutes ago, ChrisDL said:

Would you have a recommendation for a cylinder size? We are a family of four (might grow to five one day) but we are likely to have guests over, hence I have based my DHW requirements for 6 at 75l each

What temperature do you intend to store DHW at, that changes the number of kWh stored and the recharge time.

Keeping the temperature as low as possible reduces the losses and increases the CoP.

When guests stay, put the immersion heater on, cost less than a bottle of cheap wine.

[ChrisDL]

3 minutes ago, SteamyTea said:

What temperature do you intend to store DHW at, that changes the number of kWh stored and the recharge time.

Keeping the temperature as low as possible reduces the losses and increases the CoP.

When guests stay, put the immersion heater on, cost less than a bottle of cheap wine.

I had calculated at 50°C with a starting input temperature of 5°C in winter. Calculated this to require 23.57 kWh of energy to heat. However I realise now this is overcompensating as 50°C is too hot for any purpose and will be blended down. However I was originally thinking along the thermal store route..... 

In summer not a problem we can have the immersion running all the time to heat water but in winter we won't have the PV spare - back up generator yes - but converting diesel to electric back into heat isn't as economical as a straight up oil boiler... although it is simpler.... And if the ASHP still has a SCOP or 3 it will get back up closer to oils 10kWh/l

[JohnMo]

If you start over compensating things, things start to get bigger, a heat pump that's oversized is not a happy thing.  You will start to need bigger and bigger buffer, to keep it happy. The power required gets bigger, more generator run time.

 

A 12kW heat pump will need a flow rate close to 1.7m3/h to stop short cycling.

 

Why not go a hybrid solution for heat and DHW. Heat pump size and use to use down to about 5 degs, then let the other boiler take over.

 

If there is only four in the house, you would be reheating 300 lts of water, if you used 75l each, you seem to be sizing for 6 people.

[ProDave]

17 minutes ago, ChrisDL said:

I had calculated at 50°C with a starting input temperature of 5°C in winter. Calculated this to require 23.57 kWh of energy to heat. However I realise now this is overcompensating as 50°C is too hot for any purpose and will be blended down. However I was originally thinking along the thermal store route..... 

In summer not a problem we can have the immersion running all the time to heat water but in winter we won't have the PV spare - back up generator yes - but converting diesel to electric back into heat isn't as economical as a straight up oil boiler... although it is simpler.... And if the ASHP still has a SCOP or 3 it will get back up closer to oils 10kWh/l

Thermal store is a bad match for an ASHP.  Stick to an unvented cylinder.  I find DHW at 48 degrees plenty hot enough.

[ChrisDL]

Trying to calculate flow rates.... Maybe someone can correct me if I am wrong in this assumption. 

 

Worst case scenario (cold day/no solar gain)- we may need 8kWh in heat energy per hour to maintain a balmy 21°C inside. From UFH running at 29°C (I think polished concrete can't push past much more than this for fear of cracking) returning at 21°C we need 900l an hour or 15l per minute. 

 

So about half the capacity of a 12kW heat pump until we start using showers/baths etc.  So can heat pumps not run at half power? Or otherwise a buffer? Yes we are sizing for 6 - since there may well be 5 in a couple years and then guests.

 

I do want a hybrid system of sorts. Because we are off grid I need to be able to tap into a oil boiler or alternative when the PV generation is not there. But when energy is abundant is makes sense to maximise using a heat pump I guess. There are the odd crisp sunny days in winter when our batteries will be charged and we have excess electricity - original thought was to dump this into a thermal store... however @ProDave and others a Thermal store is not often recommended. Consequently still looking for this buffer and 'thermal battery' in a UVC system.

 

I feel like I am approaching the question of whether say a 500l UVC will suffice for DWH and heating ? Or do we need a buffer as well?

 

With respects to the 'professional services' one recommended a 300l buffer with a 300l UVC the other a 180l buffer and two 305l UVC's

 

In fact whilst I am at it... what even is the 'buffer' surely just a larger UVC will suffice?

 

Another question on my mind is since there is no temperature stratification in a UVC how is that optimised for UFH?

 

Sorry for all the questions.

 

23 minutes ago, JohnMo said:

 

A 12kW heat pump will need a flow rate close to 1.7m3/h to stop short cycling.

 

Why not go a hybrid solution for heat and DHW. Heat pump size and use to use down to about 5 degs, then let the other boiler take over.

 

If there is only four in the house, you would be reheating 300 lts of water, if you used 75l each, you seem to be sizing for 6 people.

[JohnMo]

UVC is full of drinking water, so if you wanted to take UFH from that cylinder you would need a coil for the UFH heat exchange.

 

Have attached a thermal store scheme with heat pump.

 

Neither scheme is that good for your CoP.

 

You can use the attached to calculate your UFH flow rates and mean temperature. A big delta T means low flow rates, a low delta T has high flow rates, you can manipulate the temperature to get same output from the floor. As its the mean temperature that determines heat given to room.

 

But there are two ways to heat the slab.  Long and slow, or is your screed is deep enough, batch charge at a slightly higher temp for about 6 hours.

 

You can also use the attached with the heating spreadsheet to work out a weather compensation line, just calculate the heat required on the spreadsheet at at +20 and -20, use the sheets to work out flow temperature there a is a just about straight line relationship between the two extremes.

 

Also remember the coldest day is a rare day, most the time the heating will be ticking over with a low demand.

 

[ChrisDL]

On 05/09/2022 at 09:49, JohnMo said:

UVC is full of drinking water, so if you wanted to take UFH from that cylinder you would need a coil for the UFH heat exchange.

 

Have attached a thermal store scheme with heat pump.

 

Neither scheme is that good for your CoP.

 

 

Thank you for the above.

Having a quick sum up I reckon my demand for heating energy and DHW will be almost equal across the year. Of course heating much greater when there is actually less PV.

 

I suppose in a ideal solution one would have two heat pumps and two cylinders. One running at 30 for UFH and another for DHW.

 

Compromising does a thermal store benefit UFH and ASHP cop more? Since it will be drawing heat from lower temperature water?

 

 

[JohnMo]

I noticed cool energy do a package consists of ASHP plus an exhaust air heat pump cylinder.  But if you have an oil or biomass, just use that with a UVC for DHW.

 

I have a theoretical heat demand of 3.2kW.  I am thinking of getting a 3kW heat pump just for my underfloor heating. No buffer.  Just need to get around how to make the gas boiler fire up if and when I hit a shortfall from the ASHP.  But it is big enough for most of the heating season, just not the extremes.  I can also then use it for cooling in the summer.  Electric demand is 0.7kWh, so would run on solar most the time.

[SteamyTea]

2 hours ago, JohnMo said:

I have a theoretical heat demand of 3.2kW.  I am thinking of getting a 3kW heat pump just for my underfloor heating. No buffer

Is it a large loop, how will it cope with just 4 or 500 W of demand?

[JohnMo]

If you heat load is that low I would suspect it would be off.  The average heat demand is around 1.8 to 2.0kW for the months of the heating season.

[Radian]

3 hours ago, JohnMo said:

I am thinking of getting a 3kW heat pump just for my underfloor heating.

Have you identified a particular model? I was looking at this Trianco Activair have you seen any others?

 

[JohnMo]

If I remember correctly the flow rate for the UFH is just over 12l/s

[JohnMo]

That's a good price, but the Trianco company has closed.  Pity.  I was looking at an inverter Dream heat pump.  It connects to smart life app so I was going to build a simple control logic in to that.

[Radian]

10 minutes ago, JohnMo said:

I was looking at an inverter Dream heat pump.

Smallest one is 5kW which is a bit too big for me without a buffer.