UPDATE ASHP OR GET GAS

[ST3VE78]

Hi, I’m having a dilemma with my heating system. So over 10 years ago We build our own home and my wife decided she wanted ashp. A firm that went bust not long after we had the system designed a system that consisted of 2 x Calorex aw9002 ashp’s, a 240l buffer tank, a 300l DHW cylinder and ufh over 3 floors with 100mm centres. The flow temp for the ufh is set to 35 and when outside temperature is above 5 degrees the house will warm to 22 deg no problem. When the outside temperature goes to 3 deg or lower, the flow temp struggles to get over 25 deg  and so the room temps struggle to get over 19 deg. This is confusing to me as the pumps will warm the DHW up to 60 deg no problem but struggle to get the ufh temp over 25. So, I’m debating getting a new ashp that will warm the water to a min of 35 deg when outside temp is below 0 deg or just get gas. 

[PeterW]

So did you use bare minimum insulation at the time ..? As that may be your issue. UFH won’t do better on gas at 35°C flow rates as it will short cycle unless the buffer is set to do long burns with multiple stats to control call for heat.

 

Start again with a heat calc but if it adds up I would stick with a new ASHP (sounds like a 16kW or similar) on R32 for the DHW and you should be fine. 

[JohnMo]

It's not set for weather compensation is it? Seen it plenty of time on here recently, where the weather compensation curve has been set the wrong way around, so as it gets colder outside the flow temp also gets colder.  Have you checked?

 

[ProDave]

I don't know those ASHP's but what is their rated power output?

 

To give any meaningful input we need to know a LOT more about the house, total size and as above insulation levels.  Did anyone do any proper heat loss calculations?  

[PeterW]

7 minutes ago, JohnMo said:

It's not set for weather compensation is it?

Doubt it as they are pretty old units - rated at nominal 9kW but at 0°C that drops to 6.5kW so by -5°C you can reckon it’s about 4.5kW or 9 for the pair assuming they are tandem.

 

 

[ST3VE78]

As peter said, no weather compensation. They do run in tandem and the total floor space is 358 m2. After years and years of researching these pumps, the only I can find is that they are good for heating swimming pools. I would like to think the insulation is really good. I’ve insulated and put 6mm ply on  all the stud walls and In Between both floors. The main issue is the heat pumps not putting out over 25 deg when it gets cold out. I have even tried turning most of the room stats off so there is less water to warm but that doesn’t make any difference. When I open the side of the ashp, the only thing I can do is adjust the ufh temp up to a max of 55 deg. I have tried this  but it doesn’t make any difference. There is no option for me to turn an internal immersion heater on. 

[ST3VE78]

I averaged the last 9 years between 18000 and 20000 kw per year, that was with the room stats set to 22 deg. I lowered the room stats to 20 this year because of the rise in electricity prices so hopefully I’ll be closer to 15000 kw this year.. For 10 months of the year the system is great, but the other 2 months when heat is needed, it’s rubbish and costs a fortune. 

[SteamyTea]

As others have said, needs a lot more information about your house.

 

How much insulation is there under the ground floor UFH?

 

You also seem to have quite a large buffer tank.

[ProDave]

9 hours ago, ST3VE78 said:

The main issue is the heat pumps not putting out over 25 deg when it gets cold out. I have even tried turning most of the room stats off so there is less water to warm but that doesn’t make any difference

This bit is puzzling me.  You say it is still able to heat the hot water.  Are you SURE it is heating the hot water only with the ASHP and not turning on the immersion heater in the HW tank?  That is something to establish first.  i.e. get it to heat the HW and while it is doing so, measure the flow and return pipe temperatures.

 

If you have turned off most of the UFH so it is only trying to heat a small area and it still can't get hotter than 25 degrees would suggest a problem with the heat pumps.  Bit do that HW check first please.

[Radian]

41 minutes ago, ProDave said:

Are you SURE it is heating the hot water only with the ASHP and not turning on the immersion heater in the HW tank? 

 

As far as I was aware, swimming pool heat pumps just aren't designed to operate year-round and can only reach mid-thirties flows. I've looked at a few because they're a fraction of the price!

 

[ProDave]

6 minutes ago, Radian said:

 

As far as I was aware, swimming pool heat pumps just aren't designed to operate year-round and can only reach mid-thirties flows. I've looked at a few because they're a fraction of the price!

 

That's what I am trying to establish.  Swimming pool heaters often don't have a defrost function as they are expected only to run in mild weather.  If that is what @ST3VE78 has then changing for a proper ASHP would work.

[ST3VE78]

I have  100mm insulation under the ground floor with 70mm liquid screed. Other 2 floors I fitted batten and put the pipes in 25mm of screed. The house was built over 10 years ago and built to a better spec than building regs. When the water comes out of the pump at 35 degrees the house warms well and keeps the heat.  In cold weather the 25 deg that comes out of the pump is too low to put any heat into the house.  The temps I get from the manifolds in the house are the same as what the heat pump says so there’s not really any heat lost through the buffer. Ashp’s Have come a long way since I the ones I have so I think it’s time for an upgrade. Any recommendations on new ashp or reliable firms that deal with these. I have all the details from the previous install. 

