could i use my existing UDHWC with proposed heat pump?

[Gordo]

I am considering a ASHP to replace my pellet boiler. I have done space heating heat loss calculations that indicate a peak load of 4.7Kwh at -1.5 degrees outside. So aiming at at 5Kwh heat pump. I have a pretty new 200l UDHWC with a 16kw / 0.7m2 primary heating coil which i am reluctant to dump. if using the pellet boiler to heat it i have to close off gate valve to tank to almost closed because the DHW demand is so great. I generally heat tank with immersion heater at night on E7 or with solar in summer and use about 2-6Kwh per night as it works out cheaper.

 

I think i would be a loooooong time payback on new HP tank so would prefer to keep my tank and heat it with the HP. Has anyone got any advice for me please?

[G and J]

I believe that with that small a coil you’ll need a high flow temp from your heat pump to heat water effectively.  That will hit your COP hard and I presume the extra load on the heat pump would increase noise and wear.  
 

Maybe some form of heat exchanger may work better but it still might be simpler and easier (and who knows maybe even cheaper) just to accept the tank replacement as part of the cost of boiler replacement. 

[Gordo]

53 minutes ago, G and J said:

I believe that with that small a coil you’ll need a high flow temp from your heat pump to heat water effectively.  That will hit your COP hard and I presume the extra load on the heat pump would increase noise and wear.  
 

Maybe some form of heat exchanger may work better but it still might be simpler and easier (and who knows maybe even cheaper) just to accept the tank replacement as part of the cost of boiler replacement. 

I hear you and agree if tank was old id replace. but at £1500 the payback would be unrealistic. Heating with immersion works out at  around £255 PA. if the heat pump managed a COP of say 2.5 id save say £102 PA with a payback of over 15 years if i replaced tank. 

 

I guess what Im wondering is if a 5Kwh heat pump could heat my existing tank with its 16Kw coil?  I assume the 16Kw is based on a flow temp of say 60 degrees. Most HPs can manage 60 degrees but probably not with a good COP. Maybe im ignorant but id assume 5Kwh DHW load with 16Kw coil at 60 degrees would give theoretical a heat recovery in the region of 30 minutes now. From experience that bears out now. But if i fit a 5Kwh heat pump that obviously changes. Surley the heat power from HP would be say 2.5Kwh maybe at 60 degrees (maybe this is silly thinking and it remains at 5Kwh with more electric consumption). This would mean a reheat time of say 2 hours at 2.5Kwh (or 1hr at 5kwh) at around 60 degrees. I assume the HP reduces the flow rate automatically to achieve the set 60 degrees. maybe iv got it all wrong. help i need educating

Edited November 8 by Gordo

[Dillsue]

Some options might be-

Speak to the cylinder manufacturer and ask them about recovery/reheat times at various flow temperatures

Turn the pellet flow temp down to 50degrees, if possible, and test the reheat time

Temporarily fit a blending valve on the flow to the cylinder and test a various flow temps. Bit of work but may save your existing cylinder

[JohnMo]

Think you will struggle to get the heat transfer. The 16kW rating comes from flow temps circa 80 degs.

 

The alternative to replacing the cylinder is to use a plate exchanger. One side goes to the heat pump. The other to the hot and cold tapping at bottom and top of cylinder. You would have a circulation pump suitable for DHW on the cylinder side.

 

When you get a call for heat the heat pump starts and so does the circulation pump.

 

The bigger the heat exchanger the better the CoP.

 

So something like this

https://beetbg.com/products/dhw-plate-loading-20xplate-kit

[JoeBano]

I’m using my old unvented 180l cylinder megaflow on my 10kw grant aerona3 heat pump. I can warm it up from cold to 48c in 45-50 mins, my wife had a 10 min shower last night about 15-20 mins after she got out I checked the store temperature was fully charged again. I do believe it has a better coil than a standard cylinder. Like said above I would turn your pellet boiler down to 55c flow temperature and to a timed test. I’m not sure if your will get short cycle on a dwh run with your desired heat pump

[JamesPa]

40 minutes ago, JohnMo said:

Think you will struggle to get the heat transfer. The 16kW rating comes from flow temps circa 80 degs.

 

The alternative to replacing the cylinder is to use a plate exchanger. One side goes to the heat pump. The other to the hot and cold tapping at bottom and top of cylinder. You would have a circulation pump suitable for DHW on the cylinder side.

