Boiler temperature for hybrid geothermal heat pump system

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Hi eQuesters,

I have a problem regarding hybrid geothermal heat pump system built in a heating dominated building. From ASHRAE Handbook, it suggests that boiler be installed separately from geothermal heat exchanger loop due to high temperature of boiler.

In addition, one report says that even if there are some cases that boiler has to be installed into GHX loop, the setpoint shall be 5-10F lower than GHX setpoint( e.g. 45F). That is because if the boiler is installed in the GHX loop, it is likely to dump heat back into the ground if boiler setpoint is higher than GHX setpoint.

However, heat exchange is related to both temperature and flow rate. With smaller size of boiler, can we allow to install boiler with higher setpoint higher than GHX setpoint into the GHX loop? Thanks!

All the best,
Siliang

via Equest-users's picture
Joined: 2016-07-15
Reputation: 400

Hi Siliang,

The Water loop set-point that you have got as may be 70 ?F is the water
temperature that is 21.11 ?C.

It is obvious to take set-point of the boiler below the Ground source HX as
then it will operate.

The ground source steam temperature may shoot up-to 360-370 ?F.

So may be taking steam boiler or steam plus hybrid boiler that is
electricity and keeping the set-point from 180 to 230 ?F can be a take.

One can keep the set-point of the ground water heating loop as more than
230 ?F to 370 ?F may be 360 ?F.

Then the functioning will be okay.

Generally the boiler set-point of electrical heater is 180 ?F for steam
water to 230 ?F which is nearly 82 ?C (180 ?F ).

*Thanks*,
Sharad.

---------- Forwarded message ----------
From: Siliang Lu via Equest-users
To: "equest-users at lists.onebuilding.org" Cc:
Date: Sat, 6 Aug 2016 16:14:27 -0400
Subject: [Equest-users] Boiler temperature for hybrid geothermal heat pump
system
Hi eQuesters,

I have a problem regarding hybrid geothermal heat pump system built in a
heating dominated building. From ASHRAE Handbook, it suggests that boiler
be installed separately from geothermal heat exchanger loop due to high
temperature of boiler.

In addition, one report says that even if there are some cases that boiler
has to be installed into GHX loop, the setpoint shall be 5-10F lower than
GHX setpoint( e.g. 45F). That is because if the boiler is installed in the
GHX loop, it is likely to dump heat back into the ground if boiler setpoint
is higher than GHX setpoint.

However, heat exchange is related to both temperature and flow rate. With
smaller size of boiler, can we allow to install boiler with higher setpoint
higher than GHX setpoint into the GHX loop? Thanks!

All the best,
Siliang

via Equest-users's picture
Joined: 2016-07-15
Reputation: 400

Hi Sharad,
Thanks for your reply.
My design is ambient geothermal heat pump system. The water loop
temperature ranges between 55-85F and the ground temperature is not so high
as 360-370F. What I am wondering is that if the boiler setpoint could be
much higher than ground heat exchanger setpoint if the size of boiler is
much smaller than that of GHX? Thanks a lot!

Have a great day!

All the best,
Siliang

Siliang

via Equest-users's picture
Joined: 2016-07-15
Reputation: 400

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via Equest-users's picture
Joined: 2016-07-15
Reputation: 400

Hi Siliang,

I think that i was right in the discussion.

The boiler set-point should be lower than the heating ground water steam
exchanger.

See the following cases:

1. The boiler set-point can be higher than the ground steam water heat
exchanger if you apply some extra heat or fuel and you apply more than one
fuel then it can be a hybrid boiler. So electricity can be the fuel to
enhance the heat.

2. If the boiler is of less size then there can not be any scope to
increase the set-point of the boiler as there can not be any extra heat
added to the boiler as it is only again a heat exchanger.

The temperature that the 360 or 370 ?F i am talking about is the steam
coming out of the earth crust.

*Thanks,*
Sharad.

---------- Forwarded message ----------
From: Siliang Lu via Equest-users
To: Sharad Kumar
Cc: equest-users ,
equest-users-request at lists.onebuilding.org
Date: Mon, 8 Aug 2016 08:58:55 -0400
Subject: Re: [Equest-users] Boiler temperature for hybrid geothermal heat
pump system
Hi Sharad,
Thanks for your reply.
My design is ambient geothermal heat pump system. The water loop
temperature ranges between 55-85F and the ground temperature is not so high
as 360-370F. What I am wondering is that if the boiler setpoint could be
much higher than ground heat exchanger setpoint if the size of boiler is
much smaller than that of GHX? Thanks a lot!

Have a great day!

All the best,
Siliang

via Equest-users's picture
Joined: 2016-07-15
Reputation: 400

Thank you all!

Best,
Siliang

Siliang

via Equest-users's picture
Joined: 2016-07-15
Reputation: 400