[ST3VE78]

Hi Dave, have read a lot of your posts over the years of researching my pumps, all good may I add, My DHW has an internal 3kw immersion that I have a timer on that I set through the winter. It is separate from the heat pumps. When it’s really cold I just heat the DHW from the immersion only so the heat pumps can focus on the ufh. Not that that makes much difference tho. The pumps do start to defrost when it gets cold, I’m thinking around 7 deg.  

[ST3VE78]

That’s my set up. The temp will go up to 35 easy enough today as it’s about 9 deg. It’s only just came on as it does the DHW 8-10 am. 

[Andehh]

Afraid I can't help much other then to say Bravo, there's some serious history in those ASHP!! 😨

 

Are you able to add any more insulation anywhere in the house? As with laptops & mobile phones, what was high end 10 years ago has sadly moved on now, insulation standards being no different.

[ST3VE78]

More insulation is not possible sadly. 

[ProDave]

So it looks like it is the old ASHP's not able to do the job then.

 

A modern ASHP will work in sub zero weather, mine has not let us down and continues working at -10 outside.  You need a good estimate of the heat input required to properly size a replacement.  Have any calculations been done?

[Conor]

Do you live by the sea? You might want to upgrade to a coastal rated one next 🤣

[ST3VE78]

When my architect completed the SAP he said space heating kWh per year 21,158 and water heating kWh per year 2,578.  When it was -8 the other week, the heat pumps wouldn’t put out more the 19 deg 🥶 

I’ve been meaning to paint them. Only 2 years ago I put the covers above them. Bit like shutting the stable door after the horse has bolted 😂

[SteamyTea]

Have you taken the rusty covers off and made sure the heat exchangers are clear?

[billt]

4 hours ago, ProDave said:

Swimming pool heaters often don't have a defrost function as they are expected only to run in mild weather.  If that is what @ST3VE78 has then changing for a proper ASHP would work.

 

I think that you'll find that most swimming pool heat pumps have a defrost function. Mine has, as did th eancient Calorex model that it replaced. They can be used in the spring/autumn when you can have low early/late temperatues and icing is going to occur, even though midday temeratures may be mild.

 

I'd be very surprised if the OPs heat pumps were designed for swimming pools though.

 

A quick search finds this http://www.gasapplianceguide.co.uk/Heat Pumps/Technical Info/Calorex heat pumps 1.pdf

which shows that it is designed for normal heating purposes but only has an output of 6.56kW at 0C air temp/ 35C water temp, IOW it;s not going to be capable enough in very low temperatures. Earlier in the brochure it suggests that it should use a supplementary boiler in very cold weather.

[ProDave]

37 minutes ago, billt said:

A quick search finds this http://www.gasapplianceguide.co.uk/Heat Pumps/Technical Info/Calorex heat pumps 1.pdf

which shows that it is designed for normal heating purposes but only has an output of 6.56kW at 0C air temp/ 35C water temp, IOW it;s not going to be capable enough in very low temperatures. Earlier in the brochure it suggests that it should use a supplementary boiler in very cold weather.

But that PDF says at 0 degree air, it can output 6.58kW at 35 degree water, or 5.63kW at 55 degree water.  And @ST3VE78 has two of them so double that.  So at 0 degrees he should get 13kW of 35 degree water.

 

So either his house is consuming WAY more that 13kW of heat and it can't cope, or the units are faulty.

 

I don't think anyone has asked the obvious question yet, has it EVER worked properly?

[PeterW]

@ST3VE78 from memory those calorex units have a built in pump - is there a Y strainer anywhere in the system and have you checked if it’s blocked ..? Also worth checking the pumps themselves as they could be running very slowly and increasing speed may help

 

Also worth getting hold of 4 cheap digital thermometers and putting them on all 4 of the buffer pipes at about 1m from the buffer itself.
 

have to say though the units look on their last legs !!
 

[PeterW]

Actually these were rebadged by Ideal - sold as secondary to a boiler 

 

http://www.gasapplianceguide.co.uk/Heat Pumps/Technical Info/Airtherm User Manual 1.pdf

 

@ST3VE78 where are the pumps installed in the diagram you’ve got as there is only one shown..?? Are they on the supply or return..?

 

 

 

[ST3VE78]

No is the answer Dave. They have always struggled when the outside temperature drops. When it’s warm out your feet are toasty on the ufh. Because the company went bust soon after I fitted them and lack of knowledge in my area with ashp’’s I’ve just put up with them. It took me nearly 2 years after fitting to get someone who was MCS accredited to sign them off so I could receive the grant. 

 

I have not  checked the the heat exchangers steamy tea but I did have a local guy come to look at them 2 years ago as 1 stopped working. It turned out a wire had burned the plastic in the big red on off switch.

 

There are 2 pumps in the drawing. One above the other. I presume they both supply and return as I can have one to go off at say 30 when it’s warm out and the other will take the water to 35+ easy. I think I’ve been sold a duff system by the look of it. 

 

As andehh said, these ashp’s have come along way in the last 10+ years so I’d like to think there’s something out there that would work well with the rest of my system.