 

When you get a call for heat the heat pump starts and so does the circulation pump.

 

The bigger the heat exchanger the better the CoP.

 

So something like this

https://beetbg.com/products/dhw-plate-loading-20xplate-kit

Its good to see somebody is now offering this as a kit, which sort of brings the option a bit more mainstream.

[G and J]

8 hours ago, Gordo said:

I hear you and agree if tank was old id replace. but at £1500 the payback would be unrealistic. Heating with immersion works out at  around £255 PA. if the heat pump managed a COP of say 2.5 id save say £102 PA with a payback of over 15 years if i replaced tank. 

 

I guess what Im wondering is if a 5Kwh heat pump could heat my existing tank with its 16Kw coil?  I assume the 16Kw is based on a flow temp of say 60 degrees. Most HPs can manage 60 degrees but probably not with a good COP. Maybe im ignorant but id assume 5Kwh DHW load with 16Kw coil at 60 degrees would give theoretical a heat recovery in the region of 30 minutes now. From experience that bears out now. But if i fit a 5Kwh heat pump that obviously changes. Surley the heat power from HP would be say 2.5Kwh maybe at 60 degrees (maybe this is silly thinking and it remains at 5Kwh with more electric consumption). This would mean a reheat time of say 2 hours at 2.5Kwh (or 1hr at 5kwh) at around 60 degrees. I assume the HP reduces the flow rate automatically to achieve the set 60 degrees. maybe iv got it all wrong. help i need educating

I’m hoping (having read round the subject lots) for a COP of 4+ on DHW,  even better on heating.   
 

I think that turns your water heating cost of £255 per year into less than £65, reducing the payback period considerably.  

How long are you staying in your house and what’s your guess re future lecky prices?

I like the plate exchanger idea but I’d shy away from having another pump and more stuff to go wrong.   But I’m tight so I’d try v hard to sell the cylinder.  

[PhilT]

9 hours ago, Gordo said:

4.7Kwh at -1.5 degrees outside. So aiming at at 5Kwh heat pump.

That would be risky in an area which gets as low as -11°C to -18°C. The heating would be off for much of the time during lots of defrost cycles and while the heat pump struggles to get your hot water up to temperature. You talk of "payback" but what value do you put on comfort, convenience and lack of worry?

[G and J]

2 minutes ago, PhilT said:

gets as low as -11°C to -18°C

Crumbs.  I’m glad I live in Suffolk!

[JohnMo]

38 minutes ago, G and J said:

COP of 4+ on DHW

Dream on, unless you do a massive coil and heat slowly over an hour or two, then you may get that. 3 to 3.5 is more normal.

[G and J]

38 minutes ago, JohnMo said:

Dream on, unless you do a massive coil and heat slowly over an hour or two, then you may get that. 3 to 3.5 is more normal.

We certainly intend to get a cylinder with a coil specifically for a heat pump, but heating slowly might conflict with the idea of heating in bursts using the two or three low rate tariff periods each day.  I think that reinforces the case for a solar diverter or a battery (but not both).

[PhilT]

1 hour ago, G and J said:

 I think that reinforces the case for a solar diverter or a battery (but not both).

Neither can be justified on purely financial grounds if you have any kind of export and off peak tariff. It costs me 4p/kWh (12p/COP of 3) to heat my DHW. I get 15p/kWh credit for excess solar exported so it would cost me 15p/kWh to heat my DHW if I used a diverter, not to mention the up front cost. The up front commercial cost of a battery just takes too long to pay back, but do the spreadsheet to satisfy yourself that is the case. 

Edited November 8 by PhilT

[sharpener]

3 hours ago, PhilT said:

That would be risky in an area which gets as low as -11°C to -18°C.

 

I think OP is in N Ireland so quite mild.

 

5 hours ago, JohnMo said:

Think you will struggle to get the heat transfer. The 16kW rating comes from flow temps circa 80 degs.

 

I have trodden this path ahead of you and agree with this opinion, see this thread for full gory details. Heat exchanger a better technical option, less disruptive to install and prob. cheaper than the marginal cost of upgrading the tank even if you can find a buyer for the old one.

 

However it will be interesting to see what the kit costs, it is based on a SWEP HX, reputable brand, lots of further info here, water needs a pH of 7.5 or more to avoid corrosion (which my rainwater supply can't meet).

[PhilT]

2 hours ago, sharpener said:

 

I think OP is in N Ireland so quite mild

-18degC that's what the met office weather record is for Northern Ireland

[G and J]

3 minutes ago, PhilT said:

-18degC that's what the met office weather record is for Northern Ireland

But what should we sensibly dimension for?  We’ve known -15C in Suffolk once, many years ago methinks. But if I dimensioned for that I’d over engineer it. 

[JohnMo]

8 minutes ago, PhilT said:

18degC that's what the met office weather record

So not a heat pump design case then.

 

Even the MCS design guide says

 

"External design temperatures should reflect typical low

temperatures experienced in the winter heating season and as a guide should be exceeded for 99.6% of the year."

 

Edited November 8 by JohnMo

[G and J]

5 minutes ago, JohnMo said:

So not a heat pump design case then.

 

Even the MCS design guide says

 

"External design temperatures should reflect typical low

temperatures experienced in the winter heating season and as a guide should be exceeded for 99.6% of the year."

 

 

So roughly ignore the worst two days each year.  

[G and J]

Just now, G and J said:

So roughly ignore the worst two days each year.  

And that’s an organisation who it appears, at least to me, habitually oversized heat pumps….

[Conor]

@Gordo how old is your boiler and are you having any issues with it? Might be a case of waiting for something to go bang, then replace it with an ASHP.

 

And for sizing an ASHP here in NI... A huge factor in performance is frosting. It's so bloody humid here, my pump struggles most around just above freezing. I'm in Holywood and I can count the number of frosts each year on my hands. We don't do cold and dry here. Worth a bigger heat pump just to mitigate that issue as that's when you'll be needing the heat the most.

Edited November 8 by Conor

[Gordo]

9 hours ago, JoeBano said:

I’m using my old unvented 180l cylinder megaflow on my 10kw grant aerona3 heat pump. I can warm it up from cold to 48c in 45-50 mins, my wife had a 10 min shower last night about 15-20 mins after she got out I checked the store temperature was fully charged again. I do believe it has a better coil than a standard cylinder. Like said above I would turn your pellet boiler down to 55c flow temperature and to a timed test. I’m not sure if your will get short cycle on a dwh run with your desired heat pump

would you mind looking at your data label on UDHWC and let me know the Kw rating of primary heating coil for comparison.

[Gordo]

2 hours ago, PhilT said:

-18degC that's what the met office weather record is for Northern Ireland

yes N. Ireland. i have historical data for past few years which show it rarely goes below 0. So if i have the option i will design for 0 degrees, but suspect MCS engineer would refuse to do that. MCS state design for -1.5 degrees for Belfast. in the few days it goes below 0 degrees im happy to fire up the stove.

[Gordo]

30 minutes ago, Conor said:

@Gordo how old is your boiler and are you having any issues with it? Might be a case of waiting for something to go bang, then replace it with an ASHP.

 

And for sizing an ASHP here in NI... A huge factor in performance is frosting. It's so bloody humid here, my pump struggles most around just above freezing. I'm in Holywood and I can count the number of frosts each year on my hands. We don't do cold and dry here. Worth a bigger heat pump just to mitigate that issue as that's when you'll be needing the heat the most.

the boiler is only about 10 years old. it has quite some life left in it yet. I hear grant will be coming in next year or two so just planning a head. I didnt want to over size the pump as the average temperature is around 7 degrees. dont know much about frosting up, so ill have to research that end. Does an oversized ASHP frost up less?

Edited November 8 by Gordo

[JohnMo]

24 minutes ago, Gordo said:

the boiler is only about 10 years old. it has quite some life left in it yet. I hear grant will be coming in next year or two so just planning a head. I didnt want to over size the pump as the average temperature is around 7 degrees. dont know much about frosting up, so ill have to research that end. Does an oversized ASHP frost up less?

I quite like hybrid systems, no frosting issues, no heating system changes needed, no cylinder changes. Dinky heat pump needed, heat pump does nearly all the heating, use the boiler for DHW and when temps get to 3 or so and below.

 

You won't get a grant, but you only need a small heat pump, and would probably cost less, compared to you than getting a grant and the final bill.

[JoeBano]

On 08/11/2024 at 18:15, Gordo said:

would you mind looking at your data label on UDHWC and let me know the Kw rating of primary heating coil for comparison.

Here is the label off unvented cylinder

Coil rating

@ 15l/min/80°C: 19.4kW